task_manager.h

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/* Copyright (C) 2012 Chris Vine

The library comprised in this file or of which this file is part is
distributed by Chris Vine under the GNU Lesser General Public
License as follows:

   This library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public License
   as published by the Free Software Foundation; either version 2.1 of
   the License, or (at your option) any later version.

   This library is distributed in the hope that it will be useful, but
   WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License, version 2.1, for more details.

   You should have received a copy of the GNU Lesser General Public
   License, version 2.1, along with this library (see the file LGPL.TXT
   which came with this source code package in the c++-gtk-utils
   sub-directory); if not, write to the Free Software Foundation, Inc.,
   51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.

However, it is not intended that the object code of a program whose
source code instantiates a template from this file or uses macros or
inline functions (of any length) should by reason only of that
instantiation or use be subject to the restrictions of use in the GNU
Lesser General Public License.  With that in mind, the words "and
macros, inline functions and instantiations of templates (of any
length)" shall be treated as substituted for the words "and small
macros and small inline functions (ten lines or less in length)" in
the fourth paragraph of section 5 of that licence.  This does not
affect any other reason why object code may be subject to the
restrictions in that licence (nor for the avoidance of doubt does it
affect the application of section 2 of that licence to modifications
of the source code in this file).

*/

#ifndef CGU_TASK_MANAGER_H
#define CGU_TASK_MANAGER_H

#include <deque>
#include <utility>     // for std::pair, std::move and std::forward
#include <exception>   // for std::exception
#include <memory>      // for std::unique_ptr
#include <type_traits> // for std::remove_reference and std::remove_const

#include <c++-gtk-utils/callback.h>
#include <c++-gtk-utils/thread.h>
#include <c++-gtk-utils/mutex.h>
#include <c++-gtk-utils/async_queue.h>
#include <c++-gtk-utils/shared_ptr.h>
#include <c++-gtk-utils/async_result.h>
#include <c++-gtk-utils/emitter.h>
#include <c++-gtk-utils/cgu_config.h>

namespace Cgu {

namespace Thread {

struct TaskError: public std::exception {
  virtual const char* what() const throw() {return "TaskError\n";}
};

/**
 * @class Cgu::Thread::TaskManager task_manager.h c++-gtk-utils/task_manager.h
 * @brief A thread-pool class for managing tasks in multi-threaded programs.
 * @sa Cgu::Thread::Future Cgu::AsyncResult Cgu::AsyncQueueDispatch Cgu::Callback::post()
 *
 * Cgu::Thread::Future operates on the principle of there being one
 * worker thread per task.  In some cases however, it may be better to
 * have a limited pool of worker threads executing a larger number of
 * tasks.  This class implements this approach via a thread pool.
 *
 * One common approach for thread pools of this kind is to set the
 * maximum number of threads to the number of cores, or some number
 * less than the number of cores, available on the local machine.  How
 * that can be determined is system specific (on linux it can be
 * obtained by, for example, inspecting the 'siblings' and 'cpu cores'
 * fields in /proc/cpuinfo or by using sysconf with the glibc
 * extension for _SC_NPROCESSORS_ONLN).
 *
 * Where the task needs to provide a result, two approaches can be
 * adopted.  First, the task callback can have a Cgu::AsyncResult
 * object held by Cgu::SharedLockPtr (or by std::shared_ptr having a
 * thread safe reference count) bound to it.  Alternatively, a task
 * can provide a result asynchronously to a glib main loop by calling
 * Cgu::Callback::post() when it is ready to do so.  From version
 * 2.0.13, the TaskManager::make_task_result(),
 * TaskManager::make_task_when(), TaskManager::make_task_when_full()
 * and TaskManager::make_task_compose() convenience wrapper methods
 * are provided which will set this up for you (including constructing
 * appropriate task callbacks) for target functions which return a
 * value.  Tasks can add other tasks, enabling the composition of an
 * arbitrary number of tasks to obtain a final result.
 * 
 * TaskManager objects do not provide thread cancellation.  Thread
 * cancellation is incompatible with the task-centred thread pool
 * model.  If task cancellation is wanted, use a Cgu::Thread::Future
 * (or Cgu::Thread::Thread or Cgu::Thread::JoinableHandle) object
 * instead, and have a dedicated thread for the cancelable task.
 *
 * If glib < 2.32 is installed, g_thread_init() must be called before
 * any TaskManager objects are constructed, which in turn means that
 * with glib < 2.32 TaskManager objects may not be constructed as
 * static objects in global namespace (that is, before g_thread_init()
 * has been called in the program).
 *
 * Any exceptions which propagate from a task will be consumed to
 * protect the TaskManager object, and to detect whether this has
 * happened there is a version of the TaskManager::add_task() method
 * which takes a second argument comprising a 'fail' callback.  If an
 * exception propagates from the 'fail' callback that is also consumed
 * and a g_critical() message issued.
 *
 * Tasks can be aborted by throwing Cgu::Thread::Exit (as well as any
 * other exception).  Where a thread is managed by a TaskManager
 * object, throwing Cgu::Thread::Exit will only terminate the task and
 * not the thread on which it is running (and will cause the 'fail'
 * callback to be executed, if there is one).
 *
 * TaskManager objects have no copy constructor or copy assignment
 * operator, as copying them would have no obvious semantic meaning.
 * Whilst swapping or moving TaskManager objects would be meaningful,
 * this is not implemented either because it would require an
 * additional internal lock to be thread safe, and the circumstances
 * in which moving or swapping would be useful are limited.  Where a
 * move option is wanted, a TaskManager object can be constructed on
 * free store and held by std::unique_ptr.
 *
 * Here is a compilable example of the calculator class referred to in
 * the documentation on the AsyncResult but which uses a TaskManager
 * object so that the calculator class can run more than one thread to
 * service its calculations:
 *
 * @code 
 *   #include <vector>
 *   #include <numeric>
 *   #include <ostream>
 *   #include <iostream>
 *
 *   #include <glib.h>
 *
 *   #include <c++-gtk-utils/task_manager.h>
 *   #include <c++-gtk-utils/async_result.h>
 *   #include <c++-gtk-utils/shared_ptr.h>
 *   #include <c++-gtk-utils/callback.h>
 *
 *   using namespace Cgu;
 *
 *   class Calcs {
 *     Thread::TaskManager tm;
 *   public:
 *     SharedLockPtr<AsyncResult<double>> mean(const std::vector<double>& nums) {
 *       SharedLockPtr<AsyncResult<double>> res(new AsyncResult<double>);
 *       tm.add_task(Callback::lambda<>([=]() {
 *             if (nums.empty()) res->set(0.0);
 *             else res->set(std::accumulate(nums.begin(), nums.end(), 0.0)/nums.size());
 *           }));
 *       return res;
 *     }
 *
 *     // ... other calculation methods here
 *   };
 *
 *   int main () {
 *
 *     g_thread_init(0);
 *     Calcs calcs;
 *     auto res1 = calcs.mean(std::vector<double>({1, 2, 8, 0}));
 *     auto res2 = calcs.mean(std::vector<double>({101, 53.7, 87, 1.2}));
 *
 *     // ... do something else
 *     std::cout << res1->get() << std::endl;
 *     std::cout << res2->get() << std::endl;
 *
 *   }
 * @endcode
 *
 * @b The @b TaskManager::make_task_result(), @b TaskManager::make_task_when_full(), @b TaskManager::make_task_when() and @b TaskManager::make_task_compose() @b functions
 *
 * From version 2.0.13 the TaskManager::make_task_result(),
 * TaskManager::make_task_when(), TaskManager::make_task_when_full()
 * and TaskManager::make_task_compose() convenience wrapper methods
 * are provided, which construct a task from a target function
 * returning a value by calling TaskManager::add_task() with an
 * appropriate callback object.  TaskManager::make_task_result()
 * returns a Cgu::AsyncResult object held by Cgu::SharedLockPtr which
 * will hold the result provided by the function;
 * TaskManager::make_task_when(), TaskManager::make_task_when_full()
 * and TaskManager::make_task_compose() execute a callback in a glib
 * main loop when the task has completed by passing the callback the
 * target function's return value.  The wrappers therefore provide a
 * similar interface to the one provided by Cgu::Thread::Future
 * objects.  These wrapper methods can make it easier to compose the
 * results of a number of different tasks.
 *
 * The TaskManager::make_task_result(), TaskManager::make_task_when()
 * and TaskManager::make_task_when_full() can take a plain function,
 * static member function or non-static member function as the target
 * function, and can take up to three arguments in the case of a
 * non-static member function, and four arguments in the case of any
 * other function. In the case of a non-static member function, the
 * referenced object whose member function is to be called must remain
 * in existence until the task concerned has completed.
 * Alternatively, a callable object such as a std::function object, a
 * lambda or the return value of std::bind can be passed, which can
 * have any number of arguments using lambda capture or std::bind (and
 * which can also bind the referenced object of a non-static member
 * function by taking a copy of it where that is necessary).
 * TaskManager::make_task_compose() only takes a function object for
 * its task.
 *
 * Where a callable object is not passed, internal moving/copying of
 * arguments for the target function to be represented by the task
 * takes place (once by invoking the rvalue move constructor or lvalue
 * copy constructor, as appropriate, when the wrapper methods are
 * called and, if the argument is not a const reference argument, once
 * when the task is dispatched by the TaskManager object). Therefore,
 * if a non-trivial class object is to be received by the target
 * function as an argument, it is best either (a) if it has a move
 * constructor, to pass it to the TaskManager::make_task_result(),
 * TaskManager::make_task_when() or TaskManager::make_task_when_full()
 * wrapper method as a temporary and have the target function take a
 * const reference argument, or (b) for it to be constructed on free
 * store and for the target function to receive it by pointer, by
 * Cgu::SharedLockPtr, or by a std::shared_ptr implementation which
 * has a thread-safe reference count.  Note also that constructing
 * callable objects using std::bind will cause copies of arguments to
 * be made, as will lambda capture, so when not using
 * TaskManager::make_task_compose() ordinarily it is better to pass a
 * function pointer with arguments to the wrapper methods rather than
 * a function object.
 *
 * Copying of the return value of the target function represented by
 * the task may also take place. When a task completes, the return
 * value will be stored, either in a Cgu::AsyncResult object (if
 * TaskManager::make_task_result() is called) or for the purposes of
 * executing the 'when' callback in a glib main loop (if
 * TaskManager::make_task_when(), TaskManager::make_task_when_full()
 * or TaskManager::make_task_compose() are called).  This storage will
 * therefore cause the return value type's assignment operator or copy
 * constructor to be called once unless that type has a move
 * assignment operator or move constructor, in which case a move
 * operation will be made.  Note that a 'when' callback takes the
 * stored return value by reference to const and so without any
 * additional copying upon the 'when' callback being executed in the
 * main loop.
 *
 * With version 2.0.13 of the library, if a callable object which was
 * not a std::function object (such as a lambda) was passed, the
 * return value had to be explicitly stated in the call to
 * make_task_*().  So, if a lambda expression returning an int was to
 * be executed as a task, TaskManager::make_task_result<int>(),
 * TaskManager::make_task_when_full<int>(),
 * TaskManager::make_task_when<int>() or
 * TaskManager::make_task_compose<int>() had to be called.  This is no
 * longer necessary with version 2.0.14: the return value will be
 * deduced automatically if it is not stated.
 *
 * Here is a compilable example of the calculator class using
 * TaskManager::make_task_result():
 * @code 
 *   #include <vector>
 *   #include <numeric>
 *   #include <ostream>
 *   #include <iostream>
 *
 *   #include <glib.h>
 *
 *   #include <c++-gtk-utils/task_manager.h>
 *   #include <c++-gtk-utils/async_result.h>
 *   #include <c++-gtk-utils/shared_ptr.h>
 *   #include <c++-gtk-utils/callback.h>
 *
 *   using namespace Cgu;
 *
 *   class Calcs {
 *     Thread::TaskManager tm;
 *   public:
 *     SharedLockPtr<AsyncResult<double>> mean(const std::vector<double>& nums) {
 *       return tm.make_task_result([=]() {
 *           if (nums.empty()) return 0.0;
 *           return std::accumulate(nums.begin(), nums.end(), 0.0)/nums.size();
 *         });
 *     }
 *
 *     // ... other calculation methods here
 *   };
 *
 *   int main () {
 *
 *     g_thread_init(0);
 *     Calcs calcs;
 *     auto res1 = calcs.mean(std::vector<double>({1, 2, 8, 0}));
 *     auto res2 = calcs.mean(std::vector<double>({101, 53.7, 87, 1.2}));
 *
 *     // ... do something else
 *     std::cout << res1->get() << std::endl;
 *     std::cout << res2->get() << std::endl;
 *
 *   }
 * @endcode
 *
 * Here is a reimplementation, using TaskManager::make_task_when(), of
 * the Number class example with get_primes() method given in the
 * documentation for Cgu::Thread::Future:
 * @code 
 *   class Numbers {
 *   public:
 *     std::vector<long> get_primes(int n); // calculates the first n primes
 *                                          // and puts them in a vector
 *    ...
 *   };
 *
 *   void print_primes(const std::vector<long>& result) {
 *     std::for_each(result.begin(), result.end(), [](long l) {std::cout << l << std::endl;});
 *   }
 *
 *   Numbers obj;
 *
 *   // get the first 1,000 primes
 *   using namespace Cgu;
 *   Thread::TaskManager tm;
 *   std::unique_ptr<const Callback::CallbackArg<const std::vector<long>&>> when(
 *     Callback::make(&print_primes)
 *   );
 *   tm.make_task_when(std::move(when),
 *                     0,                    // default main loop context
 *                     obj,
 *                     &Numbers::get_primes,
 *                     1000);
 * @endcode
 *
 * Where a member function or ordinary function to be represented by a
 * task is overloaded, this will cause difficulties in template type
 * deduction when TaskManager::make_task_result(),
 * TaskManager::make_task_when() or TaskManager::make_task_when_full()
 * are called.  Explicit disambiguation would be required, for
 * example:
 * @code
 *   class Numbers {
 *   public:
 *     int calc(int i);
 *     int calc(double d);
 *    ...
 *   };
 *
 *   Numbers obj;
 *
 *   using namespace Cgu;
 *
 *   Thread::TaskManager tm;
 *
 *   int i = 1;
 *   double d = 2.0;
 *
 *   auto res1 =
 *     tm.make_task_result(obj, static_cast<int (Numbers::*)(int)>(&Numbers::calc), i);
 *   auto res2 = 
 *     tm.make_task_result(obj, static_cast<int (Numbers::*)(double)>(&Numbers::calc), d);
 * @endcode
 */

