The following is from the documentation published at www.boost.org. Surely that "!" in the "throws" description should not be there, right? Weston functionN invocation result_type operator()(arg1_type a1, arg2_type a2, ... , argN_type aN) const; Effects: f(a1, a2, ..., aN), where f is the target of *this. Returns: if R is void, nothing is returned; otherwise, the return value of the call to f is returned. Throws: bad_function_call if !this->empty(). Otherwise, may through any exception thrown by the target function f.