[boost] [config] unexpected explicit operators conversion on some compilers.

Subject: [boost] [config] unexpected explicit operators conversion on some compilers.
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2012-03-31 02:49:51

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