Hello all, I've been considering adding a .except(...) construct to Boost.Contract that will allow to specify conditions to check at function exit but when the function body throws (in contrast to postconditions that are checked at function exit but only when the function body does not throw). I think .except(...) could be used to assert strong exception safety guarantees... (note that class invariants are already checked at exit of public functions and also when the public function bodies throw, but invariants are suited to assert only basic, and not strong, exception safety guarantees). That might help addressing a point made in P0147R0 under the "Contracts and Exceptions" section: ``Some operations make stronger exception safety claims. In particular, they claim that an exception thrown from a member function will leave the state of an object unchanged from its state on function entry. If we invalidate postconditions on exceptions, we appear to have no mechanism to describe strong exception safety.'' For example, the following swap ensures to leave its arguments unchanged when it throws: void swap(std::string& a, std::string& b) { boost::contract::old_ptr<std::string> old_a = BOOST_CONTRACT_OLDOF(a); boost::contract::old_ptr<std::string> old_b = BOOST_CONTRACT_OLDOF(b); boost::contract::guard c = boost::contract::function() .postcondition([&] { // Checked at function exit but only if body does not throw. BOOST_CONTRACT_ASSERT(a == *old_b); BOOST_CONTRACT_ASSERT(b == *old_a); }) .except([&] { // Checked if body throws (strong exception safety guarantees). BOOST_CONTRACT_ASSERT(a == *old_a); BOOST_CONTRACT_ASSERT(b == *old_b); }) ; ... // Body implementation (which might throw). } Would this be beneficial? Thanks, --Lorenzo