boostorg/optional — pull request #3 by MarcelRaad (2014-04-24 08:02:16+00:00)
Title: Use BOOST_EXPLICIT_OPERATOR_BOOL for optional

I often have the problem that when I change a std::wstring to boost::optional\ 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.