The predicate_result class combines a boolean result from a predicate with a detailed context message for a failed predicate evaluation. Usually this class is only used in the internal implementation of the test framework, but a user can define their own custom predicates that return a predicate_result to support detailed failure messages when an assertion fails.

predicate_result is constructable and assignable from bool. The message() method returns an output string stream that can be used to build up a context message.

namespace boost { namespace test_tools {

class BOOST_TEST_DECL predicate_result {
public:
    // Constructor
    predicate_result( bool pv_ )
    : p_predicate_value( pv_ )
    {}

    template<typename BoolConvertable>
    predicate_result( BoolConvertable const& pv_ ) : p_predicate_value( !!pv_ ) {}

    // Access methods
    bool                operator!() const           { return !p_predicate_value; }
    void                operator=( bool pv_ )       { p_predicate_value.value = pv_; }
    operator            safe_bool() const           { return !!p_predicate_value ? &dummy::nonnull : 0; }

    // Public properties
    BOOST_READONLY_PROPERTY( bool, (predicate_result) ) p_predicate_value;

    // Access methods
    bool                has_empty_message() const   { return !m_message; }
    wrap_stringstream&  message()
    const_string        message() const             { return !m_message ? const_string() : const_string( m_message->str() ); }
};

} }

See BOOST_REQUIRE_MESSAGE for an example using predicate_result.