Index: libs/utility/operators.htm =================================================================== RCS file: /cvsroot/boost/boost/libs/utility/operators.htm,v retrieving revision 1.12 diff -u -r1.12 operators.htm --- libs/utility/operators.htm 4 Nov 2002 01:59:31 -0000 1.12 +++ libs/utility/operators.htm 27 Feb 2003 10:51:17 -0000 @@ -340,6 +340,8 @@
  • indexable<>
    • +
  • bool_testable<>
    • +
  • Any composite operator template that includes at least one of the above
    • @@ -528,6 +530,16 @@ + bool_testable<T> + + operator unspecified-bool-type() const + + !t.
    + Return convertible to bool. + + + addable<T>
    addable1<T> @@ -974,6 +986,45 @@ that don't implement the NRVO.

    +

    Bool Testable Note

    + +

    bool_testable provides a + conversion operator to an unspecified value that, when used in boolean + contexts, is equivalent to !(!t). This allows + expressions such as :

    + + 
    +    if (object)
+       ...
+
    + +

    bool_testable differs in its + use slightly in that it is necessary to publicly derive from it. This is + because conversion operators must be members. e.g:

    + +
    +class Stream : public boost::bool_testable<Stream>
+{
+public:
+    explicit Stream(const char * source);
+    bool operator!() const;  // (Could also be a non-member function)
+    // operator unspecified-bool-type() auto-generated 
+    // by bool_testable<>
+};
+
+void f()
+{
+    if (Stream stream("source.txt"))
+    {
+        // use stream ...
+    }
+}
+
    + +

    Although it would perhaps have made more semantic sense to provide + bool operator!() given operator bool(), it is + difficult to implement a safe boolean conversion operator (that prevents + accidental conversion to integer types).

    Grouped Arithmetic Operators