Code Pitfalls - Frequently made mistakes

Comparing TriBool values

Comparing two TriBool values is not as easy as it might look like, because the TriBool::operator== and TriBool::operator!= return a TriBool.

For example is (indeterminate==indeterminate) not true, but indeterminate. That's why the following snippet does not do what the author expected:

    // buggy option class
    struct Option
    {
      public:
        Option()
          : _value( indeterminate )
        {}

        bool get() const
        { return ( _value == indeterminate ) ? true : bool(_value); }

        void set( bool new_r )
        { _value = new_r; }

      private:
        tribool _value;
    };

You find that get() returns false as long as the option is unset, and not true as the code suggests. That's because (_value==indeterminate) returns indeterminate.

Note:

Always use indeterminate(_value) to test whether a TriBools value is indeterminate:

        bool get() const
        { return indeterminate( _value ) ? true : bool(_value); }

        tribool _value;
        ...
        if ( indeterminate( _value ) )
        { ... }              // indeterminate
        else if ( _value )
        { ... }              // true
        else
        { ... }              // false

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