Assert with dynamic message?

You could define your own macro

#define ASSERT_WITH_MSG(cond, msg) do \
{ if (!(cond)) { std::ostringstream str; str << msg; std::cerr << str.str(); std::abort(); } \
} while(0)

It's relatively trivial to achieve this.

BOOST_ASSERT_MSG( length >= 0, (std::stringstream() << "No positive length found! It is " << length).str().c_str() )