Added bool_testable by Sam Partington

[SVN r18413]

include/boost/operators.hpp

@@ -9,6 +9,7 @@
 //  See http://www.boost.org/libs/utility/operators.htm for documentation.
 
 //  Revision History
+//  04 May 05 Added operator class bool_testable.  (Sam Partington)
 //  21 Oct 02 Modified implementation of operators to allow compilers with a
 //            correct named return value optimization (NRVO) to produce optimal
 //            code.  (Daniel Frey)
@@ -308,6 +309,18 @@ struct indexable : B
   }
 };
 
+//  bool_testable -----------------------------------------------------------//
+//  (contributed by Sam Partington, David Abrahams and Daniel Frey) ---------//
+
+template <class T, class B = ::boost::detail::empty_base>
+struct bool_testable : B
+{
+    friend bool operator!(const T& t) { return !static_cast<bool>(t); }
+private:
+    typedef signed char private_number_type;
+    operator private_number_type() const;
+};
+ 
 //  More operator classes (contributed by Daryle Walker) --------------------//
 //  (NRVO-friendly implementation contributed by Daniel Frey) ---------------//