// TODO: this is a work-around for gcc < 4.7, which has a bug which
// requires a function whose return value is determined by decltype,
// such as make_task_result(Func&&), to be inline.  At a suitable
// API/ABI break when gcc requirements are updated, this should be
// moved to task_manager.tpp.
namespace TaskManagerHelper {

template <class Ret, class FType>
struct FunctorResultWrapper {
  static void exec(FType& f,
                   const SharedLockPtr<AsyncResult<Ret>>& ret) {
    ret->set(f());
  }
  static void do_fail(const SharedLockPtr<AsyncResult<Ret>>& ret) {
    ret->set_error(); // won't throw
  }
};

/* 
 * The FunctorResultExec class is a specialised class which is
 * necessary because the 'functor member needs to be declared mutable
 * so that it can bind to the reference to non-const argument of
 * FunctorResultWrapper::exec(), and thus so that a mutable lambda can
 * be executed by that function.  Because it is so specialised, it is
 * not suitable for inclusion in the generic interfaces provided in
 * callback.h.  (Except in this specialised usage, it can also be
 * dangerous, as it allows a member of the callback object to be
 * mutated: normally this would be undesirable.)  An alternative would
 * have been to put the 'functor' member in a wrapper struct like
 * MemfunWhenWrapperArgs or FunWhenWrapperArgs, but if 'functor' were
 * an lvalue that would mean it being copied twice.  This is the most
 * efficient implementation.
 */
template <class Ret, class FType>
class FunctorResultExec: public Cgu::Callback::Callback {
  mutable FType functor;
  SharedLockPtr<AsyncResult<Ret>> ret;
public:
  void dispatch() const {FunctorResultWrapper<Ret, FType>::exec(functor, ret);}
  // we don't need to templatize 'ret_' for perfect forwarding - it is
  // always passed as a lvalue
  template <class FunctorArg>
  FunctorResultExec(FunctorArg&& functor_,
                    const SharedLockPtr<AsyncResult<Ret>>& ret_): functor(std::forward<FunctorArg>(functor_)),
                                                                  ret(ret_) {}
};

} // namespace TaskManagerHelper


class TaskManager {
 public:
  enum StopMode {wait_for_running, wait_for_all}; 
 private:
  typedef std::pair<std::unique_ptr<const Callback::Callback>,
                    std::unique_ptr<const Callback::Callback>> QueueItemType;

  struct RefImpl; // reference counted implementation class
  // it is fine holding RefImpl by plain pointer and not by
  // IntrusivePtr: it is the only data member this class has, so it
  // can safely manage that member in its own destructor and other
  // methods
  RefImpl* ref_impl;

  void set_max_threads_impl(unsigned int max);
 public:
/**
 * This class cannot be copied.  The copy constructor is deleted.
 */
  TaskManager(const TaskManager&) = delete;

/**
 * This class cannot be copied.  The assignment operator is deleted.
 */
  TaskManager& operator=(const TaskManager&) = delete;

 /**
  * Gets the maximum number of threads which the TaskManager object is
  * currently set to run in the thread pool. This value is established
  * initially by the 'max' argument passed to the TaskManager
  * constructor and can subequently be changed by calling
  * set_max_threads() or change_max_threads().  The default value is 8.
  * This method will not throw and is thread safe.
  * @return The maximum number of threads.
  *
  * Since 2.0.12
  */
  unsigned int get_max_threads() const;

 /**
  * Gets the minimum number of threads which the TaskManager object
  * will run in the thread pool (these threads will last until
  * stop_all() is called or the TaskManager object is destroyed).
  * This value is established by the 'min' argument passed to the
  * TaskManager constructor and cannot subequently be changed.  The
  * default is 0.  This method will not throw and is thread safe.
  * @return The minimum number of threads.
  *
  * Since 2.0.12
  */
  unsigned int get_min_threads() const;

 /**
  * Gets the number of threads which the TaskManager object is
  * currently running in the thread pool, including those blocking
  * waiting for a task.  This value could be greater than the number
  * returned by get_max_threads() if set_max_threads() has recently
  * been called with a value which is less than that number but not
  * enough tasks have since completed to reduce the number of running
  * threads to the new value set.  This method will not throw and is
  * thread safe.
  * @return The number of threads running in the thread pool,
  * including those blocking waiting for a task.
  *
  * Since 2.0.12
  */
  unsigned int get_used_threads() const;

 /**
  * Gets the number of tasks which the TaskManager object is at
  * present either running in the thread pool or has queued for
  * execution.  This value will be less than the number returned by
  * get_used_threads() if threads in the thread pool are currently
  * waiting to receive tasks for execution.  This method will not
  * throw and is thread safe.
  * @return The number of tasks either running or queued for
  * execution.
  *
  * Since 2.0.12
  */
  unsigned int get_tasks() const;

 /**
  * Sets the maximum number of threads which the TaskManager object
  * will currently run in the thread pool.  If this is less than the
  * current number of running threads, the number of threads actually
  * running will only be reduced as tasks complete, or as idle
  * timeouts expire.  This method does nothing if stop_all() has
  * previously been called.  This method is thread safe.
  * @param max The maximum number of threads which the TaskManager
  * object will currently run in the thread pool.  This method will
  * not set the maximum value of threads to a value less than that
  * returned by get_min_threads(), nor to a value less than 1.
  * @exception std::bad_alloc If this call is passed a value for 'max'
  * which increases the maximum number of threads from its previous
  * setting and tasks are currently queued for execution, new threads
  * will be started for the queued tasks, so this exception may be
  * thrown on starting the new threads if memory is exhausted and the
  * system throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError If this call is passed a value
  * for 'max' which increases the maximum number of threads from its
  * previous setting and tasks are currently queued for execution, new
  * threads will be started for the queued tasks, so this exception
  * may be thrown on starting the new threads if a thread fails to
  * start correctly (this would mean that memory is exhausted, the
  * pthread thread limit has been reached or pthread has run out of
  * other resources to start new threads).
  *
  * Since 2.0.12
  */
  void set_max_threads(unsigned int max);

 /**
  * This will increase, or if 'delta' is negative reduce, the maximum
  * number of threads which the TaskManager object will currently run
  * in the thread pool by the value of 'delta'.  The purpose of this
  * is to enable a task to increment the maximum thread number where
  * it is about to enter a call which may block for some time, with a
  * view to decrementing it later when it has finished making blocking
  * calls, so as to enable another thread to keep a core active.  If
  * 'delta' is negative and results in a max_threads value of less
  * than the current number of running threads, the number of threads
  * actually running will only be reduced as tasks complete, or as
  * idle timeouts expire.  This method does nothing if stop_all() has
  * previously been called.  This method is thread safe.
  * @param delta The change (positive or negative) to the maximum
  * number of threads which the TaskManager object will currently run
  * in the thread pool.  This method will not set the maximum value of
  * threads to a value less than that returned by get_min_threads(),
  * nor to a value less than 1.
  * @exception std::bad_alloc If this call is passed a positive value
  * and tasks are currently queued for execution, a new thread or
  * threads will be started for the queued tasks, so this exception
  * may be thrown on starting a new thread if memory is exhausted and
  * the system throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError If this call is passed a
  * positive value and tasks are currently queued for execution, a new
  * thread or threads will be started for the queued tasks, so this
  * exception may be thrown on starting a new thread if it fails to
  * start correctly (this would mean that memory is exhausted, the
  * pthread thread limit has been reached or pthread has run out of
  * other resources to start new threads).
  *
  * Since 2.0.14
  */
  void change_max_threads(int delta);

