[/
    Copyright (c) 2019 Vinnie Falco (vinnie.falco@gmail.com)

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

    Official repository: https://github.com/boostorg/json
]

[/-----------------------------------------------------------------------------]

[section Background]

The first version of allocators in C++ defined the named requirement
__Allocator__, and made each standard container a class template
parameterized on the allocator type. For example, here is the
declaration for __std_vector__:

[doc_background_1]

The standard allocator is __DefaultConstructible__. To support stateful
allocators, containers provide additional constructor overloads taking
an allocator instance parameter.

[doc_background_2]

While the system works, it has some usability problems:

* The container must be a class template.
* Parameterizing the allocator on the element type is clumsy.
* The system of allocator traits, especially POCCA and POCMA,
  is complicated and error-prone.

Allocator-based programs which use multiple allocator types
incur a greater number of function template instantiations and
are generally slower to compile because class template function
definitions must be visible at all call sites.

[heading Polymorphic Allocators]

C++17 improves the allocator model by representing the low-level
allocation operation with an abstract interface called `memory_resource`,
which is not parameterized on the element type, and has no traits:

[doc_background_3]

The class template `polymorphic_allocator` wraps a `memory_resource`
pointer and meets the requirements of __Allocator__, allowing it to be
used where an allocator is expected. The standard provides type aliases
using the polymorphic allocator for standard containers:

[doc_background_4]

A polymorphic allocator constructs with a pointer to a memory resource:

[doc_background_5]

The memory resource is passed by pointer; ownership is not transferred.
The caller is responsible for extending the lifetime of the memory
resource until the last container which is using it goes out of scope,
otherwise the behavior is undefined. Sometimes this is the correct model,
such as in this example which uses a monotonic resource constructed from
a local stack buffer:

[doc_background_6]

However, sometimes shared ownership is needed. Specifically, that the
lifetime extension of the memory resource should be automatic. For
example, if a library wants to return a container which owns an
instance of the library's custom memory resource as shown below:

[doc_background_7]

This can be worked around by declaring the container to use a custom allocator
(perhaps using a `std::shared_ptr< std::pmr::memory_resource >` as
a data member). This hinders library composition; every library now exports
unique, incompatible container types. A raw memory resource pointer is also
easy to misuse:

[doc_background_8]

Workarounds for this problem are worse than the problem itself. The library
could return a pair with the vector and
`std::unique_ptr<std::pmr::memory_resource>` which the caller must
manage. Or the library could change its function signatures to accept a
`memory_resource*` provided by the caller, where the library also makes public
the desired memory resources (`my_resource` above).

[heading Problem Statement]

We would like an allocator model using a single type `T` with the
following properties:

* `T` is not a class template
* `T` references a `memory_resource`
* `T` supports both reference semantics or shared ownership
* `T` interoperates with code already using `std::pmr`

Boost.JSON solves this problem by introducing a new smart pointer
called __storage_ptr__ which builds upon C++17's memory allocation
interfaces, accomplishing the goals above. As a result, libraries
which use this type compose more easily and enjoy faster compilation,
as member functions for containers which use the type can be defined
out-of-line.

[endsect]
