Re: [boost] [foreach] copying rvalues

From: Eric Niebler (eric_at_[hidden])
Date: 2007-07-05 22:00:10

Lewis Hyatt wrote:
> I'm not quite sure I understand the behavior of this simple test of 
> BOOST_FOREACH:

<snip>

> TEST 2: RVALUE
>          starting foreach
>          container was constructed
>          container was copied
>          container was destructed
>          container was destructed
> -------------
> 
> So in either case, using BOOST_FOREACH requires one extra copy. Why is 
> this necessary? I guess the macro is making a copy of the argument to 
> avoid evaluating it more than once, but isn't it possible just to bind 
> the temporary to a const reference instead?

Let me be more precise: using BOOST_FOREACH *with rvalue containers* 
requires one "extra" copy. When the container is an lvalue, no copies 
are made, ever. You suggested binding the rvalue to a const reference. 
That would work, if you knew both that the object was an rvalue *and* 
what the type of the object was. But you don't. Instead, you have to put 
the object in a variant of sorts, and store a const reference to that. 
That's where the extra copy comes from.

For all the details, see: http://www.artima.com/cppsource/foreach.html

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com