On Sunday, August 26, 2012 14:47:38 Jens Mueller wrote: > What is the ref situation? > I thought ref is used as an optimization that allows passing lvalues > more efficiently whereas without ref is needed for rvalues.
ref doesn't necessarily have anything to do with optimizations. It makes it so that any operations done on the function parameter inside the function are also done on the variable passed in. The parameter is essentially an alias for the variable passed in. You can choose to use ref in order to avoid a copy, but it's also arguably bad practice to have a function take an argument by ref if it's not actually going to mutate the original, since that's confusing. _const_ ref is more for avoiding the copy, since it's like ref except that you can't actually mutate the variable. So, it makes sense to use const ref specifically to avoid unnecessary copies. However, while in C++, you can pass rvalues to const&, in D, you can't. It doesn't matter if the ref is const or not, it still requires an lvalue. This solves certain issues caused by C++'s choice (issues primarily related to function overloading IIRC), but it makes it rather difficult to declare a single function which takes either an lvalue or an rvalue but which tries to avoid unnecessary copying. D's auto ref essentially does it (rvalues are then always passed by rvalue and lvalues are always passed by lvalue), but it only works with templates, because it has to create two versions of the function. So, what we'd _like_ to have is a solution where you can mark a parameter in a way similar to auto ref or C++s const& and have the function not care whether it's passed an lvalue or rvalue and have it avoid copying as much as possible and have it work with _all_ function types, not just templated ones, but we haven't sorted out how to do that yet. _That's_ the ref situation that needs to be solved. - Jonathan M Davis
