Re: [Boost-users] Cpp-Next - slightly OT

Subject: Re: [Boost-users] Cpp-Next - slightly OT
From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2009-08-21 02:55:33

Robert Jones wrote:
> There's a terrific article about RVO and copy elision by Dave A at
>
> http://cpp-next.com/
>
> If I understand this correctly a decent compiler (say VS2008) will
> compile this
> sequence
>
> using namespace std;
> typedef vector< string > V;
> V get_names( );
>
> V sorted(V names)
> {
>     sort(names);
>     V ret;
>     swap(ret, names);
>     return ret;
> }
>
> V v = sorted( get_names( ) );
>
> to generate zero copies - the final value of v will be the actual
> object generated by get_names().
No, (without inlining and a compiler that really understands the
semantics of swapping vectors) the final value will be the object
constructed at the line "V ret;". We have the following object identities:
V1: The return value of "get_names" and the parameter "names".
V2: The variable "ret", the return value of "sorted", and the variable "v".
Data is moved from V1 to V2 by the swap, so you have no expensive
copies, but there are still two objects.

Sebastian