Use BOOST_EXPLICIT_OPERATOR_BOOL for optional

I often have the problem that when I change a std::wstring to boost::optional<std::wstring> and the variable is used as a parameter with Boost.Format, the result silently changes from the string contents to "1".

This change prevents implicit conversion to bool if the compiler supports explicit conversion operators.

test/optional_test_fail_implicit_bool_convert.cpp

@@ -0,0 +1,28 @@
+// Copyright (C) 2014, Andrzej Krzemienski.
+//
+// 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)
+//
+// See http://www.boost.org/lib/optional for documentation.
+//
+// You are welcome to contact the author at:
+//  akrzemi1@gmail.com
+//
+#include "boost/optional.hpp"
+
+//
+// THIS TEST SHOULD FAIL TO COMPILE
+//
+
+#if !defined(BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS)
+
+bool test_implicit_conversion_to_bool()
+{
+  boost::optional<int> opt;
+  return opt;
+}
+
+#else
+#  error "Test skipped: this compiler does not support explicit conversion operators."
+#endif