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From: Peter Dimov (pdimov_at_[hidden])
Date: 2008-04-05 19:15:16

Eric Niebler:

> Now take a look at what happens when I do this:
> BigObject const obj;
> compose< identity, make_tuple >()( obj );
> (And assume that make_tuple is implemented as you implemented it above.)
> Now it looks to me like the wrong thing is going to happen here. In the
> return type calculation, make_tuple is being told it is going to get an
> lvalue (BigObject const &). But the rvalue overload of
> make_tuple::operator() is going to be selected, causing BigObject to be
> copied when it shouldn't be -- if it even compiles at all.

Doesn't make_tuple()(obj) have the exact same problem?
compose<identity,make_tuple> faithfully reproduces it.

> So where have I screwed up my logic?

Nowhere. But I don't think that this problem is somehow caused by result_of.
If you replace every result_of occurence with <magic happens here> the
result would be the same.

Can you give an example in which compose<identity,make_tuple>, implemented
within your new framework, is able to avoid the problem?

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