BOOST_FAIL(message) always fails and emits the given message at the REQUIRE assertion level. Other assertion macros are generally preferred over BOOST_FAIL because they emit better diagnostics in the case of failure.

There are two situations where you might be tempted to use BOOST_FAIL: testing the situations where the system under test will throw exceptions or comparing against the values computed by the system under test. For testing exception generation, use BOOST_REQUIRE_THROW, BOOST_REQUIRE_NO_THROW or BOOST_REQUIRE_EXCEPTION. For testing against values computed by the system under test, use the comparing assertions.

Example

BOOST_AUTO_TEST_CASE(example_fail)
{
    BOOST_FAIL("This should never happen.");
}