stl for_each makes two additional copies of functor

[Sean]

#include <iostream>
#include <algorithm>
struct functor {
~functor() {std::cout << 'D';}
void operator()(int a) {std::cout << a;}
};
int main()
{
int a[] = {1,2,3,4,5};
functor f;
std::for_each(a ,a+5,f);
}
// produces : 12345DDD

notice the three D's in the output.

The functor is copied once as pass-by-value to for_each()
and once again as the return value from for_each().

Is there a way to use for_each without introducing these
extra copies of the functor? Perhaps this is just a
g++ implementation issue?

Regards,
Sean

[Victor Bazarov]

Re: stl for_each makes two additional copies of functor

"Sean" <seanlkml@roger s.com> wrote...[color=blue]
> #include <iostream>
> #include <algorithm>
> struct functor {
> ~functor() {std::cout << 'D';}
> void operator()(int a) {std::cout << a;}
> };
> int main()
> {
> int a[] = {1,2,3,4,5};
> functor f;
> std::for_each(a ,a+5,f);
> }
> // produces : 12345DDD
>
> notice the three D's in the output.
>
> The functor is copied once as pass-by-value to for_each()
> and once again as the return value from for_each().
>
> Is there a way to use for_each without introducing these
> extra copies of the functor? Perhaps this is just a
> g++ implementation issue?[/color]


Try

int a[] = {1,2,3,4,5};
const functor &f = functor();
std::for_each(a ,a+5,f);

Victor

[Sean]

Re: stl for_each makes two additional copies of functor


"Howard Hinnant" <hinnant@metrow erks.com> wrote in message
news:3006200319 12177840%hinnan t@metrowerks.co m...
<snip>[color=blue]
>
> You could use explicit template arguments to specify pass-by-reference
> for for_each:
>
> #include <iostream>
> #include <algorithm>
>
> struct functor
> {
> ~functor() {std::cout << 'D';}
> void operator()(int a) const {std::cout << a;}
> };
>
> int main()
> {
> int a[] = {1,2,3,4,5};
> functor f;
> std::for_each<i nt*, functor&>(a,a+5 ,f);
> }
>
> 12345D
>
> Not all algorithms may behave as you want, even with this extra effort.
> For example see:
>
> http://anubis.dkuug.dk/jtc1/sc22/wg2...active.html#92
>
> However, if you happen to be a Metrowerks customer, and you're getting
> unwanted functor copying even when passing by reference via explicit
> template arguments, feel free to report it as a bug to me. We endeavor
> to make this idiom work.
>
> --
> Howard Hinnant
> Metrowerks[/color]

Thanks for this helpful information Howard. It seems to me that
pass-by-reference should be the default behavior for for_each. But after
considering the information at the link you provided i can understand
why it is not.

Cheers,
Sean