Hi, Given the following explicit bool conversion template<typename Mutex> class unique_lock { public: explicit operator bool() const BOOST_NOEXCEPT I was expecting the following code to compile fail (note the int cast) boost::unique_lock<boost::mutex> lk0; int i = int(lk0); It fails with gcc-4.7.0, but compile with gcc-4.6.2 and clang-3.0 (I have not tested yet with msvc). When BOOST_NO_EXPLICIT_CONVERSION_OPERATORS is defined I use the bool safe idiom to emulate the conversion #if defined(BOOST_NO_EXPLICIT_CONVERSION_OPERATORS) typedef void (unique_lock::*bool_type)(); operator bool_type() const BOOST_NOEXCEPT; bool operator!() const BOOST_NOEXCEPT; #else explicit operator bool() const BOOST_NOEXCEPT; #endif and in this case the compile fail as desired. Do we need a new macro to state that even if explicit operators are provide, there are some unexpected conversions that could be introduced? Best, Vicente