move enable_if tests into their own subdirectory

[SVN r25649]

enable_if/test/lazy.cpp

@@ -0,0 +1,82 @@
+// Boost enable_if library
+
+// Copyright 2003 <20> 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 J<>rvi (jajarvi at osl.iu.edu)
+//             Jeremiah Willcock (jewillco at osl.iu.edu)
+//             Andrew Lumsdaine (lums at osl.iu.edu)
+
+#include <boost/test/minimal.hpp>
+
+#include <boost/utility/enable_if.hpp>
+#include <boost/type_traits/is_same.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 test_main(int, char*[])
+{
+
+
+  BOOST_CHECK(mult(1, 2) == 2);
+
+  BOOST_CHECK(mult(1, 3.0) == 3.0);
+
+  return 0;
+}
+