From: Andrea Torsello (torsello_at_[hidden])
Date: 2004-04-16 14:32:44
"David Abrahams" <dave_at_[hidden]> 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?
> 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
based on the current standard, it means that relying on one interpretation
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
for pleasure. I started using your approach but the templated constructor
interactions with other 2 parameter constructors. I cannot remember the case
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.
> 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
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.
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