1 | |
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 */ |
77 | |