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/detail/workaround.hpp> |
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11 | #include <boost/mpl/identity.hpp> |
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12 | |
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13 | #include <boost/iterator/detail/config_def.hpp> |
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14 | |
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15 | // |
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16 | // Boost iterators uses its own enable_if cause we need |
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17 | // special semantics for deficient compilers. |
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18 | // 23/02/03 thw |
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19 | // |
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20 | |
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21 | namespace boost |
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22 | { |
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23 | |
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24 | namespace iterators |
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25 | { |
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26 | // |
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27 | // Base machinery for all kinds of enable if |
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28 | // |
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29 | template<bool> |
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30 | struct enabled |
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31 | { |
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32 | template<typename T> |
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33 | struct base |
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34 | { |
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35 | typedef T type; |
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36 | }; |
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37 | }; |
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38 | |
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39 | // |
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40 | // For compilers that don't support "Substitution Failure Is Not An Error" |
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41 | // enable_if falls back to always enabled. See comments |
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42 | // on operator implementation for consequences. |
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43 | // |
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44 | template<> |
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45 | struct enabled<false> |
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46 | { |
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47 | template<typename T> |
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48 | struct base |
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49 | { |
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50 | #ifdef BOOST_NO_SFINAE |
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51 | |
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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 | : mpl::identity<Return> |
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73 | # endif |
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74 | { |
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75 | # if BOOST_WORKAROUND(BOOST_MSVC, < 1300) |
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76 | typedef Return type; |
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77 | # endif |
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78 | }; |
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79 | |
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80 | } // namespace iterators |
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81 | |
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82 | } // namespace boost |
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83 | |
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84 | #include <boost/iterator/detail/config_undef.hpp> |
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85 | |
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86 | #endif // BOOST_ENABLE_IF_23022003THW_HPP |
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