BOOST_EXPLICIT_OPERATOR_BOOL() and BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL() are compatibility helper macros that expand to an explicit conversion operator to bool. For compilers not supporting explicit conversion operators introduced in C++11 the macros expand to a conversion operator that implements the safe bool idiom. In case if the compiler is not able to handle safe bool idiom well the macros expand to a regular conversion operator to bool.

Both macros are intended to be placed within a user's class definition. The generated conversion operators will be implemented in terms of operator!() that should be defined by user in this class. In case of BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL() the generated conversion operator will be declared constexpr which requires the corresponding operator!() to also be constexpr.

template< typename T >
class my_ptr
{
    T* m_p;

public:
    BOOST_EXPLICIT_OPERATOR_BOOL()

    bool operator!() const
    {
        return !m_p;
    }
};

Now my_ptr can be used in conditional expressions, similarly to a regular pointer:

my_ptr< int > p;
if (p)
    std::cout << "true" << std::endl;

boost 1.55