 /**
  * Gets the length of time in milliseconds that threads greater in
  * number than the minimum and not executing any tasks will remain in
  * existence waiting for new tasks.  This value is established
  * initially by the 'idle' argument passed to the TaskManager
  * constructor and can subequently be changed by calling
  * set_idle_time().  The default value is 10000 (10 seconds).  This
  * method will not throw and is thread safe.
  * @return The idle time in milliseconds.
  *
  * Since 2.0.12
  */
  unsigned int get_idle_time() const;

 /**
  * Sets the length of time in milliseconds that threads greater in
  * number than the minimum and not executing any tasks will remain in
  * existence waiting for new tasks.  This will only have effect for
  * threads in the pool which begin waiting for new tasks after this
  * method is called.  This method will not throw and is thread safe.
  * @param idle The length of the idle time in milliseconds during
  * which threads will remain waiting for new tasks.
  *
  * Since 2.0.12
  */
  void set_idle_time(unsigned int idle);

 /**
  * Gets the current blocking setting, which determines whether calls
  * to stop_all() and the destructor will block waiting for all
  * remaining tasks to complete.  This value is established initially
  * by the 'blocking' argument passed to the TaskManager constructor
  * and can subequently be changed by calling set_blocking().  This
  * method will not throw and is thread safe.
  * @return The current blocking setting.
  *
  * Since 2.0.12
  */
  bool get_blocking() const;

 /**
  * Sets the current blocking setting, which determines whether calls
  * to stop_all() and the destructor will block waiting for all
  * remaining tasks to complete.  This method cannot be called after
  * stop_all() has been called (if that is attempted,
  * Cgu::Thread::TaskError will be thrown).  It is thread safe.
  * @param blocking The new blocking setting.
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.
  *
  * Since 2.0.12
  */
  void set_blocking(bool blocking);

 /**
  * Gets the current StopMode setting (either
  * Cgu::Thread::TaskManager::wait_for_running or
  * Cgu::Thread::TaskManager::wait_for_all) executed when running
  * stop_all() or when the destructor is called.  See the
  * documentation on stop_all() for an explanation of the setting.
  * This value is established initially by the 'mode' argument passed
  * to the TaskManager constructor and can subequently be changed by
  * calling set_stop_mode().  This method will not throw and is thread
  * safe.
  * @return The current StopMode setting.
  *
  * Since 2.0.12
  */
  StopMode get_stop_mode() const;

 /**
  * Sets the current StopMode setting (either
  * Cgu::Thread::TaskManager::wait_for_running or
  * Cgu::Thread::TaskManager::wait_for_all) executed when running
  * stop_all() or when the destructor is called.  See the
  * documentation on stop_all() for an explanation of the setting.
  * This method will not throw and is thread safe.
  * @param mode The new StopMode setting.
  *
  * Since 2.0.12
  */
  void set_stop_mode(StopMode mode);

 /**
  * This will cause the TaskManager object to stop running tasks.  The
  * precise effect depends on the current StopMode and blocking
  * settings.  If StopMode is set to
  * Cgu::Thread::TaskManager::wait_for_running, all queued tasks which
  * are not yet running on a thread will be dispensed with, but any
  * already running will be left to complete normally.  If StopMode is
  * set to Cgu::Thread::TaskManager::wait_for_all, both already
  * running tasks and all tasks already queued will be permitted to
  * execute and complete normally.  If the blocking setting is set to
  * true, this method will wait until all the tasks still to execute
  * have finished before returning, and if false it will return
  * straight away.
  *
  * After this method has been called, any attempt to add further
  * tasks with the add_task() method will fail, and add_task() will
  * throw Cgu::Thread::TaskError.
  *
  * This method is thread safe (any thread may call it) unless the
  * blocking setting is true, in which case no task running on the
  * TaskManager object may call this method.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the system throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called, unless that previous call
  * threw std::bad_alloc: if std::bad_alloc is thrown, this method may
  * be called again to stop all threads, once the memory deficiency is
  * dealt with, but no other methods of the TaskManager object should
  * be called.
  *
  * Since 2.0.12
  */
  void stop_all();

 /**
  * This method adds a new task.  If one or more threads in the pool
  * are currently blocking and waiting for a task, then the task will
  * begin executing immediately in one of the threads.  If not, and
  * the value returned by get_used_threads() is less than the value
  * returned by get_max_threads(), a new thread will start and the
  * task will execute immediately in the new thread.  Otherwise, the
  * task will be queued for execution as soon as a thread becomes
  * available.  Tasks will be executed in the order in which they are
  * added to the ThreadManager object.  This method is thread safe
  * (any thread may call it, including any task running on the
  * TaskManager object).
  *
  * A task may terminate itself prematurely by throwing
  * Cgu::Thread::Exit.  In addition, the implementation of TaskManager
  * will consume any other exception escaping from the task callback
  * and safely terminate the task concerned in order to protect the
  * integrity of the TaskManager object.  Where detecting any of these
  * outcomes is important (usually it won't be), the two argument
  * version of this method is available so that a 'fail' callback can
  * be executed in these circumstances.
  *
  * @param task A callback representing the new task, as constructed
  * by the Callback::make(), Callback::make_ref() or
  * Callback::lambda() factory functions.  Ownership is taken of this
  * callback, and it will be disposed of when it has been finished
  * with.  The destructors of any bound arguments in the callback must
  * not throw.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).  If this exception
  * is thrown, the 'task' callback will be disposed of.
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)  If this exception is thrown, the 'task' callback will be
  * disposed of.
  * 
  * Since 2.0.12
  */
  void add_task(const Callback::Callback* task) {
#ifdef CGU_USE_AUTO_PTR
    add_task(std::auto_ptr<const Callback::Callback>(task),
             std::auto_ptr<const Callback::Callback>());
#else
    add_task(std::unique_ptr<const Callback::Callback>(task),
             std::unique_ptr<const Callback::Callback>());
#endif
  }

 /**
  * This method adds a new task.  If one or more threads in the pool
  * are currently blocking and waiting for a task, then the task will
  * begin executing immediately in one of the threads.  If not, and
  * the value returned by get_used_threads() is less than the value
  * returned by get_max_threads(), a new thread will start and the
  * task will execute immediately in the new thread.  Otherwise, the
  * task will be queued for execution as soon as a thread becomes
  * available.  Tasks will be executed in the order in which they are
  * added to the ThreadManager object.  This method is thread safe
  * (any thread may call it, including any task running on the
  * TaskManager object).
  *
  * A task may terminate itself prematurely by throwing
  * Cgu::Thread::Exit.  In addition, the implementation of TaskManager
  * will consume any other exception escaping from the task callback
  * and safely terminate the task concerned in order to protect the
  * integrity of the TaskManager object.  Where detecting any of these
  * outcomes is important (usually it won't be), a callback can be
  * passed to the 'fail' argument which will execute if, and only if,
  * either Cgu::Thread::Exit is thrown or some other exception has
  * propagated from the task.  This 'fail' callback is different from
  * the 'fail' callback of Cgu::Thread::Future objects (programming
  * for many tasks to a lesser number of threads requires different
  * approaches from programming for one thread per task), and it
  * executes in the task thread rather than executing in a glib main
  * loop (however, the 'fail' callback can of course call
  * Cgu::Callback::post() to execute another callback in a main loop,
  * if that is what is wanted).
  *
  * @param task A callback representing the new task, as constructed
  * by the Callback::make(), Callback::make_ref() or
  * Callback::lambda() factory functions.
  * @param fail A callback which will be executed if the function
  * executed by the 'task' callback exits by throwing Thread::Exit or
  * some other exception.  If an exception propagates from the
  * function represented by the 'fail' callback, this will be consumed
  * to protect the TaskManager object, and a g_critical() warning will
  * be issued.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note 1. Question: why does the single argument version of
  * add_task() take a pointer, and this version take the callbacks by
  * std::unique_ptr?  Answer: The two argument version of add_task()
  * takes its arguments by std::unique_ptr in order to be exception
  * safe if the first callback to be constructed is constructed
  * correctly but construction of the second callback object throws.
  * @note 2. If the library is compiled using the --with-auto-ptr
  * configuration option, then this method's signature is
  * add_task(std::auto_ptr<const Callback::Callback>,
  * std::auto_ptr<const Callback::Callback>) in order to retain
  * compatibility with the 1.2 series of the library.
  *
  * Since 2.0.12
  */
#ifdef CGU_USE_AUTO_PTR
  void add_task(std::auto_ptr<const Callback::Callback> task,
                std::auto_ptr<const Callback::Callback> fail);
#else
  void add_task(std::unique_ptr<const Callback::Callback> task,
                std::unique_ptr<const Callback::Callback> fail);
#endif

 /**
  * This will return true if a thread required by the thread pool has
  * failed to start correctly because of memory exhaustion or because
  * pthread has run out of other resources to start new threads, or
  * because an internal operation has thrown std::bad_alloc.  (On
  * systems with over-commit/lazy-commit combined with virtual memory
  * (swap), it is rarely useful to check for memory exhaustion, and
  * even more so where glib is used, as that terminates a program if
  * memory cannot be obtained from the operating system, but there may
  * be some specialized cases where the return value of this method is
  * useful - this class does not use any glib functions which might
  * cause such termination.)  This method will not throw and is thread
  * safe.
  * 
  * Since 2.0.12
  */
  bool is_error() const;

 /**
  * This is a wrapper which will take a member function pointer to a
  * member function which returns a value, together with arguments,
  * and constructs a TaskManager task which will execute that function
  * by calling add_task() with an appropriate callback object, and
  * returns a Cgu::AsyncResult object (held by Cgu::SharedLockPtr)
  * which will provide the value that the function returns.  It is
  * thread safe (any thread may call this method, including another
  * task running on the TaskManager object).  Apart from the absence
  * of a 'one thread per task' model, this method therefore provides a
  * similar interface to the one provided by Cgu::Thread::Future.  See
  * the documentation on add_task() for further information about how
  * task execution works.
  *
  * This method can take up to three bound arguments for the target
  * member function.
  *
  * If the function passed to this method exits by throwing
  * Thread::Exit or some other exception, then the returned
  * Cgu::AsyncResult object's get() method will unblock and its
  * get_error() method will return -1.
  *
  * @param t The object whose member function passed to this method is
  * to execute as a task.
  * @param func The member function to be executed as a task.
  * @param args The arguments to be passed to that member function.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note This method will also throw if the copy or move constructor
  * of a bound argument throws.
  *
  * Since 2.0.13
  */

  template <class Ret, class... Params, class... Args, class T>
  Cgu::SharedLockPtr<Cgu::AsyncResult<Ret>> make_task_result(T& t,
                                                             Ret (T::*func)(Params...),
                                                             Args&&... args);

