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2 | // (C) Copyright Edward Diener 2011-2015 | |
3 | // Use, modification and distribution are subject to the Boost Software License, | |
4 | // Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at | |
5 | // http://www.boost.org/LICENSE_1_0.txt). | |
6 | ||
7 | #if !defined(BOOST_VMD_IDENTITY_HPP) | |
8 | #define BOOST_VMD_IDENTITY_HPP | |
9 | ||
10 | #include <boost/vmd/detail/setup.hpp> | |
11 | ||
12 | #if BOOST_PP_VARIADICS | |
13 | ||
14 | #if BOOST_VMD_MSVC | |
15 | #include <boost/preprocessor/cat.hpp> | |
16 | #endif | |
17 | #include <boost/vmd/empty.hpp> | |
18 | ||
19 | /* | |
20 | ||
21 | The succeeding comments in this file are in doxygen format. | |
22 | ||
23 | */ | |
24 | ||
25 | /** \file | |
26 | */ | |
27 | ||
28 | /** \def BOOST_VMD_IDENTITY(item) | |
29 | ||
30 | \brief Macro which expands to its argument when invoked with any number of parameters. | |
31 | ||
32 | item = any single argument | |
33 | ||
34 | When BOOST_VMD_IDENTITY(item) is subsequently invoked with any number of parameters it expands | |
35 | to 'item'. Subsequently invoking the macro is done as 'BOOST_VMD_IDENTITY(item)(zero_or_more_arguments)'. | |
36 | ||
37 | The macro is equivalent to the Boost PP macro BOOST_PP_IDENTITY(item) with the difference | |
38 | being that BOOST_PP_IDENTITY(item) is always invoked with no arguments, as in | |
39 | 'BOOST_VMD_IDENTITY(item)()' whereas BOOST_VMD_IDENTITY can be invoked with any number of | |
40 | arguments. | |
41 | ||
42 | The macro is meant to be used in BOOST_PP_IF and BOOST_PP_IIF statements when only one | |
43 | of the clauses needs to be invoked with calling another macro and the other is meant to | |
44 | return an 'item'. | |
45 | ||
46 | returns = the macro as 'BOOST_VMD_IDENTITY(item)', when invoked with any number of parameters | |
47 | as in '(zero_or_more_arguments)', returns 'item'. The macro itself returns | |
48 | 'item BOOST_VMD_EMPTY'. | |
49 | ||
50 | */ | |
51 | ||
52 | #define BOOST_VMD_IDENTITY(item) item BOOST_VMD_EMPTY | |
53 | ||
54 | /** \def BOOST_VMD_IDENTITY_RESULT(result) | |
55 | ||
56 | \brief Macro which wraps any result which can return its value using BOOST_VMD_IDENTITY or 'item BOOST_VMD_EMPTY'. | |
57 | ||
58 | result = any single result returned when BOOST_VMD_IDENTITY is used or 'item BOOST_VMD_EMPTY'. | |
59 | ||
60 | The reason for this macro is to smooth over a problem when using VC++ with BOOST_VMD_IDENTITY. | |
61 | If your BOOST_VMD_IDENTITY macro can be used where VC++ is the compiler then you need to | |
62 | surround your macro code which could return a result with this macro in order that VC++ handles | |
63 | BOOST_VMD_IDENTITY correctly. | |
64 | ||
65 | If you are not using VC++ you do not have to use this macro, but doing so does no harm. | |
66 | ||
67 | */ | |
68 | ||
69 | #if BOOST_VMD_MSVC | |
70 | #define BOOST_VMD_IDENTITY_RESULT(result) BOOST_PP_CAT(result,) | |
71 | #else | |
72 | #define BOOST_VMD_IDENTITY_RESULT(result) result | |
73 | #endif | |
74 | ||
75 | #endif /* BOOST_PP_VARIADICS */ | |
76 | #endif /* BOOST_VMD_IDENTITY_HPP */ |