| 1 | // (C) Copyright David Abrahams 2002. |
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| 2 | // (C) Copyright Jeremy Siek 2002. |
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| 3 | // (C) Copyright Thomas Witt 2002. |
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| 4 | // Distributed under the Boost Software License, Version 1.0. (See |
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| 5 | // accompanying file LICENSE_1_0.txt or copy at |
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| 6 | // http://www.boost.org/LICENSE_1_0.txt) |
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| 7 | #ifndef BOOST_ENABLE_IF_23022003THW_HPP |
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| 8 | #define BOOST_ENABLE_IF_23022003THW_HPP |
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| 9 | |
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| 10 | #include <boost/config.hpp> |
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| 11 | #include <boost/iterator/detail/config_def.hpp> |
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| 12 | #if defined(BOOST_NO_SFINAE) || defined(BOOST_NO_IS_CONVERTIBLE) |
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| 13 | #include <boost/type_traits/type_identity.hpp> |
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| 14 | #endif |
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| 15 | |
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| 16 | // |
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| 17 | // Boost iterators uses its own enable_if cause we need |
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| 18 | // special semantics for deficient compilers. |
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| 19 | // 23/02/03 thw |
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| 20 | // |
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| 21 | |
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| 22 | namespace boost |
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| 23 | { |
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| 24 | |
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| 25 | namespace iterators |
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| 26 | { |
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| 27 | // |
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| 28 | // Base machinery for all kinds of enable if |
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| 29 | // |
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| 30 | template<bool> |
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| 31 | struct enabled |
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| 32 | { |
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| 33 | template<typename T> |
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| 34 | struct base |
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| 35 | { |
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| 36 | typedef T type; |
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| 37 | }; |
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| 38 | }; |
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| 39 | |
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| 40 | // |
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| 41 | // For compilers that don't support "Substitution Failure Is Not An Error" |
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| 42 | // enable_if falls back to always enabled. See comments |
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| 43 | // on operator implementation for consequences. |
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| 44 | // |
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| 45 | template<> |
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| 46 | struct enabled<false> |
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| 47 | { |
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| 48 | template<typename T> |
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| 49 | struct base |
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| 50 | { |
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| 51 | #ifdef BOOST_NO_SFINAE |
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| 52 | typedef T type; |
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| 53 | |
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| 54 | // This way to do it would give a nice error message containing |
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| 55 | // invalid overload, but has the big disadvantage that |
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| 56 | // there is no reference to user code in the error message. |
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| 57 | // |
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| 58 | // struct invalid_overload; |
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| 59 | // typedef invalid_overload type; |
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| 60 | // |
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| 61 | #endif |
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| 62 | }; |
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| 63 | }; |
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| 64 | |
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| 65 | |
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| 66 | template <class Cond, |
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| 67 | class Return> |
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| 68 | struct enable_if |
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| 69 | # if !defined(BOOST_NO_SFINAE) && !defined(BOOST_NO_IS_CONVERTIBLE) |
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| 70 | : enabled<(Cond::value)>::template base<Return> |
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| 71 | # else |
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| 72 | : boost::type_identity<Return> |
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| 73 | # endif |
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| 74 | { |
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| 75 | }; |
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| 76 | |
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| 77 | } // namespace iterators |
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| 78 | |
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| 79 | } // namespace boost |
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| 80 | |
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| 81 | #include <boost/iterator/detail/config_undef.hpp> |
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| 82 | |
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| 83 | #endif // BOOST_ENABLE_IF_23022003THW_HPP |
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| 84 | |
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