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Extracted BOOST_EXPLICIT_OPERATOR_BOOL macro from Boost.Log.

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[SVN r85543]
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Andrey Semashev
2013-09-01 16:31:16 +00:00
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[/
/ 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]