Den 14-02-2012 10:43, Hite, Christopher skrev:
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); } } }
I have a similar use-case, where an optional<std::exception&> is used as an argument, and its important to preserve reference semantics.
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&>.
The main benefit is that optional<T&> is /way/ more clear than T*; people use T* even when it can't be null. In such a code base, T* is ambiguous to read, whereas optional<T&> is not. And no, we can't just clean up hundreds of k lines of source code to make the interpretation unambiguous. Anyway, it should be trivial to make a new optional<T&> as efficient as T*. -Thorsten