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1.. Metafunctions/Composition and Argument Binding//lambda |20
2
3lambda
4======
5
6Synopsis
7--------
8
9.. parsed-literal::
10
11    template<
12          typename X
13        , typename Tag = |unspecified|
14        >
15    struct lambda
16    {
17        typedef |unspecified| type;
18    };
19
20
21
22Description
23-----------
24
25If ``X`` is a |placeholder expression|, transforms ``X`` into a corresponding
26|Metafunction Class|, otherwise ``X`` is returned unchanged.
27
28
29Header
30------
31
32.. parsed-literal::
33
34    #include <boost/mpl/lambda.hpp>
35
36
37Parameters
38----------
39
40+---------------+-----------------------+-----------------------------------------------+
41| Parameter     | Requirement           | Description                                   |
42+===============+=======================+===============================================+
43| ``X``         | Any type              | An expression to transform.                   |
44+---------------+-----------------------+-----------------------------------------------+
45| ``Tag``       | Any type              | A tag determining transform semantics.        |
46+---------------+-----------------------+-----------------------------------------------+
47
48Expression semantics
49--------------------
50
51For arbitrary types ``x`` and ``tag``:
52
53
54.. parsed-literal::
55
56    typedef lambda<x>::type f;
57
58:Return type:
59    |Metafunction Class|.
60
61:Semantics:
62    If ``x`` is a |placeholder expression| in a general form ``X<a1,...an>``, where
63    ``X`` is a class template and ``a1``,... ``an`` are arbitrary types, equivalent
64    to
65
66    .. parsed-literal::
67
68        typedef protect< bind<
69              quote\ *n*\ <X>
70            , lambda<a1>::type,\ |...| lambda<a\ *n*\ >::type
71            > > f;
72
73    otherwise, ``f`` is identical to ``x``.
74
75.. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
76
77.. parsed-literal::
78
79    typedef lambda<x,tag>::type f;
80
81:Return type:
82    |Metafunction Class|.
83
84:Semantics:
85    If ``x`` is a |placeholder expression| in a general form ``X<a1,...an>``, where
86    ``X`` is a class template and ``a1``,... ``an`` are arbitrary types, equivalent
87    to
88
89    .. parsed-literal::
90
91        typedef protect< bind<
92              quote\ *n*\ <X,tag>
93            , lambda<a1,tag>::type,\ |...| lambda<a\ *n*\ ,tag>::type
94            > > f;
95
96    otherwise, ``f`` is identical to ``x``.
97
98
99Example
100-------
101
102.. parsed-literal::
103
104    template< typename N1, typename N2 > struct int_plus
105        : int_<( N1::value + N2::value )>
106    {
107    };
108
109    typedef lambda< int_plus<_1, int_<42> > >::type f1;
110    typedef bind< quote\ ``2``\ <int_plus>, _1, int_<42> > f2;
111
112    typedef f1::apply<42>::type r1;
113    typedef f2::apply<42>::type r2;
114
115    BOOST_MPL_ASSERT_RELATION( r1::value, ==, 84 );
116    BOOST_MPL_ASSERT_RELATION( r2::value, ==, 84 );
117
118
119See also
120--------
121
122|Composition and Argument Binding|, |Invocation|, |Placeholders|, |bind|, |quote|, |protect|, |apply|
123
124
125.. copyright:: Copyright �  2001-2009 Aleksey Gurtovoy and David Abrahams
126   Distributed under the Boost Software License, Version 1.0. (See accompanying
127   file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
128