Hello, I have a test that uses many BOOST_CHECKs (~ 10 thousands or so). Running that test is very slow and I have noticed most of the time is spent by formatting the description string to be passed to check_impl, only to be ignored, because I don't want to log successful checks (and they are successful). I have improved this by using some kind of expression template for this (called lazy_output). It works like this: lazy_output is a class that implements an expression template for "x << y << z" expressions. The expression o << a << b << c << (make_lazy_output() << x << y << z) is the same as o << a << b << c << x << y << z which is similar to o << a << b << c << (wrap_stringstream().ref() << x << y << z).ref() The difference is that, in the call f(make_lazy_output() << x << y << z), the data is formatted only if f outputs them to an ostream. This reduced the time needed for running my test from ~12 minutes to ~7 minutes. There is more room for similar (but more complicated) improvements. Any comments or suggestions? Regards Jiri Palecek