 /**
  * This is a wrapper which will take a member function pointer to a
  * member function which returns a value, together with arguments,
  * and constructs a TaskManager task which will execute that function
  * by calling add_task() with an appropriate callback object, and
  * causes the 'when' callback passed as an argument to this method to
  * be executed by a glib main loop if and when the task finishes
  * correctly - the 'when' callback is passed the member function's
  * return value when it is invoked.  It is thread safe (any thread
  * may call this method, including another task running on the
  * TaskManager object).  Apart from the absence of a 'one thread per
  * task' model, this method therefore provides a similar interface to
  * the one provided by Cgu::Thread::Future.  See the documentation on
  * add_task() for further information about how task execution works.
  *
  * This method can take up to three bound arguments for the target
  * member function.
  *
  * Note that unlike add_task(), but like the 'fail' callback of
  * Cgu::Thread::Future objects, if a fail callback is provided to
  * this method and it executes, it will execute in the glib main loop
  * whose GMainContext object is passed to the 'context' argument of
  * this method.
  *
  * Note also that if releasers are provided for the 'when' or 'fail'
  * callbacks, these are passed by pointer and not by reference (this
  * is so that a NULL pointer can indicate that no releaser is to be
  * provided).  If provided, a releaser will enable automatic
  * disconnection of the 'when' or 'fail' callback, if the object
  * having the callback function as a member is destroyed. For this to
  * be race free, the lifetime of that object must be controlled by
  * the thread in whose main loop the 'when' or 'fail' callback will
  * execute.
  *
  * The make_task_when() method is similar to this method but provides
  * an abbreviated set of paramaters suitable for most cases.  This
  * method is for use where releasers or a 'fail' callback are
  * required.
  *
  * @param when A callback which will be executed if and when the
  * function passed to this method finishes correctly. The callback is
  * passed that function's return value when it is invoked.  It will
  * execute in the glib main loop whose GMainContext object is passed
  * to the 'context' argument of this method.
  * @param when_releaser A pointer to a releaser object for automatic
  * disconnection of the 'when' callback if the object of which the
  * callback function is a member is destroyed.  A value of
  * 0/NULL/nullptr indicates no releaser.
  * @param fail A callback which will be executed if the 'when'
  * callback does not execute.  This would happen if the function
  * passed to this method exits by throwing Thread::Exit or some other
  * exception or the copy constructor of a non-reference argument of
  * that function throws, or if the 'when' callback does not execute
  * because the internal implementation of this wrapper throws
  * std::bad_alloc (which will not happen if the library has been
  * installed using the –with-glib-memory-slices-no-compat
  * configuration option: instead glib will terminate the program if
  * it is unable to obtain memory from the operating system).  If an
  * exception propagates from the function represented by the 'fail'
  * callback, this will be consumed to protect the TaskManager object,
  * and a g_critical() warning will be issued.  The callback will
  * execute in the glib main loop whose GMainContext object is passed
  * to the 'context' argument of this method.  An empty
  * std::unique_ptr object indicates no 'fail' callback.
  * @param fail_releaser A pointer to a releaser object for automatic
  * disconnection of the 'fail' callback if the object of which the
  * callback function is a member is destroyed.  A value of
  * 0/NULL/nullptr indicates no releaser.
  * @param priority The priority to be given in the main loop to the
  * 'when' callback or any 'fail' callback. In ascending order of
  * priorities, priorities are G_PRIORITY_LOW,
  * G_PRIORITY_DEFAULT_IDLE, G_PRIORITY_HIGH_IDLE, G_PRIORITY_DEFAULT
  * and G_PRIORITY_HIGH. This determines the order in which the
  * callback will appear in the event list in the main loop, not the
  * priority which the OS will adopt.
  * @param context The glib main context of the main loop in which the
  * 'when' callback or any 'fail' callback is to be executed.  A value
  * 0/NULL/nullptr will cause the callback to be executed in the main
  * program loop.
  * @param t The object whose member function passed to this method is
  * to execute as a task.
  * @param func The member function to be executed as a task.
  * @param args The arguments to be passed to that member function.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note 1. This method will also throw if the copy or move
  * constructor of a bound argument throws.
  * @note 2. If a 'when_releaser' or a 'fail_releaser' argument is
  * provided, it is in theory possible (if memory is exhausted and the
  * system throws in that case) that an internal SafeEmitterArg object
  * will throw std::bad_alloc when emitting/executing the 'when' or
  * 'fail' callback in the glib main loop, with the result that the
  * relevant callback will not execute (instead the exception will be
  * consumed and a g_critical() warning will be issued).  This is
  * rarely of any relevance because glib will abort the program if it
  * is itself unable to obtain memory from the operating system.
  * However, where it is relevant, design the program so that it is
  * not necessary to provide a releaser object.
  * @note 3. If the library is compiled using the --with-auto-ptr
  * configuration option, then this method uses std::auto_ptr in place
  * of std::unique_ptr in its signature in order to retain
  * compatibility with the 1.2 series of the library.
  *
  * Since 2.0.13
  */
  template <class Ret, class... Params, class... Args, class T>
#ifdef CGU_USE_AUTO_PTR
  void make_task_when_full(std::auto_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                           Cgu::Releaser* when_releaser,
                           std::auto_ptr<const Cgu::Callback::Callback> fail,
                           Cgu::Releaser* fail_releaser,
                           gint priority,
                           GMainContext* context,
                           T& t,
                           Ret (T::*func)(Params...),
                           Args&&... args);
#else
  void make_task_when_full(std::unique_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                           Cgu::Releaser* when_releaser,
                           std::unique_ptr<const Cgu::Callback::Callback> fail,
                           Cgu::Releaser* fail_releaser,
                           gint priority,
                           GMainContext* context,
                           T& t,
                           Ret (T::*func)(Params...),
                           Args&&... args);
#endif

 /**
  * This is an abbreviated version of make_task_when_full(), which is
  * for use when it is known that the member function passed to this
  * method, and the copy constructors of any non-reference bound
  * arguments passed to it, do not throw, and the user is not
  * interested in std::bad_alloc and does not need a Cgu::Releaser
  * object for the 'when' callback (which is likely to cover the
  * majority of uses, particularly when composing tasks using glib
  * because glib terminates the program if it is unable to obtain
  * memory).
  *
  * This method can take up to three bound arguments for the target
  * member function.
  *
  * Like make_task_when_full(), this method is a wrapper which will
  * take a member function pointer to a member function which returns
  * a value, together with arguments, and constructs a TaskManager
  * task which will execute that function by calling add_task() with
  * an appropriate callback object, and causes the 'when' callback
  * passed as an argument to this method to be executed by a glib main
  * loop if and when the task finishes correctly - the 'when' callback
  * is passed the member function's return value when it is invoked.
  * It is thread safe (any thread may call this method, including
  * another task running on the TaskManager object).  Apart from the
  * absence of a 'one thread per task' model, this method therefore
  * provides a similar interface to the one provided by
  * Cgu::Thread::Future.  See the documentation on add_task() for
  * further information about how task execution works.
  *
  * The 'when' callback will execute with G_PRIORITY_DEFAULT priority
  * in the main loop.
  *
  * @param when A callback which will be executed if and when the
  * function passed to this method finishes correctly. The callback is
  * passed that function's return value when it is invoked.  It will
  * execute in the glib main loop whose GMainContext object is passed
  * to the 'context' argument of this method.
  * @param context The glib main context of the main loop in which the
  * 'when' callback is to be executed.  A value 0/NULL/nullptr will
  * cause the callback to be executed in the main program loop.
  * @param t The object whose member function passed to this method is
  * to execute as a task.
  * @param func The member function to be executed as a task.
  * @param args The arguments to be passed to that member function.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note 1. This method will also throw if the copy or move
  * constructor of a bound argument throws.
  * @note 2. If the library is compiled using the --with-auto-ptr
  * configuration option, then this method uses std::auto_ptr in place
  * of std::unique_ptr in its signature in order to retain
  * compatibility with the 1.2 series of the library.
  *
  * Since 2.0.13
  */
  template <class Ret, class... Params, class... Args, class T>
#ifdef CGU_USE_AUTO_PTR
  void make_task_when(std::auto_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                      GMainContext* context,
                      T& t,
                      Ret (T::*func)(Params...),
                      Args&&... args) {
    static_assert(sizeof...(Args) < 4,
                  "No greater than three bound arguments can be passed to "
                  "TaskManager::make_task_when() taking a member function.");

    make_task_when_full(when,
                        0,
                        std::auto_ptr<const Cgu::Callback::Callback>(),
                        0,
                        G_PRIORITY_DEFAULT,
                        context,
                        t,
                        func,
                        std::forward<Args>(args)...);
  }
#else
  void make_task_when(std::unique_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                      GMainContext* context,
                      T& t,
                      Ret (T::*func)(Params...),
                      Args&&... args) {
    static_assert(sizeof...(Args) < 4,
                  "No greater than three bound arguments can be passed to "
                  "TaskManager::make_task_when() taking a member function.");

    make_task_when_full(std::move(when),
                        0,
                        std::unique_ptr<const Cgu::Callback::Callback>(),
                        0,
                        G_PRIORITY_DEFAULT,
                        context,
                        t,
                        func,
                        std::forward<Args>(args)...);
  }
#endif

 /**
  * This is a wrapper which will take a member function pointer to a
  * member function which returns a value, together with arguments,
  * and constructs a TaskManager task which will execute that function
  * by calling add_task() with an appropriate callback object, and
  * returns a Cgu::AsyncResult object (held by Cgu::SharedLockPtr)
  * which will provide the value that the function returns.  It is
  * thread safe (any thread may call this method, including another
  * task running on the TaskManager object).  Apart from the absence
  * of a 'one thread per task' model, this method therefore provides a
  * similar interface to the one provided by Cgu::Thread::Future.  See
  * the documentation on add_task() for further information about how
  * task execution works.
  *
  * This method can take up to three bound arguments for the target
  * member function.
  *
  * If the function passed to this method exits by throwing
  * Thread::Exit or some other exception, then the returned
  * Cgu::AsyncResult object's get() method will unblock and its
  * get_error() method will return -1.
  *
  * @param t The object whose member function passed to this method is
  * to execute as a task.
  * @param func The member function to be executed as a task.
  * @param args The arguments to be passed to that member function.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note This method will also throw if the copy or move constructor
  * of a bound argument throws.
  *
  * Since 2.0.13
  */

  template <class Ret, class... Params, class... Args, class T>
  Cgu::SharedLockPtr<Cgu::AsyncResult<Ret>> make_task_result(const T& t,
                                                             Ret (T::*func)(Params...) const,
                                                             Args&&... args);

