ceph/src/boost/libs/core/doc/explicit_operator_bool.qbk

   1 [/
   2  / Copyright (c) 2013 Andrey Semashev
   3  /
   4  / Distributed under the Boost Software License, Version 1.0. (See accompanying
   5  / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   6  /]
   7
   8 [section:explicit_operator_bool explicit_operator_bool]
   9
  10 [/=================]
  11 [simplesect Authors]
  12 [/=================]
  13
  14 * Andrey Semashev
  15
  16 [endsimplesect]
  17
  18 [/===============]
  19 [section Overview]
  20 [/===============]
  21
  22 Header `<boost/core/explicit_operator_bool.hpp>` provides
  23 `BOOST_EXPLICIT_OPERATOR_BOOL()`, `BOOST_EXPLICIT_OPERATOR_BOOL_NOEXCEPT()`
  24 and `BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL()` compatibility helper macros
  25 that expand to an explicit conversion operator to `bool`. For compilers not
  26 supporting explicit conversion operators introduced in C++11 the macros expand
  27 to a conversion operator that implements the
  28 [@http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Safe_bool safe bool idiom].
  29 In case if the compiler is not able to handle safe bool idiom well the macros
  30 expand to a regular conversion operator to `bool`.
  31
  32 [endsect]
  33
  34 [/===============]
  35 [section Examples]
  36 [/===============]
  37
  38 Both macros are intended to be placed within a user's class definition. The
  39 generated conversion operators will be implemented in terms of `operator!()`
  40 that should be defined by user in this class. In case of
  41 `BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL()` the generated conversion operator
  42 will be declared `constexpr` which requires the corresponding `operator!()`
  43 to also be `constexpr`.
  44
  45 ``
  46 template< typename T >
  47 class my_ptr
  48 {
  49     T* m_p;
  50
  51 public:
  52     BOOST_EXPLICIT_OPERATOR_BOOL()
  53
  54     bool operator!() const
  55     {
  56         return !m_p;
  57     }
  58 };
  59 ``
  60
  61 Now `my_ptr` can be used in conditional expressions, similarly to a regular pointer:
  62
  63 ``
  64 my_ptr< int > p;
  65 if (p)
  66     std::cout << "true" << std::endl;
  67 ``
  68
  69 [endsect]
  70
  71 [/==============]
  72 [section History]
  73 [/==============]
  74
  75 [heading boost 1.56]
  76
  77 * Added new macros `BOOST_EXPLICIT_OPERATOR_BOOL_NOEXCEPT` and `BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL` to define `noexcept` and `constexpr` operators.
  78 * The header moved to Boost.Core.
  79
  80 [heading boost 1.55]
  81
  82 * The macro was extracted from Boost.Log.
  83
  84 [endsect]
  85
  86 [endsect]