safe-bool.h

 
#ifndef COMMON_SAFE_BOOL_H
#define COMMON_SAFE_BOOL_H
 
namespace Common {
    namespace impl {
        template <typename T>
        struct no_base {};
 
        template <typename T>
        struct safe_bool_impl {
            typedef T *TP; // workaround to make parsing easier
            TP stub;
            typedef TP safe_bool_impl::*type;
        };
    }
 
    template <typename DerivedT, typename BaseT = impl::no_base<DerivedT> >
    struct SafeBool : BaseT {
    private:
        typedef impl::safe_bool_impl<DerivedT> impl_t;
        typedef typename impl_t::type bool_type;
 
    public:
        operator bool_type() const {
            return static_cast<const DerivedT *>(this)->operator_bool() ?
            &impl_t::stub : nullptr;
        }
 
        operator bool_type() {
            return static_cast<DerivedT *>(this)->operator_bool() ?
            &impl_t::stub : 0;
        }
    };
} // End of namespace Common
 
#endif