 /**
  * This is a wrapper which will take a member function pointer to a
  * member function which returns a value, together with arguments,
  * and constructs a TaskManager task which will execute that function
  * by calling add_task() with an appropriate callback object, and
  * causes the 'when' callback passed as an argument to this method to
  * be executed by a glib main loop if and when the task finishes
  * correctly - the 'when' callback is passed the member function's
  * return value when it is invoked.  It is thread safe (any thread
  * may call this method, including another task running on the
  * TaskManager object).  Apart from the absence of a 'one thread per
  * task' model, this method therefore provides a similar interface to
  * the one provided by Cgu::Thread::Future.  See the documentation on
  * add_task() for further information about how task execution works.
  *
  * This method can take up to three bound arguments for the target
  * member function.
  *
  * Note that unlike add_task(), but like the 'fail' callback of
  * Cgu::Thread::Future objects, if a fail callback is provided to
  * this method and it executes, it will execute in the glib main loop
  * whose GMainContext object is passed to the 'context' argument of
  * this method.
  *
  * Note also that if releasers are provided for the 'when' or 'fail'
  * callbacks, these are passed by pointer and not by reference (this
  * is so that a NULL pointer can indicate that no releaser is to be
  * provided).  If provided, a releaser will enable automatic
  * disconnection of the 'when' or 'fail' callback, if the object
  * having the callback function as a member is destroyed. For this to
  * be race free, the lifetime of that object must be controlled by
  * the thread in whose main loop the 'when' or 'fail' callback will
  * execute.
  *
  * The make_task_when() method is similar to this method but provides
  * an abbreviated set of paramaters suitable for most cases.  This
  * method is for use where releasers or a 'fail' callback are
  * required.
  *
  * @param when A callback which will be executed if and when the
  * function passed to this method finishes correctly. The callback is
  * passed that function's return value when it is invoked.  It will
  * execute in the glib main loop whose GMainContext object is passed
  * to the 'context' argument of this method.
  * @param when_releaser A pointer to a releaser object for automatic
  * disconnection of the 'when' callback if the object of which the
  * callback function is a member is destroyed.  A value of
  * 0/NULL/nullptr indicates no releaser.
  * @param fail A callback which will be executed if the 'when'
  * callback does not execute.  This would happen if the function
  * passed to this method exits by throwing Thread::Exit or some other
  * exception or the copy constructor of a non-reference argument of
  * that function throws, or if the 'when' callback does not execute
  * because the internal implementation of this wrapper throws
  * std::bad_alloc (which will not happen if the library has been
  * installed using the –with-glib-memory-slices-no-compat
  * configuration option: instead glib will terminate the program if
  * it is unable to obtain memory from the operating system).  If an
  * exception propagates from the function represented by the 'fail'
  * callback, this will be consumed to protect the TaskManager object,
  * and a g_critical() warning will be issued.  The callback will
  * execute in the glib main loop whose GMainContext object is passed
  * to the 'context' argument of this method.  An empty
  * std::unique_ptr object indicates no 'fail' callback.
  * @param fail_releaser A pointer to a releaser object for automatic
  * disconnection of the 'fail' callback if the object of which the
  * callback function is a member is destroyed.  A value of
  * 0/NULL/nullptr indicates no releaser.
  * @param priority The priority to be given in the main loop to the
  * 'when' callback or any 'fail' callback. In ascending order of
  * priorities, priorities are G_PRIORITY_LOW,
  * G_PRIORITY_DEFAULT_IDLE, G_PRIORITY_HIGH_IDLE, G_PRIORITY_DEFAULT
  * and G_PRIORITY_HIGH. This determines the order in which the
  * callback will appear in the event list in the main loop, not the
  * priority which the OS will adopt.
  * @param context The glib main context of the main loop in which the
  * 'when' callback or any 'fail' callback is to be executed.  A value
  * 0/NULL/nullptr will cause the callback to be executed in the main
  * program loop.
  * @param t The object whose member function passed to this method is
  * to execute as a task.
  * @param func The member function to be executed as a task.
  * @param args The arguments to be passed to that member function.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note 1. This method will also throw if the copy or move
  * constructor of a bound argument throws.
  * @note 2. If a 'when_releaser' or a 'fail_releaser' argument is
  * provided, it is in theory possible (if memory is exhausted and the
  * system throws in that case) that an internal SafeEmitterArg object
  * will throw std::bad_alloc when emitting/executing the 'when' or
  * 'fail' callback in the glib main loop, with the result that the
  * relevant callback will not execute (instead the exception will be
  * consumed and a g_critical() warning will be issued).  This is
  * rarely of any relevance because glib will abort the program if it
  * is itself unable to obtain memory from the operating system.
  * However, where it is relevant, design the program so that it is
  * not necessary to provide a releaser object.
  * @note 3. If the library is compiled using the --with-auto-ptr
  * configuration option, then this method uses std::auto_ptr in place
  * of std::unique_ptr in its signature in order to retain
  * compatibility with the 1.2 series of the library.
  *
  * Since 2.0.13
  */
  template <class Ret, class... Params, class... Args, class T>
#ifdef CGU_USE_AUTO_PTR
  void make_task_when_full(std::auto_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                           Cgu::Releaser* when_releaser,
                           std::auto_ptr<const Cgu::Callback::Callback> fail,
                           Cgu::Releaser* fail_releaser,
                           gint priority,
                           GMainContext* context,
                           const T& t,
                           Ret (T::*func)(Params...) const,
                           Args&&... args);
#else
  void make_task_when_full(std::unique_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                           Cgu::Releaser* when_releaser,
                           std::unique_ptr<const Cgu::Callback::Callback> fail,
                           Cgu::Releaser* fail_releaser,
                           gint priority,
                           GMainContext* context,
                           const T& t,
                           Ret (T::*func)(Params...) const,
                           Args&&... args);
#endif

 /**
  * This is an abbreviated version of make_task_when_full(), which is
  * for use when it is known that the member function passed to this
  * method, and the copy constructors of any non-reference bound
  * arguments passed to it, do not throw, and the user is not
  * interested in std::bad_alloc and does not need a Cgu::Releaser
  * object for the 'when' callback (which is likely to cover the
  * majority of uses, particularly when composing tasks using glib
  * because glib terminates the program if it is unable to obtain
  * memory).
  *
  * This method can take up to three bound arguments for the target
  * member function.
  *
  * Like make_task_when_full(), this method is a wrapper which will
  * take a member function pointer to a member function which returns
  * a value, together with arguments, and constructs a TaskManager
  * task which will execute that function by calling add_task() with
  * an appropriate callback object, and causes the 'when' callback
  * passed as an argument to this method to be executed by a glib main
  * loop if and when the task finishes correctly - the 'when' callback
  * is passed the member function's return value when it is invoked.
  * It is thread safe (any thread may call this method, including
  * another task running on the TaskManager object).  Apart from the
  * absence of a 'one thread per task' model, this method therefore
  * provides a similar interface to the one provided by
  * Cgu::Thread::Future.  See the documentation on add_task() for
  * further information about how task execution works.
  *
  * The 'when' callback will execute with G_PRIORITY_DEFAULT priority
  * in the main loop.
  *
  * @param when A callback which will be executed if and when the
  * function passed to this method finishes correctly. The callback is
  * passed that function's return value when it is invoked.  It will
  * execute in the glib main loop whose GMainContext object is passed
  * to the 'context' argument of this method.
  * @param context The glib main context of the main loop in which the
  * 'when' callback is to be executed.  A value 0/NULL/nullptr will
  * cause the callback to be executed in the main program loop.
  * @param t The object whose member function passed to this method is
  * to execute as a task.
  * @param func The member function to be executed as a task.
  * @param args The arguments to be passed to that member function.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note 1. This method will also throw if the copy or move constructor
  * of a bound argument throws.
  * @note 2. If the library is compiled using the --with-auto-ptr
  * configuration option, then this method uses std::auto_ptr in place
  * of std::unique_ptr in its signature in order to retain
  * compatibility with the 1.2 series of the library.
  *
  * Since 2.0.13
  */
  template <class Ret, class... Params, class... Args, class T>
#ifdef CGU_USE_AUTO_PTR
  void make_task_when(std::auto_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                      GMainContext* context,
                      const T& t,
                      Ret (T::*func)(Params...) const,
                      Args&&... args) {
    static_assert(sizeof...(Args) < 4,
                  "No greater than three bound arguments can be passed to "
                  "TaskManager::make_task_when() taking a member function.");

    make_task_when_full(when,
                        0,
                        std::auto_ptr<const Cgu::Callback::Callback>(),
                        0,
                        G_PRIORITY_DEFAULT,
                        context,
                        t,
                        func,
                        std::forward<Args>(args)...);
  }
#else
  void make_task_when(std::unique_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                      GMainContext* context,
                      const T& t,
                      Ret (T::*func)(Params...) const,
                      Args&&... args) {
    static_assert(sizeof...(Args) < 4,
                  "No greater than three bound arguments can be passed to "
                  "TaskManager::make_task_when() taking a member function.");

    make_task_when_full(std::move(when),
                        0,
                        std::unique_ptr<const Cgu::Callback::Callback>(),
                        0,
                        G_PRIORITY_DEFAULT,
                        context,
                        t,
                        func,
                        std::forward<Args>(args)...);
  }
#endif

 /**
  * This is a wrapper which will take a pointer to a function which
  * returns a value, together with arguments, and constructs a
  * TaskManager task which will execute that function by calling
  * add_task() with an appropriate callback object, and returns a
  * Cgu::AsyncResult object (held by Cgu::SharedLockPtr) which will
  * provide the value that the function returns.  It is thread safe
  * (any thread may call this method, including another task running
  * on the TaskManager object).  Apart from the absence of a 'one
  * thread per task' model, this method therefore provides a similar
  * interface to the one provided by Cgu::Thread::Future.  See the
  * documentation on add_task() for further information about how task
  * execution works.
  *
  * This method can take up to four bound arguments for the target
  * function.
  *
  * If the function passed to this method exits by throwing
  * Thread::Exit or some other exception, then the returned
  * Cgu::AsyncResult object's get() method will unblock and its
  * get_error() method will return -1.
  *
  * @param func The function to be executed as a task.
  * @param args The arguments to be passed to that function.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note This method will also throw if the copy or move constructor
  * of a bound argument throws.
  *
  * Since 2.0.13
  */
  template <class Ret, class... Params, class... Args>
  Cgu::SharedLockPtr<Cgu::AsyncResult<Ret>> make_task_result(Ret (*func)(Params...),
                                                             Args&&... args);

