I've only been using C++ for a few months, but it's my opinion that all function parameters should have been "const &" by default (except for maybe primitive types). Const correctness has helped me spot some nasty bugs, and it's too easy to accidentally forget the "&" and pass horrendously large structures by value, which must go through a copy constructor and destructor with every call. Plus, it gets really tedious typing "const Mat &src" a million times (guess which library I'm using, lol).