forked from boostorg/utility
75 lines
2.0 KiB
C++
75 lines
2.0 KiB
C++
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//
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// Copyright 2003 <20> The Trustees of Indiana University. All rights
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// reserved.
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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 J<>rvi (jajarvi@osl.iu.edu)
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// Jeremiah Willcock (jewillco@osl.iu.edu)
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// Andrew Lumsdaine (lums@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_arithmetic.hpp>
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using boost::enable_if;
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using boost::enable_if_lazy;
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using boost::is_arithmetic;
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// This class provides a reduced example of a traits class for computing
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// the result of multiplying two types. The exists constant is true when a
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// multiplication operator exists between two types, and the type member is
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// defined to the return type of this operator. The return type member is
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// not defined when a multiplication operator does not exist.
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template <class T, class U>
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struct mult_traits {
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BOOST_STATIC_CONSTANT(bool, exists = false);
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};
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template <>
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struct mult_traits<int, int> {
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BOOST_STATIC_CONSTANT(bool, exists = true);
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typedef int type;
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};
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template <>
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struct mult_traits<double, double> {
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BOOST_STATIC_CONSTANT(bool, exists = true);
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typedef double type;
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};
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// Next, a forwarding function mult() is defined. It is enabled only when
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// mult_traits<T, U>::exists is true. The first version, using normal
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// enable_if, works with only some compilers (it does not work in g++):
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#if 0
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template <class T, class U>
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typename enable_if<
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mult_traits<T, U>::exists,
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typename mult_traits<T, U>::type>::type
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mult(const T& x, const U& y) {return x * y;}
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#endif
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// An improved version that works with more compilers uses enable_if_lazy.
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// This template removes compiler errors from invalid code used as an
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// argument to enable_if.
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template <class T, class U>
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typename enable_if_lazy<
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mult_traits<T, U>::exists,
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mult_traits<T, U> >::type
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mult(const T& x, const U& y) {return x * y;}
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int test_main(int, char*[])
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{
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BOOST_TEST(mult(1, 2) == 2);
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BOOST_TEST(mult(1.0, 3.0) == 3.0);
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return 0;
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}
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