 /**
  * This is a wrapper which will take a pointer to a function which
  * returns a value, together with arguments, and constructs a
  * TaskManager task which will execute that function by calling
  * add_task() with an appropriate callback object, and causes the
  * 'when' callback passed as an argument to this method to be
  * executed by a glib main loop if and when the task finishes
  * correctly - the 'when' callback is passed the function's return
  * value when it is invoked.  It is thread safe (any thread may call
  * this method, including another task running on the TaskManager
  * object).  Apart from the absence of a 'one thread per task' model,
  * this method therefore provides a similar interface to the one
  * provided by Cgu::Thread::Future.  See the documentation on
  * add_task() for further information about how task execution works.
  *
  * This method can take up to four bound arguments for the target
  * function.
  *
  * Note that unlike add_task(), but like the 'fail' callback of
  * Cgu::Thread::Future objects, if a fail callback is provided to
  * this method and it executes, it will execute in the glib main loop
  * whose GMainContext object is passed to the 'context' argument of
  * this method.
  *
  * Note also that if releasers are provided for the 'when' or 'fail'
  * callbacks, these are passed by pointer and not by reference (this
  * is so that a NULL pointer can indicate that no releaser is to be
  * provided).  If provided, a releaser will enable automatic
  * disconnection of the 'when' or 'fail' callback, if the object of
  * which the releaser is a member is destroyed. For this to be race
  * free, the lifetime of that object must be controlled by the thread
  * in whose main loop the 'when' or 'fail' callback will execute.
  *
  * The make_task_when() method is similar to this method but provides
  * an abbreviated set of paramaters suitable for most cases.  This
  * method is for use where releasers or a 'fail' callback are
  * required.
  *
  * @param when A callback which will be executed if and when the
  * function passed to this method finishes correctly. The callback is
  * passed that function's return value when it is invoked.  It will
  * execute in the glib main loop whose GMainContext object is passed
  * to the 'context' argument of this method.
  * @param when_releaser A pointer to a releaser object for automatic
  * disconnection of the 'when' callback if the object of which the
  * releaser object is a member is destroyed.  A value of
  * 0/NULL/nullptr indicates no releaser.
  * @param fail A callback which will be executed if the 'when'
  * callback does not execute.  This would happen if the function
  * passed to this method exits by throwing Thread::Exit or some other
  * exception or the copy constructor of a non-reference argument of
  * that function throws, or if the 'when' callback does not execute
  * because the internal implementation of this wrapper throws
  * std::bad_alloc (which will not happen if the library has been
  * installed using the –with-glib-memory-slices-no-compat
  * configuration option: instead glib will terminate the program if
  * it is unable to obtain memory from the operating system).  If an
  * exception propagates from the function represented by the 'fail'
  * callback, this will be consumed to protect the TaskManager object,
  * and a g_critical() warning will be issued.  The callback will
  * execute in the glib main loop whose GMainContext object is passed
  * to the 'context' argument of this method.  An empty
  * std::unique_ptr object indicates no 'fail' callback.
  * @param fail_releaser A pointer to a releaser object for automatic
  * disconnection of the 'fail' callback if the object of which the
  * releaser object is a member is destroyed.  A value of
  * 0/NULL/nullptr indicates no releaser.
  * @param priority The priority to be given in the main loop to the
  * 'when' callback or any 'fail' callback. In ascending order of
  * priorities, priorities are G_PRIORITY_LOW,
  * G_PRIORITY_DEFAULT_IDLE, G_PRIORITY_HIGH_IDLE, G_PRIORITY_DEFAULT
  * and G_PRIORITY_HIGH. This determines the order in which the
  * callback will appear in the event list in the main loop, not the
  * priority which the OS will adopt.
  * @param context The glib main context of the main loop in which the
  * 'when' callback or any 'fail' callback is to be executed.  A value
  * 0/NULL/nullptr will cause the callback to be executed in the main
  * program loop.
  * @param func The function to be executed as a task.
  * @param args The arguments to be passed to that function.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note 1. This method will also throw if the copy or move
  * constructor of a bound argument throws.
  * @note 2. If a 'when_releaser' or a 'fail_releaser' argument is
  * provided, it is in theory possible (if memory is exhausted and the
  * system throws in that case) that an internal SafeEmitterArg object
  * will throw std::bad_alloc when emitting/executing the 'when' or
  * 'fail' callback in the glib main loop, with the result that the
  * relevant callback will not execute (instead the exception will be
  * consumed and a g_critical() warning will be issued).  This is
  * rarely of any relevance because glib will abort the program if it
  * is itself unable to obtain memory from the operating system.
  * However, where it is relevant, design the program so that it is
  * not necessary to provide a releaser object.
  * @note 3. If the library is compiled using the --with-auto-ptr
  * configuration option, then this method uses std::auto_ptr in place
  * of std::unique_ptr in its signature in order to retain
  * compatibility with the 1.2 series of the library.
  *
  * Since 2.0.13
  */
  template <class Ret, class... Params, class... Args>
#ifdef CGU_USE_AUTO_PTR
  void make_task_when_full(std::auto_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                           Cgu::Releaser* when_releaser,
                           std::auto_ptr<const Cgu::Callback::Callback> fail,
                           Cgu::Releaser* fail_releaser,
                           gint priority,
                           GMainContext* context,
                           Ret (*func)(Params...),
                           Args&&... args);
#else
  void make_task_when_full(std::unique_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                           Cgu::Releaser* when_releaser,
                           std::unique_ptr<const Cgu::Callback::Callback> fail,
                           Cgu::Releaser* fail_releaser,
                           gint priority,
                           GMainContext* context,
                           Ret (*func)(Params...),
                           Args&&... args);
#endif

 /**
  * This is an abbreviated version of make_task_when_full(), which is
  * for use when it is known that the function passed to this method,
  * and the copy constructors of any non-reference bound arguments
  * passed to it, do not throw, and the user is not interested in
  * std::bad_alloc and does not need a Cgu::Releaser object for the
  * 'when' callback (which is likely to cover the majority of uses,
  * particularly when composing tasks using glib because glib
  * terminates the program if it is unable to obtain memory).
  *
  * This method can take up to four bound arguments for the target
  * function.
  *
  * Like make_task_when_full(), this method is a wrapper which will
  * take a pointer to a function which returns a value, together with
  * arguments, and constructs a TaskManager task which will execute
  * that function by calling add_task() with an appropriate callback
  * object, and causes the 'when' callback passed as an argument to
  * this method to be executed by a glib main loop if and when the
  * task finishes correctly - the 'when' callback is passed the
  * function's return value when it is invoked.  It is thread safe
  * (any thread may call this method, including another task running
  * on the TaskManager object).  Apart from the absence of a 'one
  * thread per task' model, this method therefore provides a similar
  * interface to the one provided by Cgu::Thread::Future.  See the
  * documentation on add_task() for further information about how task
  * execution works.
  *
  * The 'when' callback will execute with G_PRIORITY_DEFAULT priority
  * in the main loop.
  *
  * @param when A callback which will be executed if and when the
  * function passed to this method finishes correctly. The callback is
  * passed that function's return value when it is invoked.  It will
  * execute in the glib main loop whose GMainContext object is passed
  * to the 'context' argument of this method.
  * @param context The glib main context of the main loop in which the
  * 'when' callback is to be executed.  A value 0/NULL/nullptr will
  * cause the callback to be executed in the main program loop.
  * @param func The function to be executed as a task.
  * @param args The arguments to be passed to that function.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note 1. This method will also throw if the copy or move constructor
  * of a bound argument throws.
  * @note 2. If the library is compiled using the --with-auto-ptr
  * configuration option, then this method uses std::auto_ptr in place
  * of std::unique_ptr in its signature in order to retain
  * compatibility with the 1.2 series of the library.
  *
  * Since 2.0.13
  */
  template <class Ret, class... Params, class... Args>
#ifdef CGU_USE_AUTO_PTR
  void make_task_when(std::auto_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                      GMainContext* context,
                      Ret (*func)(Params...),
                      Args&&... args) {
    static_assert(sizeof...(Args) < 5,
                  "No greater than four bound arguments can be passed to "
                  "TaskManager::make_task_when() taking a function.");

    make_task_when_full(when,
                        0,
                        std::auto_ptr<const Cgu::Callback::Callback>(),
                        0,
                        G_PRIORITY_DEFAULT,
                        context,
                        func,
                        std::forward<Args>(args)...);
#else
  void make_task_when(std::unique_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                      GMainContext* context,
                      Ret (*func)(Params...),
                      Args&&... args) {
    static_assert(sizeof...(Args) < 5,
                  "No greater than four bound arguments can be passed to "
                  "TaskManager::make_task_when() taking a function.");

    make_task_when_full(std::move(when),
                        0,
                        std::unique_ptr<const Cgu::Callback::Callback>(),
                        0,
                        G_PRIORITY_DEFAULT,
                        context,
                        func,
                        std::forward<Args>(args)...);
#endif
  }

 /**
  * This is a wrapper which will take a callable object (such as a
  * std::function object, a lambda or the return value of std::bind)
  * representing a function which returns a value, and constructs a
  * TaskManager task which will execute that function by calling
  * add_task() with an appropriate callback object, and returns a
  * Cgu::AsyncResult object (held by Cgu::SharedLockPtr) which will
  * provide the value that the function returns.  It is thread safe
  * (any thread may call this method, including another task running
  * on the TaskManager object).  Apart from the absence of a 'one
  * thread per task' model, this method therefore provides a similar
  * interface to the one provided by Cgu::Thread::Future.  See the
  * documentation on add_task() for further information about how task
  * execution works.
  *
  * From version 2.0.14, this method takes the callable object as a
  * template parameter, and in version 2.0.13 it took it as a
  * std::function object.  In version 2.0.13 it was necessary to
  * specify the return value of any callable object which was not a
  * std::function object as a specific template parameter: this is not
  * necessary in version 2.0.14, as it is deduced automatically.
  *
  * If the function passed to this method exits by throwing
  * Thread::Exit or some other exception, then the returned
  * Cgu::AsyncResult object's get() method will unblock and its
  * get_error() method will return -1.
  *
  * @param f The callable object to be executed as a task.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note 1. This method will also throw if the copy or move
  * constructor of a bound argument throws.
  * @note 2. If the callable object passed as an argument has both
  * const and non-const operator()() methods, the non-const version
  * will be called even if the callable object passed is a const
  * object.
  *
  * Since 2.0.13
  */
  // we don't need this version of make_task_result() for syntactic
  // reasons - the version taking a single template parameter will do
  // by itself syntactically because it can use decltype.  However, we
  // include this version in order to be API compatible with
  // c++-gtk-utils < 2.0.14, which required the return type to be
  // specified when this method is passed something other than a
  // std::function object.  SFINAE will take care of the rest, except
  // with a corner case where all of the following apply: (i) a
  // function object is passed whose operator()() method returns a
  // copy of the function object (or another function object of the
  // same type), (ii) the function object is passed to this method as
  // a rvalue and not a lvalue, and (iii) the user specifically states
  // the return type when instantiating this template function.  This
  // would give rise to an ambiguity, but its happening is extremely
  // unlikely, and cannot happen with a lambda or the return value of
  // std::bind, because those types are only known to the compiler,
  // and cannot happen with other objects if the user lets template
  // deduction take its course.
  template <class Ret, class Func>
  Cgu::SharedLockPtr<Cgu::AsyncResult<Ret>> make_task_result(Func&& f);

