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
19The 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]]
23the same thing as a /pointer to a function/.
24
25[tip
26function_traits is intended to introspect only C++ functions of the
27form R (), R( A1 ), R ( A1, ... etc. ) and not function pointers or
28class member functions. To convert a function pointer type to a suitable
29type 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[endsect]
56
57