mirror of
https://github.com/boostorg/utility.git
synced 2025-11-04 02:11:45 +01:00
75 lines
2.0 KiB
C++
75 lines
2.0 KiB
C++
//
|
|
// Copyright 2003 © The Trustees of Indiana University. All rights
|
|
// reserved.
|
|
//
|
|
// See the file enable_if_LICENSE for licensing conditions.
|
|
//
|
|
// Authors: Jaakko Järvi (jajarvi@osl.iu.edu)
|
|
// Jeremiah Willcock (jewillco@osl.iu.edu)
|
|
// Andrew Lumsdaine (lums@osl.iu.edu)
|
|
//
|
|
|
|
#include <boost/test/minimal.hpp>
|
|
|
|
#include <boost/utility/enable_if.hpp>
|
|
#include <boost/type_traits/is_arithmetic.hpp>
|
|
|
|
using boost::enable_if;
|
|
using boost::enable_if_lazy;
|
|
using boost::is_arithmetic;
|
|
|
|
// This class provides a reduced example of a traits class for computing
|
|
// the result of multiplying two types. The exists constant is true when a
|
|
// multiplication operator exists between two types, and the type member is
|
|
// defined to the return type of this operator. The return type member is
|
|
// not defined when a multiplication operator does not exist.
|
|
|
|
template <class T, class U>
|
|
struct mult_traits {
|
|
BOOST_STATIC_CONSTANT(bool, exists = false);
|
|
};
|
|
|
|
template <>
|
|
struct mult_traits<int, int> {
|
|
BOOST_STATIC_CONSTANT(bool, exists = true);
|
|
typedef int type;
|
|
};
|
|
|
|
template <>
|
|
struct mult_traits<double, double> {
|
|
BOOST_STATIC_CONSTANT(bool, exists = true);
|
|
typedef double type;
|
|
};
|
|
|
|
// Next, a forwarding function mult() is defined. It is enabled only when
|
|
// mult_traits<T, U>::exists is true. The first version, using normal
|
|
// enable_if, works with only some compilers (it does not work in g++):
|
|
|
|
#if 0
|
|
template <class T, class U>
|
|
typename enable_if<
|
|
mult_traits<T, U>::exists,
|
|
typename mult_traits<T, U>::type>::type
|
|
mult(const T& x, const U& y) {return x * y;}
|
|
#endif
|
|
|
|
// An improved version that works with more compilers uses enable_if_lazy.
|
|
// This template removes compiler errors from invalid code used as an
|
|
// argument to enable_if.
|
|
|
|
template <class T, class U>
|
|
typename enable_if_lazy<
|
|
mult_traits<T, U>::exists,
|
|
mult_traits<T, U> >::type
|
|
mult(const T& x, const U& y) {return x * y;}
|
|
|
|
int test_main(int, char*[])
|
|
{
|
|
|
|
BOOST_TEST(mult(1, 2) == 2);
|
|
BOOST_TEST(mult(1.0, 3.0) == 3.0);
|
|
|
|
return 0;
|
|
}
|
|
|