Re: [boost] [foreach] copying rvalues

From: David Abrahams (dave_at_[hidden])
Date: 2007-07-05 20:58:58

on Thu Jul 05 2007, Lewis Hyatt <lhyatt-AT-princeton.edu> wrote:

> 	cerr << "TEST 2: RVALUE" << endl;
> 	{
> 		cerr << "\tstarting foreach" << endl;
> 		BOOST_FOREACH(int const& i, get_vec(5));
> 	}
...
> If I compile it with the RVO enabled, I get the following output:
> -------------
> TEST 1: LVALUE
>          container was constructed
>          starting foreach
>          container was destructed
>
> TEST2: LVALUE, REFERENCE
>          container was constructed
>          starting foreach
>          container was destructed
> 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. nt to 

I'm not so sure.  If you want to avoid seeing the copy that happens
when get_vec returns, just do

                BOOST_FOREACH(int const& i, noisy_vector(5,0));

That's as good an rvalue as any-----^^^^^^^^^^^^^^^^^
and might clarify things for you a bit.

> isn't it possible just to bind 
> the temporary to a const reference instead?

That's up to the implementation.  At least according to C++98, it's
allowed to make a copy when you do that :(

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com
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