1/* Copyright 2006-2018 Joaquin M Lopez Munoz.
2 * Distributed under the Boost Software License, Version 1.0.
3 * (See accompanying file LICENSE_1_0.txt or copy at
4 * http://www.boost.org/LICENSE_1_0.txt)
5 *
6 * See http://www.boost.org/libs/flyweight for library home page.
7 */
8
9#ifndef BOOST_FLYWEIGHT_DETAIL_NOT_PLACEHOLDER_EXPR_HPP
10#define BOOST_FLYWEIGHT_DETAIL_NOT_PLACEHOLDER_EXPR_HPP
11
12#if defined(_MSC_VER)
13#pragma once
14#endif
15
16/* BOOST_FLYWEIGHT_NOT_A_PLACEHOLDER_EXPRESSION can be inserted at the end
17 * of a class template parameter declaration:
18 * template<
19 * typename X0,...,typename Xn
20 * BOOST_FLYWEIGHT_NOT_A_PLACEHOLDER_EXPRESSION
21 * >
22 * struct foo...
23 * to prevent instantiations from being treated as MPL placeholder
24 * expressions in the presence of placeholder arguments; this is useful
25 * to avoid masking of a metafunction class nested ::apply during
26 * MPL invocation.
27 */
28
29#include <boost/config.hpp> /* keep it first to prevent nasty warns in MSVC */
30#include <boost/detail/workaround.hpp>
31
32#if BOOST_WORKAROUND(__GNUC__, <4)||\
33 BOOST_WORKAROUND(__GNUC__,==4)&&(__GNUC_MINOR__<2)||\
34 BOOST_WORKAROUND(__GNUC__, ==7)&&( __cplusplus>=201703L)||\
35 BOOST_WORKAROUND(__GNUC__, >=8)&&( __cplusplus>=201103L)
36/* The default trick on which the macro is based, namely adding a int=0
37 * defaulted template parameter, does not work in GCC prior to 4.2 due to
38 * an unfortunate compiler non-standard extension, as explained in
39 * http://lists.boost.org/boost-users/2007/07/29866.php
40 * As it happens, GCC 7 in C++17 mode and GCC 8 (and presumably later) in
41 * C++11 mode (and presumably later) go back to this old behavior, anticipating
42 * the resolution of CWG DR 150 (see P0522R0).
43 * In these cases we resort to an uglier technique, adding defaulted template
44 * parameters so as to exceed BOOST_MPL_LIMIT_METAFUNCTION_ARITY.
45 */
46
47#include <boost/mpl/limits/arity.hpp>
48#include <boost/preprocessor/facilities/intercept.hpp>
49#include <boost/preprocessor/repetition/enum_trailing_params.hpp>
50
51#define BOOST_FLYWEIGHT_NOT_A_PLACEHOLDER_EXPRESSION \
52BOOST_PP_ENUM_TRAILING_PARAMS( \
53 BOOST_MPL_LIMIT_METAFUNCTION_ARITY,typename=int BOOST_PP_INTERCEPT)
54#define BOOST_FLYWEIGHT_NOT_A_PLACEHOLDER_EXPRESSION_DEF \
55BOOST_PP_ENUM_TRAILING_PARAMS( \
56 BOOST_MPL_LIMIT_METAFUNCTION_ARITY,typename BOOST_PP_INTERCEPT)
57
58#else
59#define BOOST_FLYWEIGHT_NOT_A_PLACEHOLDER_EXPRESSION ,int=0
60#define BOOST_FLYWEIGHT_NOT_A_PLACEHOLDER_EXPRESSION_DEF ,int
61#endif
62
63#endif
64

source code of boost/libs/flyweight/include/boost/flyweight/detail/not_placeholder_expr.hpp