Before I discovered optional, I used a plain reference, but that was annoying because I had to create a dummy vector to be used as the default argument. (The other alternative would have been to use a pointer, but then the caller has to use the uglier syntax of passing in "&special_cases" rather than "special_cases").
Personally I'd simplify one line like this: //void some_operation(inputs, optional<vector<case>&> special_cases = none) { void some_operation(inputs, vector<case>* special_cases = 0) { for (...) { ... if (special_case) { ... if (special_cases) special_cases->push_back(current_case); } } } That's what the language construct pointer is for. Basic C++: Use a ref when you it can't be null; use a pointer when it can: http://www.parashift.com/c++-faq-lite/references.html#faq-8.6 Sorry to pick on you Nate, but your example is just what I'd like to avoid. People redefining concepts that the language already has. Any C++ programmer reading your code should understand what T* means. He may not be familiar with boost::optional<T&>. Chris