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// Boost enable_if library
// Copyright 2003 (c) The Trustees of Indiana University.
// Use, modification, and distribution is subject to the Boost Software
// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
// Authors: Jaakko Jarvi (jajarvi at osl.iu.edu)
// Jeremiah Willcock (jewillco at osl.iu.edu)
// Andrew Lumsdaine (lums at osl.iu.edu)
#include <boost/utility/enable_if.hpp>
#include <boost/type_traits/is_same.hpp>
#include <boost/core/lightweight_test.hpp>
using boost::enable_if_c;
using boost::lazy_enable_if_c;
// This class provides a reduced example of a traits class for
// computing the result of multiplying two types. The member typedef
// 'type' in this traits class defines the return type of this
// operator. The return type member is invalid unless both arguments
// for mult_traits are values that mult_traits expects (ints in this
// case). This kind of situation may arise if a traits class only
// makes sense for some set of types, not all C++ types.
template <class T> struct is_int {
BOOST_STATIC_CONSTANT(bool, value = (boost::is_same<T, int>::value));
};
template <class T, class U>
struct mult_traits {
typedef typename T::does_not_exist type;
};
template <>
struct mult_traits<int, int> {
typedef int type;
};
// Next, a forwarding function mult() is defined. It is enabled only
// when both arguments are of type int. The first version, using
// non-lazy enable_if_c does not work.
#if 0
template <class T, class U>
typename enable_if_c<
is_int<T>::value && is_int<U>::value,
typename mult_traits<T, U>::type
>::type
mult(const T& x, const U& y) {return x * y;}
#endif
// A correct version uses lazy_enable_if_c.
// This template removes compiler errors from invalid code used as an
// argument to enable_if_c.
#if 1
template <class T, class U>
typename lazy_enable_if_c<
is_int<T>::value & is_int<U>::value,
mult_traits<T, U>
>::type
mult(const T& x, const U& y) {return x * y;}
#endif
double mult(int i, double d) { return (double)i * d; }
int main()
{
BOOST_TEST(mult(1, 2) == 2);
BOOST_TEST(mult(1, 3.0) == 3.0);
return boost::report_errors();
}
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