"David Abrahams" <dave@boost-consulting.com> writes:
I'm not sure what you mean, but AFAICT case #2 is handled just fine by conforming compilers. Did you read http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2004/n1610.html?
So did you?
Yes.
The comment was wrong; either behavior could be interpreted as correct if you stretch, but Comeau's is arguably too restrictive. Unfortunately there's no macro we can use to detect strict mode on EDG; it might mean that the implicit move optimization needs to be turned off for certain cases with those compilers.
Correct me if I am wrong, but if both behaviour could be interpreted as correct based on the current standard, it means that relying on one interpretation is undefine behaviour. At least untill a TC is issued. Besides it is not the only problem. I didn't just start to create my own version for pleasure. I started using your approach but the templated constructor had weird interactions with other 2 parameter constructors. I cannot remember the case off hand, but I will look for it and post it.
There is never a need to pass a temporary by const ref if you can move it.
What if the user of the library wants to pass the object to a function that has that signature? One of the main goal is for the move infrastructure to be as transparent as possible.
Yes.
template <class T> enable_if_same<T const, X const, void> f(T&); // lvalues
void f(move_from<X>); // temporaries
Right, didn't realize that you could use the same trick here, good stuff! Anyway, using enable_if_same as a result type convertible to the required type might cause problem with template type deduction if the result is passed to a templated function, while if it is used as a second paramater with default value (as in the constructor) it changes the arity of the function, which might cause problem with functional programming (this one is probably not a real problem). Anyway forcing an otherwise non-templated fiunction to be templated, while not a real problem is not really optimal, especially considering the difficulty most compilers have with the export keyword. Of corse, the templated class might just delegate all work to a non templated class, but still.
The code snipped you sent on the quoted message doesn't appear to allow it, but as I said, I am not sure about newer versions.
Why don't you do some research? It's all in the sandbox.
I did, and I proposed an alternative which has some advantages and some disadvantages. If you like i,t please use it, if you don't, don't! I was looking for feedback and constructive criticism, not forcing my approach to anyone.
I still don't understand why you think it's important to distinguish const temporaries from const lvalues.
As you said in another post, it is not a problem to move from a const temporary (the compiler might just copy the const temporary to a non const temporary and optimize the copy away), hece you want to move from const temporaries. On the other hand, you should certainly not move from lvalues. Best regards Andrea Torsello