BOOST_level_MESSAGE(predicate, message) emits message if the supplied predicate evaluates to false. The message is applied to an output stream with operator<< and the argument can contain additional applications of operator<< for building up complex messages. The predicate can return bool or predicate_result.

Example

static int g_i;

// simple predicate
static bool global_int_is_positive()
{
    return g_i > 0;
}

// predicate that builds up detail message
static boost::test_tools::predicate_result global_int_equal(int expected)
{
    if (g_i == expected)
    {
        return true;
    }

    boost::test_tools::predicate_result failed = false;
    failed.message() << "g_i != " << expected << "; actual: " << g_i;
    return failed;
}

BOOST_AUTO_TEST_CASE(example_message)
{
    g_i = 1;

    BOOST_REQUIRE_MESSAGE(global_int_is_positive(),
        "g_i is not positive; actual: " << g_i);
    BOOST_REQUIRE_MESSAGE(global_int_equal(1), "g_i equals 1");
}