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81 lines
2.0 KiB
C++
81 lines
2.0 KiB
C++
//
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// Copyright 2003 © The Trustees of Indiana University.
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//
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// See the file enable_if_LICENSE for licensing conditions.
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//
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// Authors: Jaakko Järvi (jajarvi at osl.iu.edu)
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// Jeremiah Willcock (jewillco at osl.iu.edu)
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// Andrew Lumsdaine (lums at osl.iu.edu)
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//
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#include <boost/test/minimal.hpp>
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#include <boost/utility/enable_if.hpp>
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#include <boost/type_traits/is_same.hpp>
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using boost::enable_if_c;
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using boost::lazy_enable_if_c;
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// This class provides a reduced example of a traits class for
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// computing the result of multiplying two types. The member typedef
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// 'type' in this traits class defines the return type of this
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// operator. The return type member is invalid unless both arguments
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// for mult_traits are values that mult_traits expects (ints in this
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// case). This kind of situation may arise if a traits class only
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// makes sense for some set of types, not all C++ types.
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template <class T> struct is_int {
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BOOST_STATIC_CONSTANT(bool, value = (boost::is_same<T, int>::value));
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};
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template <class T, class U>
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struct mult_traits {
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typedef typename T::does_not_exist type;
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};
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template <>
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struct mult_traits<int, int> {
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typedef int type;
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};
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// Next, a forwarding function mult() is defined. It is enabled only
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// when both arguments are of type int. The first version, using
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// non-lazy enable_if_c does not work.
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#if 0
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template <class T, class U>
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typename enable_if_c<
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is_int<T>::value && is_int<U>::value,
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typename mult_traits<T, U>::type
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>::type
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mult(const T& x, const U& y) {return x * y;}
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#endif
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// A correct version uses lazy_enable_if_c.
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// This template removes compiler errors from invalid code used as an
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// argument to enable_if_c.
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#if 1
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template <class T, class U>
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typename lazy_enable_if_c<
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is_int<T>::value && is_int<U>::value,
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mult_traits<T, U>
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>::type
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mult(const T& x, const U& y) {return x * y;}
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#endif
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double mult(int i, double d) { return (double)i * d; }
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int test_main(int, char*[])
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{
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BOOST_CHECK(mult(1, 2) == 2);
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BOOST_CHECK(mult(1, 3.0) == 3.0);
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return 0;
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}
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