For example, I am comparing coordinates in 2d spaces and the relative value is not important, the meaningfull value is the absolute difference between coordinates.
It is almost never true. Related error gives you much better estimate of closeness.
I agree with that most of the time but this is not always true. Just imagine the case you compare lattitude, longitude. In this case, only absolute values make sense.
Sure, 0deg East and 180deg East are physically the same, but it is a fact of floating point life that you can record/measure a point close to 0deg East to more precision than one near 180 East. This is why people always suggest using custom types (e.g. a currency type for finance) when this thread comes up.
I can probably write my own macro combining BOOST_TEST_MESSAGE() and some glues to obtain what I want but I was wondering if someone had the same problem and a solution.
Be careful using the naïve solution of something like #define MY_TEST_ABS(x,y,tol,msg) BOOST_CHECK_MESSAGE( std::abs((x)-(y)) < tol , build_test_abs_error_message(x,y,tol,msg) ) Here x and y are repeated in the macro expansion so when x = f(a,b,c) this leads to subtle bugs or unexpected performance (f is called twice). To do it properly you have to follow what is done for BOOST_CHECK_CLOSE and a define a new predicate analagous to close_at_tolerance (found in <boost/test/floating_point_comparison.hpp>)