  // we don't want to document this function: it provides the type
  // deduction of the return value of the passed functor (it deals
  // with cases where this is not specified expressly).
#ifndef DOXYGEN_PARSING
  template <class Func>
  auto make_task_result(Func&& f) -> Cgu::SharedLockPtr<Cgu::AsyncResult<decltype(f())>> {

    // TODO: this is a work-around for gcc < 4.7, which has a bug
    // which requires a function whose return value is determined by
    // decltype, such as make_task_result(Func&&), to be inline.  At a
    // suitable API/ABI break when gcc requirements are updated, this
    // should be moved to task_manager.tpp.

    // there are two types related to the functor to be executed by
    // the task.  'Func' is the transient type provided by argument
    // deduction for forwarding, and will vary depending on whether
    // the functor object is a lvalue (which will deduce it as a
    // reference type) or rvalue (which will not).  'FType' is the
    // type to be held by the callback object generated in this
    // function, and is never a reference type.  It is also never
    // const, because the FType member is marked mutable in the
    // callback object so that it can execute mutable lambdas (or
    // other functors with a non-const operator()() method).
    typedef typename std::remove_const<typename std::remove_reference<Func>::type>::type FType;
    typedef decltype(f()) Ret;
#ifdef CGU_USE_AUTO_PTR
    typedef std::auto_ptr<const Callback::Callback> CbPtr;
#else
    typedef std::unique_ptr<const Callback::Callback> CbPtr;
#endif

    SharedLockPtr<AsyncResult<Ret>> ret{new AsyncResult<Ret>};
    CbPtr exec_cb(new TaskManagerHelper::FunctorResultExec<Ret, FType>(std::forward<Func>(f), ret));
    CbPtr do_fail_cb(Callback::make_ref(&TaskManagerHelper::FunctorResultWrapper<Ret, FType>::do_fail,
                                        ret));
    add_task(std::move(exec_cb), std::move(do_fail_cb));

    return ret;
  }
#endif

 /**
  * This is a wrapper which will take a callable object (such as a
  * std::function object, a lambda or the return value of std::bind)
  * representing a function which returns a value, and constructs a
  * TaskManager task which will execute that function by calling
  * add_task() with an appropriate callback object, and causes the
  * 'when' callback passed as an argument to this method to be
  * executed by a glib main loop if and when the task finishes
  * correctly - the 'when' callback is passed the function's return
  * value when it is invoked.  It is thread safe (any thread may call
  * this method, including another task running on the TaskManager
  * object).  Apart from the absence of a 'one thread per task' model,
  * this method therefore provides a similar interface to the one
  * provided by Cgu::Thread::Future.  See the documentation on
  * add_task() for further information about how task execution works.
  *
  * From version 2.0.14, this method takes the callable object as a
  * template parameter, and in version 2.0.13 it took it as a
  * std::function object.  In version 2.0.13 it was necessary to
  * specify the return value of any callable object which was not a
  * std::function object as a specific template parameter: this is not
  * necessary in version 2.0.14, as it is deduced automatically.
  *
  * Note that unlike add_task(), but like the 'fail' callback of
  * Cgu::Thread::Future objects, if a fail callback is provided to
  * this method and it executes, it will execute in the glib main loop
  * whose GMainContext object is passed to the 'context' argument of
  * this method.
  *
  * Note also that if releasers are provided for the 'when' or 'fail'
  * callbacks, these are passed by pointer and not by reference (this
  * is so that a NULL pointer can indicate that no releaser is to be
  * provided).  If provided, a releaser will enable automatic
  * disconnection of the 'when' or 'fail' callback, if the object of
  * which the releaser is a member is destroyed. For this to be race
  * free, the lifetime of that object must be controlled by the thread
  * in whose main loop the 'when' or 'fail' callback will execute.
  *
  * The make_task_when() method is similar to this method but provides
  * an abbreviated set of paramaters suitable for most cases.  This
  * method is for use where releasers or a 'fail' callback are
  * required.
  *
  * @param when A callback which will be executed if and when the
  * function represented by the callable object passed to this method
  * finishes correctly. The callback is passed that function's return
  * value when it is invoked.  It will execute in the glib main loop
  * whose GMainContext object is passed to the 'context' argument of
  * this method.
  * @param when_releaser A pointer to a releaser object for automatic
  * disconnection of the 'when' callback if the object of which the
  * releaser object is a member is destroyed.  A value of
  * 0/NULL/nullptr indicates no releaser.
  * @param fail A callback which will be executed if the 'when'
  * callback does not execute.  This would happen if the function
  * represented by the std::function object passed to this method
  * exits by throwing Thread::Exit or some other exception or the copy
  * constructor of a non-reference argument of that function throws,
  * or if the 'when' callback does not execute because the internal
  * implementation of this wrapper throws std::bad_alloc (which will
  * not happen if the library has been installed using the
  * –with-glib-memory-slices-no-compat configuration option: instead
  * glib will terminate the program if it is unable to obtain memory
  * from the operating system).  If an exception propagates from the
  * function represented by the 'fail' callback, this will be consumed
  * to protect the TaskManager object, and a g_critical() warning will
  * be issued.  The callback will execute in the glib main loop whose
  * GMainContext object is passed to the 'context' argument of this
  * method.  An empty std::unique_ptr object indicates no 'fail'
  * callback.
  * @param fail_releaser A pointer to a releaser object for automatic
  * disconnection of the 'fail' callback if the object of which the
  * releaser object is a member is destroyed.  A value of
  * 0/NULL/nullptr indicates no releaser.
  * @param priority The priority to be given in the main loop to the
  * 'when' callback or any 'fail' callback. In ascending order of
  * priorities, priorities are G_PRIORITY_LOW,
  * G_PRIORITY_DEFAULT_IDLE, G_PRIORITY_HIGH_IDLE, G_PRIORITY_DEFAULT
  * and G_PRIORITY_HIGH. This determines the order in which the
  * callback will appear in the event list in the main loop, not the
  * priority which the OS will adopt.
  * @param context The glib main context of the main loop in which the
  * 'when' callback or any 'fail' callback is to be executed.  A value
  * 0/NULL/nullptr will cause the callback to be executed in the main
  * program loop.
  * @param f The callable object to be executed as a task.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note 1. This method will also throw if the copy or move
  * constructor of a bound argument throws.
  * @note 2. If the callable object passed as an argument has both
  * const and non-const operator()() methods, the non-const version
  * will be called even if the callable object passed is a const
  * object.
  * @note 3. If a 'when_releaser' or a 'fail_releaser' argument is
  * provided, it is in theory possible (if memory is exhausted and the
  * system throws in that case) that an internal SafeEmitterArg object
  * will throw std::bad_alloc when emitting/executing the 'when' or
  * 'fail' callback in the glib main loop, with the result that the
  * relevant callback will not execute (instead the exception will be
  * consumed and a g_critical() warning will be issued).  This is
  * rarely of any relevance because glib will abort the program if it
  * is itself unable to obtain memory from the operating system.
  * However, where it is relevant, design the program so that it is
  * not necessary to provide a releaser object.
  * @note 4. If the library is compiled using the --with-auto-ptr
  * configuration option, then this method uses std::auto_ptr in place
  * of std::unique_ptr in its signature in order to retain
  * compatibility with the 1.2 series of the library.
  *
  * Since 2.0.13
  */
  template <class Ret, class Func>
#ifdef CGU_USE_AUTO_PTR
  void make_task_when_full(std::auto_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                           Cgu::Releaser* when_releaser,
                           std::auto_ptr<const Cgu::Callback::Callback> fail,
                           Cgu::Releaser* fail_releaser,
                           gint priority,
                           GMainContext* context,
                           Func&& f);
#else
  void make_task_when_full(std::unique_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                           Cgu::Releaser* when_releaser,
                           std::unique_ptr<const Cgu::Callback::Callback> fail,
                           Cgu::Releaser* fail_releaser,
                           gint priority,
                           GMainContext* context,
                           Func&& f);
#endif

 /**
  * This is an abbreviated version of make_task_when_full(), which is
  * for use when it is known that the function represented by the
  * callable object passed to this method, and the copy constructors
  * of any non-reference bound arguments passed to it, do not throw,
  * and the user is not interested in std::bad_alloc and does not need
  * a Cgu::Releaser object for the 'when' callback (which is likely to
  * cover the majority of uses, particularly when composing tasks
  * using glib because glib terminates the program if it is unable to
  * obtain memory).
  *
  * From version 2.0.14, this method takes the callable object as a
  * template parameter, and in version 2.0.13 it took it as a
  * std::function object.  In version 2.0.13 it was necessary to
  * specify the return value of any callable object which was not a
  * std::function object as a specific template parameter: this is not
  * necessary in version 2.0.14, as it is deduced automatically.
  *
  * Like make_task_when_full(), this method is a wrapper which will
  * take a callable object representing a function which returns a
  * value, and constructs a TaskManager task which will execute that
  * function by calling add_task() with an appropriate callback
  * object, and causes the 'when' callback passed as an argument to
  * this method to be executed by a glib main loop if and when the
  * task finishes correctly - the 'when' callback is passed the
  * function's return value when it is invoked.  It is thread safe
  * (any thread may call this method, including another task running
  * on the TaskManager object).  Apart from the absence of a 'one
  * thread per task' model, this method therefore provides a similar
  * interface to the one provided by Cgu::Thread::Future.  See the
  * documentation on add_task() for further information about how task
  * execution works.
  *
  * The 'when' callback will execute with G_PRIORITY_DEFAULT priority
  * in the main loop.
  *
  * There is a similar make_task_compose() function which has the
  * callable object to be executed as a task as its first argument and
  * the 'when' callback as its last argument, in order to aid task
  * composition.
  *
  * @param when A callback which will be executed if and when the
  * function represented by the callable object passed to this method
  * finishes correctly. The callback is passed that function's return
  * value when it is invoked.  It will execute in the glib main loop
  * whose GMainContext object is passed to the 'context' argument of
  * this method.
  * @param context The glib main context of the main loop in which the
  * 'when' callback is to be executed.  A value 0/NULL/nullptr will
  * cause the callback to be executed in the main program loop.
  * @param f The callable object to be executed as a task.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note 1. This method will also throw if the copy or move
  * constructor of a bound argument throws.
  * @note 2. If the callable object passed as an argument has both
  * const and non-const operator()() methods, the non-const version
  * will be called even if the callable object passed is a const
  * object.
  * @note 3. If the library is compiled using the --with-auto-ptr
  * configuration option, then this method uses std::auto_ptr in place
  * of std::unique_ptr in its signature in order to retain
  * compatibility with the 1.2 series of the library.
  *
  * Since 2.0.13
  */
  template <class Ret, class Func>
#ifdef CGU_USE_AUTO_PTR
  void make_task_when(std::auto_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                      GMainContext* context,
                      Func&& f) {
    make_task_when_full(when,
                        0,
                        std::auto_ptr<const Cgu::Callback::Callback>(),
                        0,
                        G_PRIORITY_DEFAULT,
                        context,
                        std::forward<Func>(f));
  }
#else
  void make_task_when(std::unique_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when,
                      GMainContext* context,
                      Func&& f) {
    make_task_when_full(std::move(when),
                        0,
                        std::unique_ptr<const Cgu::Callback::Callback>(),
                        0,
                        G_PRIORITY_DEFAULT,
                        context,
                        std::forward<Func>(f));
  }
#endif

