boost_core/doc/explicit_operator_bool.qbk

[section:explicit_operator_bool Header <boost/core/explicit_operator_bool.hpp>]

[/
 / 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)
 /]

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[section Overview]
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`BOOST_EXPLICIT_OPERATOR_BOOL()`, `BOOST_EXPLICIT_OPERATOR_BOOL_NOEXCEPT()` 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`.

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[section Examples]
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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;

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[section History]
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[heading boost 1.55]

* The macro was extracted from Boost.Log.

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