Re: Do compilers optimise away copies of vectors that are returned by functions?

From:

Mathias Gaunard <loufoque@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 21 Apr 2011 20:26:06 CST

Message-ID:

<7aa64ec5-917d-49e0-b656-351776e281e9@l14g2000pre.googlegroups.com>

On Apr 21, 10:45 pm, John Reid <johnbaronr...@googlemail.com> wrote:

Suppose I have the following struct:

struct A {
  std::vector< int > v;
  std::vector< int > fn() { return v; }

}

and the following code that uses it:

A a;
for( size_t i = 0; 100 != i; ++i ) {
  a.v.push_back(0);}

a.fn()[20]; //How efficient is this line?

Will modern compilers realise that I am not altering a.v in the given
use case and not copy the vector before accessing the 20th element?
Are they bound to make the copy by the standard?

That copy is not considered observable behaviour, so a compiler is
perfectly allowed to optimize it out.

Apologies if there is a better forum to ask this message on. In
particular I'm interested in the behaviour of recent gcc versions and
Microsoft's cl.

Why not try and see?

My guess is that they won't be able to.

--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]