Even though supplied testing tools cover wide range of possible checks and provide detailed report on cause of error in some cases you may want to implement and use custom predicate that perform complex check and produce intelligent report on failure. To satisfy this need testing tools implement custom predicate support. There two layers of custom predicate support implemented by testing tools toolbox: with and without custom error message generation.

The first layer is supported by BOOST_CHECK_PREDICATE family of testing tools. You can use it to check any custom predicate that reports the result as boolean value. The values of the predicate arguments are reported by the tool automatically in case of failure.

#define BOOST_TEST_MODULE example
#include <boost/test/included/unit_test.hpp>

//____________________________________________________________________________//

bool is_even( int i )        { return i%2 == 0;  }

BOOST_AUTO_TEST_CASE( test_is_even )
{
    BOOST_CHECK_PREDICATE( is_even, (14) );

    int i = 17;
    BOOST_CHECK_PREDICATE( is_even, (i) );
}

//____________________________________________________________________________//

> example
Running 1 test case...
test.cpp(13): error in "test_is_even": check is_even( i ) failed for ( 17 )

*** 1 failure detected in test suite "example"

To use second layer your predicate have to return boost::test_tools::predicate_result. This class encapsulates boolean result value along with any error or information message you opt to report.

Usually you construct the instance of class boost::test_tools::predicate_result inside your predicate function and return it by value. The constructor expects one argument - the boolean result value. The constructor is implicit, so you can simply return boolean value from your predicate and boost::test_tools::predicate_result is constructed automatically to hold your value and empty message. You can also assign boolean value to the constructed instance. You can check the current predicate value by using operator!() or directly accessing public read-only property p_predicate_value. The error message is stored in public read-write property p_message.

#define BOOST_TEST_MODULE example
#include <boost/test/included/unit_test.hpp>

//____________________________________________________________________________//

boost::test_tools::predicate_result
compare_lists( std::list<int> const& l1, std::list<int> const& l2 )
{
    if( l1.size() != l2.size() ) {
        boost::test_tools::predicate_result res( false );

        res.message() << "Different sizes [" << l1.size() << "!=" << l2.size() << "]";

        return res;
    }

    return true;
}

//____________________________________________________________________________//

BOOST_AUTO_TEST_CASE( test_list_comparizon )
{
    std::list<int> l1, l2;
    l1.push_back( 1 );
    l1.push_back( 2 );

    BOOST_CHECK( compare_lists( l1, l2 ) );
}

//____________________________________________________________________________//

Running 1 test case...
test.cpp(28): error in "test_list_comparizon": check compare_lists( l1, l2 ) failed. Different sizes [2!=0]

*** 1 failure detected in test suite "example"