 /**
  * This is an abbreviated version of make_task_when_full(), which is
  * for use when it is known that the function represented by the
  * callable object passed to this method, and the copy
  * constructors of any non-reference bound arguments passed to it, do
  * not throw, and the user is not interested in std::bad_alloc and
  * does not need a Cgu::Releaser object for the 'when' callback
  * (which is likely to cover the majority of uses, particularly when
  * composing tasks using glib because glib terminates the program if
  * it is unable to obtain memory).
  *
  * From version 2.0.14, this method takes the callable object as a
  * template parameter, and in version 2.0.13 it took it as a
  * std::function object.  In version 2.0.13 it was necessary to
  * specify the return value of any callable object which was not a
  * std::function object as a specific template parameter: this is not
  * necessary in version 2.0.14, as it is deduced automatically.
  *
  * This method does the same as the version of make_task_when()
  * taking a function object, except that this method takes the
  * callable object to be executed as a task as its first argument and
  * the 'when' callback as its last argument in order to aid task
  * composition, and in particular so tasks compose in user code in a
  * visually ordered manner.
  *
  * More particularly, like make_task_when_full(), this method is a
  * wrapper which will take a callable object representing a function
  * which returns a value, and constructs a TaskManager task which
  * will execute that function by calling add_task() with an
  * appropriate callback object, and causes the 'when' callback passed
  * as an argument to this method to be executed by a glib main loop
  * if and when the task finishes correctly - the 'when' callback is
  * passed the function's return value when it is invoked.  It is
  * thread safe (any thread may call this method, including another
  * task running on the TaskManager object).  Apart from the absence
  * of a 'one thread per task' model, this method therefore provides a
  * similar interface to the one provided by Cgu::Thread::Future.  See
  * the documentation on add_task() for further information about how
  * task execution works.
  *
  * The 'when' callback will execute with G_PRIORITY_DEFAULT priority
  * in the main loop.
  *
  * @param f The callable object to be executed as a task.
  * @param context The glib main context of the main loop in which the
  * 'when' callback is to be executed.  A value 0/NULL/nullptr will
  * cause the callback to be executed in the main program loop.
  * @param when A callback which will be executed if and when the
  * function represented by the callable object passed to this method
  * finishes correctly. The callback is passed that function's return
  * value when it is invoked.  It will execute in the glib main loop
  * whose GMainContext object is passed to the 'context' argument of
  * this method.
  * @exception std::bad_alloc This exception will be thrown if memory
  * is exhausted and the sytem throws in that case.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion).
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * stop_all() has previously been called.  It will also be thrown if
  * is_error() would return true because this class's internal thread
  * pool loop implementation has thrown std::bad_alloc, or a thread
  * has failed to start correctly.  (On systems with
  * over-commit/lazy-commit combined with virtual memory (swap), it is
  * rarely useful to check for memory exhaustion, but there may be
  * some specialized cases where the return value of is_error() is
  * useful.)
  * @note 1. This method will also throw if the copy or move
  * constructor of a bound argument throws.
  * @note 2. If the callable object passed as an argument has both
  * const and non-const operator()() methods, the non-const version
  * will be called even if the callable object passed is a const
  * object.
  * @note 3. If the library is compiled using the --with-auto-ptr
  * configuration option, then this method uses std::auto_ptr in place
  * of std::unique_ptr in its signature in order to retain
  * compatibility with the 1.2 series of the library.
  *
  * Since 2.0.13
  */
  template <class Ret, class Func>
#ifdef CGU_USE_AUTO_PTR
  void make_task_compose(Func&& f,
                         GMainContext* context,
                         std::auto_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when) {
    make_task_when_full(when,
                        0,
                        std::auto_ptr<const Cgu::Callback::Callback>(),
                        0,
                        G_PRIORITY_DEFAULT,
                        context,
                        std::forward<Func>(f));
  }
#else
  void make_task_compose(Func&& f,
                         GMainContext* context,
                         std::unique_ptr<const Cgu::Callback::CallbackArg<const Ret&>> when) {
    make_task_when_full(std::move(when),
                        0,
                        std::unique_ptr<const Cgu::Callback::Callback>(),
                        0,
                        G_PRIORITY_DEFAULT,
                        context,
                        std::forward<Func>(f));
  }
#endif

 /**
  * If the specified minimum number of threads is greater than 0, this
  * constructor will start the required minimum number of threads.  If
  * glib < 2.32 is installed, g_thread_init() must be called before
  * any TaskManager objects are constructed
  * @param max The maximum number of threads which the TaskManager
  * object will run in the thread pool.  If the value passed as this
  * argument is less than the value passed as 'min', the maximum
  * number of threads will be set to 'min'.  A value of 0 is not
  * valid, and if this is passed the number will be set to the greater
  * of 1 and 'min'.
  * @param min The minimum number of threads which the TaskManager
  * object will run in the thread pool.
  * @param idle The length of time in milliseconds that threads
  * greater in number than 'min' and not executing any tasks will
  * remain in existence.  The default is 10000 (10 seconds).
  * @param blocking If true, calls to stop_all() and the destructor
  * will not return until the tasks remaining to be executed have
  * finished (what is meant by "the tasks remaining to be executed"
  * depends on the StopMode setting, for which see the documentation
  * on the stop_all() method).  If false, stop_all() and the
  * destructor will return straight away (which in terms of the
  * TaskManager class implementation is safe for the reasons explained
  * in the documentation on the destructor).
  * @param mode The StopMode setting (either
  * Cgu::Thread::TaskManager::wait_for_running or
  * Cgu::Thread::TaskManager::wait_for_all) executed when running
  * stop_all() or when the destructor is called.  See the
  * documentation on stop_all() for an explanation of the setting.
  * @exception std::bad_alloc This exception might be thrown if memory
  * is exhausted and the system throws in that case.
  * @exception Cgu::Thread::TaskError This exception will be thrown if
  * starting the specified minimum number of threads fails.
  * @exception Cgu::Thread::MutexError This exception might be thrown
  * if initialisation of the contained mutex fails. (It is often not
  * worth checking for this, as it means either memory is exhausted or
  * pthread has run out of other resources to create new mutexes.)
  * @exception Cgu::Thread::CondError This exception might be thrown
  * if initialisation of the contained condition variable fails. (It
  * is often not worth checking for this, as it means either memory is
  * exhausted or pthread has run out of other resources to create new
  * condition variables.)
  *
  * Since 2.0.12
  */
  TaskManager(unsigned int max = 8, unsigned int min = 0,
              unsigned int idle = 10000, bool blocking = true,
              StopMode mode = TaskManager::wait_for_all);

 /**
  * The destructor will call stop_all(), unless that method has
  * previously been called explicitly without throwing std::bad_alloc.
  * If the blocking setting is true, the destructor will not return
  * until the tasks remaining to be executed have finished (what is
  * meant by "the tasks remaining to be executed" depends on the
  * StopMode setting, for which see the documentation on the
  * stop_all() method.)  If the blocking setting is false, the
  * destructor will return straight away: this is safe, because
  * TaskManager's internals for running tasks have been implemented
  * using reference counting and will not be deleted until all threads
  * running on the TaskManager object have finished, although the
  * remaining tasks should not attempt to call any of TaskManager's
  * methods once the TaskManager object itself has been destroyed.
  *
  * The destructor is thread safe (any thread can destroy a
  * TaskManager object) unless the blocking setting is true, in which
  * case no task running on the TaskManager object may destroy the
  * TaskManager object.  Subject to that, it is not an error for a
  * thread to destroy a TaskManager object and so invoke this
  * destructor while another thread is already blocking in (if the
  * blocking setting is true) or already out of (if the blocking
  * setting is false) a call to stop_all() and remaining tasks are
  * executing: if blocking, both calls (to stop_all() and to this
  * destructor) would safely block together.  Any given thread can
  * similarly safely follow a non-blocking call to stop_all() by a
  * non-blocking call to this destructor even though remaining tasks
  * are executing.  However, it is an error for a thread to call
  * stop_all() after another thread has begun destruction of the
  * TaskManager object (that is, after this destructor has been
  * entered): there would then be an unresolvable race with the
  * destructor.
  *
  * The destructor will not throw.
  *
  * If stop_all() has not previously been called explicitly and throws
  * std::bad_alloc() when called in this destructor, the exception
  * will be caught and consumed, but then the destructor will not
  * block even if the blocking setting is true, and if the minimum
  * number of threads is not 0 some threads might remain running
  * during the entire program duration (albeit safely).  Where the
  * throwing of std::bad_alloc is a meaningful event (usually it
  * isn't) and needs to be guarded against, call stop_all() explicitly
  * before this destructor is entered, or use a minimum thread value
  * of 0 and allow for the case of the destructor not blocking.
  *
  * Since 2.0.12
  */
  ~TaskManager();

/* Only has effect if --with-glib-memory-slices-compat or
 * --with-glib-memory-slices-no-compat option picked */
  CGU_GLIB_MEMORY_SLICES_FUNCS
};

} // namespace Thread

} // namespace Cgu

#include <c++-gtk-utils/task_manager.tpp>

#endif