[/
 / Copyright (c) 2013 Andrey Semashev
 /
 / 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)
 /]

[article BOOST_EXPLICIT_OPERATOR_BOOL and BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL
    [quickbook 1.5]
    [authors [Semashev, Andrey]]
    [copyright 2013 Andrey Semashev]
    [license
        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])
    ]
]

[/===============]
[section Overview]
[/===============]

`BOOST_EXPLICIT_OPERATOR_BOOL()` and `BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL()` are compatibility helper macros that expand to an explicit conversion operator to `bool`. For compilers not supporting explicit conversion operators introduced in C++11 the macros expand to a conversion operator that implements the [@http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Safe_bool safe bool idiom]. In case if the compiler is not able to handle safe bool idiom well the macros expand to a regular conversion operator to `bool`.

[endsect]


[/===============]
[section Examples]
[/===============]

Both macros are intended to be placed within a user's class definition. The generated conversion operators will be implemented in terms of `operator!()` that should be defined by user in this class. In case of `BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL()` the generated conversion operator will be declared `constexpr` which requires the corresponding `operator!()` to also be `constexpr`.

    template< typename T >
    class my_ptr
    {
        T* m_p;

    public:
        BOOST_EXPLICIT_OPERATOR_BOOL()

        bool operator!() const
        {
            return !m_p;
        }
    };

Now `my_ptr` can be used in conditional expressions, similarly to a regular pointer:

    my_ptr< int > p;
    if (p)
        std::cout << "true" << std::endl;

[endsect]

[/===============]
[section History]
[/===============]

[heading boost 1.55]

* The macro was extracted from Boost.Log.

[endsect]