boost_core/doc/explicit_operator_bool.qbk

[/
 / 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)
 /]

[section:explicit_operator_bool explicit_operator_bool]

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[simplesect Authors]
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* Andrey Semashev

[endsimplesect]

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[section Overview]
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Header `<boost/core/explicit_operator_bool.hpp>` provides
`BOOST_EXPLICIT_OPERATOR_BOOL()`, `BOOST_EXPLICIT_OPERATOR_BOOL_NOEXCEPT()`
and `BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL()` 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]

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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;
``

[endsect]

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[section History]
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[heading boost 1.56]

* Added new macros `BOOST_EXPLICIT_OPERATOR_BOOL_NOEXCEPT` and `BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL` to define `noexcept` and `constexpr` operators.
* The header moved to Boost.Core.

[heading boost 1.55]

* The macro was extracted from Boost.Log.

[endsect]

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