ceph/src/boost/libs/type_traits/doc/function_traits.qbk

   1 [/
   2   Copyright 2007 John Maddock.
   3   Distributed under the Boost Software License, Version 1.0.
   4   (See accompanying file LICENSE_1_0.txt or copy at
   5   http://www.boost.org/LICENSE_1_0.txt).
   6 ]
   7
   8 [section:function_traits function_traits]
   9 [def __argN '''arg<replaceable>N</replaceable>_type''']
  10
  11    template <class F>
  12    struct function_traits
  13    {
  14       static const std::size_t    arity = __below;
  15       typedef __below           result_type;
  16       typedef __below           __argN;
  17    };
  18
  19 The class template function_traits will only compile if:
  20
  21 * The compiler supports partial specialization of class templates.
  22 * The template argument `F` is a /function type/, note that this ['[*is not]]
  23 the same thing as a /pointer to a function/.
  24
  25 [tip
  26 function_traits is intended to introspect only C++ functions of the
  27 form R (), R( A1 ), R ( A1, ... etc. ) and not function pointers or
  28 class member functions. To convert a function pointer type to a suitable
  29 type use __remove_pointer.]
  30
  31 [table Function Traits Members
  32 [[Member] [Description]]
  33 [[`function_traits<F>::arity`]
  34    [An integral constant expression that gives the number of arguments accepted by the function type `F`.]]
  35 [[`function_traits<F>::result_type`]
  36    [The type returned by function type `F`.]]
  37 [[`function_traits<F>::__argN`]
  38    [The '''<replaceable>N</replaceable>th''' argument type of function type `F`, where `1 <= N <= arity` of `F`.]]
  39 ]
  40
  41 [table Examples
  42 [[Expression] [Result]]
  43 [[`function_traits<void (void)>::arity`] [An integral constant expression that has the value 0.]]
  44 [[`function_traits<long (int)>::arity`] [An integral constant expression that has the value 1.]]
  45 [[`function_traits<long (int, long, double, void*)>::arity`] [An integral constant expression that has the value 4.]]
  46 [[`function_traits<void (void)>::result_type`] [The type `void`.]]
  47 [[`function_traits<long (int)>::result_type`] [The type `long`.]]
  48 [[`function_traits<long (int)>::arg1_type`] [The type `int`.]]
  49 [[`function_traits<long (int, long, double, void*)>::arg4_type`] [The type `void*`.]]
  50 [[`function_traits<long (int, long, double, void*)>::arg5_type`] [A compiler error: there is no `arg5_type` since there are only four arguments.]]
  51 [[`function_traits<long (*)(void)>::arity`] [A compiler error: argument type is a /function pointer/, and not a /function type/.]]
  52
  53 ]
  54
  55 [all_compilers]
  56
  57 [endsect]
  58