What kinds of code need to be aware of rvalue references?

Question

I see lots of people having problems understanding rvalue references. Will "ordinary" C++ code (e.g., that uses the standard library, but doesn't implement it) need to know about rvalue references once the new standard comes out, or can the people/code just pretend rvalue references don't exist? Do rvalue references primarily concern library implementors or all programmers?

Answer 1

I didn't see anything much to disagree with in GMan's (aka gmannickg) blog article on the subject:

http://blackninjagames.com/?p=95

Summary: anyone who knows about copy-and-swap (and in general the rule of three), should know about move semantics (and hence rvalue references), because the best-practice idioms change as a result. I think, therefore, that's anyone who cares at all about C++.

I don't think you have to know, because of course copy-and-swap still works, it's still exception-safe, and so on. And I suppose someone could argue whether "ordinary" C++ code and programmers even need to know copy-and-swap. I'd hope that the answer is "well, duh, obviously", but you never know.

Of course, the point at which one can start using C++0x will vary from place to place. It's probably not a good idea to unlearn C++03 any time soon.

For example, returning huge collections by value in contexts that aren't valid for copy elision becomes a better bet if you can rely on move assignment. This isn't a case of something that's invalid in C++03 becoming valid in C++0x, it's code that's heinously slow in C++03, that you'd work around (even if only with a well-placed swap), becoming fine in C++0x. So if you pretend rvalue references don't exist, you'll carry on working around something you don't need to. If you make the jump then it might backport cleanly to C++03 but run dog-slow, which is a pain.

So the smart thing might be to know about it, and in many contexts pretend it doesn't exist.