| 1 | // Boost enable_if library |
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| 2 | |
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| 3 | // Copyright 2003 (c) The Trustees of Indiana University. |
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| 4 | |
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| 5 | // Use, modification, and distribution is subject to the Boost Software |
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| 6 | // License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at |
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| 7 | // http://www.boost.org/LICENSE_1_0.txt) |
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| 8 | |
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| 9 | // Authors: Jaakko Jarvi (jajarvi at osl.iu.edu) |
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| 10 | // Jeremiah Willcock (jewillco at osl.iu.edu) |
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| 11 | // Andrew Lumsdaine (lums at osl.iu.edu) |
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| 12 | |
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| 13 | #include <boost/utility/enable_if.hpp> |
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| 14 | #include <boost/type_traits/is_same.hpp> |
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| 15 | #include <boost/core/lightweight_test.hpp> |
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| 16 | |
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| 17 | using boost::enable_if_c; |
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| 18 | using boost::lazy_enable_if_c; |
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| 19 | |
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| 20 | // This class provides a reduced example of a traits class for |
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| 21 | // computing the result of multiplying two types. The member typedef |
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| 22 | // 'type' in this traits class defines the return type of this |
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| 23 | // operator. The return type member is invalid unless both arguments |
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| 24 | // for mult_traits are values that mult_traits expects (ints in this |
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| 25 | // case). This kind of situation may arise if a traits class only |
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| 26 | // makes sense for some set of types, not all C++ types. |
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| 27 | |
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| 28 | template <class T> struct is_int { |
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| 29 | BOOST_STATIC_CONSTANT(bool, value = (boost::is_same<T, int>::value)); |
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| 30 | }; |
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| 31 | |
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| 32 | template <class T, class U> |
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| 33 | struct mult_traits { |
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| 34 | typedef typename T::does_not_exist type; |
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| 35 | }; |
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| 36 | |
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| 37 | template <> |
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| 38 | struct mult_traits<int, int> { |
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| 39 | typedef int type; |
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| 40 | }; |
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| 41 | |
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| 42 | |
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| 43 | // Next, a forwarding function mult() is defined. It is enabled only |
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| 44 | // when both arguments are of type int. The first version, using |
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| 45 | // non-lazy enable_if_c does not work. |
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| 46 | |
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| 47 | #if 0 |
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| 48 | template <class T, class U> |
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| 49 | typename enable_if_c< |
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| 50 | is_int<T>::value && is_int<U>::value, |
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| 51 | typename mult_traits<T, U>::type |
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| 52 | >::type |
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| 53 | mult(const T& x, const U& y) {return x * y;} |
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| 54 | #endif |
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| 55 | |
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| 56 | // A correct version uses lazy_enable_if_c. |
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| 57 | // This template removes compiler errors from invalid code used as an |
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| 58 | // argument to enable_if_c. |
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| 59 | |
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| 60 | #if 1 |
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| 61 | template <class T, class U> |
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| 62 | typename lazy_enable_if_c< |
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| 63 | is_int<T>::value & is_int<U>::value, |
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| 64 | mult_traits<T, U> |
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| 65 | >::type |
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| 66 | mult(const T& x, const U& y) {return x * y;} |
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| 67 | #endif |
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| 68 | |
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| 69 | double mult(int i, double d) { return (double)i * d; } |
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| 70 | |
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| 71 | int main() |
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| 72 | { |
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| 73 | |
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| 74 | |
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| 75 | BOOST_TEST(mult(1, 2) == 2); |
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| 76 | |
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| 77 | BOOST_TEST(mult(1, 3.0) == 3.0); |
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| 78 | |
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| 79 | return boost::report_errors(); |
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| 80 | } |
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| 81 | |
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| 82 | |
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