[boost] Re: Collection concept

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2000-01-13 09:24:21

Darin Adler wrote on 1/13/2000 2:52 AM
>> with a statement like:
>> 
>>     swap(*i, *j);
>
>Wasn't this the motivation for iter_swap? But for some reason it was
>insufficient?

Probably (I wasn't there when iter_swap was introduced).  But if 
iter_swap is implemented like:

template <class ForwardIterator1, class ForwardIterator2>
void
iter_swap(ForwardIterator1 a, ForwardIterator2 b)
{
        typedef typename iterator_traits<ForwardIterator1>::value_type 
value_type;
        value_type tmp(*a);
        *a = *b;
        *b = tmp;
}

the performance is poor for heavy value_types that have otherwise 
specialized swap.  But it does (usually) work with proxied references.

A higher performance iter_swap is to just call swap.  But that breaks 
proxied references unless the proxy specializes swap.

One could retain the lower performance iter_swap, but recommend that 
iterators of heavy value_types overload iter_swap.  But it seems easier 
to just overload swap in the proxy when required.  Still, you could 
overload the higher performance iter_swap as another way to get around a 
broken proxy.

-Howard