lazy corrections

[SVN r1730]

test/enable_if_lazy.cpp

@@ -11,64 +11,68 @@
 #include <boost/test/minimal.hpp>
 
 #include <boost/utility/enable_if.hpp>
-#include <boost/type_traits/is_arithmetic.hpp>
+#include <boost/type_traits/is_same.hpp>
 
 using boost::enable_if_c;
 using boost::lazy_enable_if_c;
-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.
+// 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 {
-  BOOST_STATIC_CONSTANT(bool, exists = false);
+  typedef typename T::does_not_exist type;
 };
 
 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_c, works with only some compilers (it does not work in g++):
+// 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<
-	  mult_traits<T, U>::exists, 
-	  typename mult_traits<T, U>::type>::type
+  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
 
-// An improved version that works with more compilers uses lazy_enable_if_c.
+// 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<
-	  mult_traits<T, U>::exists, 
-	  mult_traits<T, U> >::type
+  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(1.0, 3.0) == 3.0);
+
   BOOST_CHECK(mult(1, 3.0) == 3.0);
 
   return 0;

test/enable_if_lazy_test.cpp

@@ -8,11 +8,13 @@
 //             Andrew Lumsdaine (lums at osl.iu.edu)
 //
 
+// Testing all variations of lazy_enable_if.
+
 #include <boost/test/minimal.hpp>
 #include <boost/mpl/not.hpp>
 
 #include <boost/utility/enable_if.hpp>
-#include <boost/type_traits/is_arithmetic.hpp>
+#include <boost/type_traits/is_same.hpp>
 
 using boost::lazy_enable_if;
 using boost::lazy_disable_if;
@@ -20,22 +22,26 @@ using boost::lazy_disable_if;
 using boost::lazy_enable_if_c;
 using boost::lazy_disable_if_c;
 
-using boost::is_arithmetic;
+
+template <class T>
+struct is_int_or_double {
+  BOOST_STATIC_CONSTANT(bool, 
+    value = (boost::is_same<T, int>::value || 
+	     boost::is_same<T, double>::value));
+};
 
 template <class T>
 struct some_traits {
-  BOOST_STATIC_CONSTANT(bool, value = false);
+  typedef typename T::does_not_exist type;
 };
 
 template <>
 struct some_traits<int> {
-  BOOST_STATIC_CONSTANT(bool, value = true);
   typedef bool type;
 };
 
 template <>
 struct some_traits<double> {
-  BOOST_STATIC_CONSTANT(bool, value = true);
   typedef bool type;
 };
 
@@ -53,21 +59,21 @@ struct make_bool<double> {};
 namespace A {
 
   template<class T>
-  typename lazy_enable_if<some_traits<T>, some_traits<T> >::type
+  typename lazy_enable_if<is_int_or_double<T>, some_traits<T> >::type
   foo(T t) { return true; }
 
   template<class T>
-  typename lazy_enable_if_c<some_traits<T>::value, some_traits<T> >::type
+  typename lazy_enable_if_c<is_int_or_double<T>::value, some_traits<T> >::type
   foo2(T t) { return true; }
 }
 
 namespace B {
   template<class T>
-  typename lazy_disable_if<some_traits<T>, make_bool<T> >::type
+  typename lazy_disable_if<is_int_or_double<T>, make_bool<T> >::type
   foo(T t) { return false; }
 
   template<class T>
-  typename lazy_disable_if_c<some_traits<T>::value, make_bool<T> >::type
+  typename lazy_disable_if_c<is_int_or_double<T>::value, make_bool<T> >::type
   foo2(T t) { return false; }
 }