Why doesn't iter_swap use swap?

[chris]

Hello self :)

Just to pad out my earlier mail, it looks like intel's implementation 
does use swap to implement iter_swap, and stlport's implementation 
requires the two parameters to iter_swap must have the same type (unlike 
the standard).

There is also a defect report against iter_swap ( 
http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html , defect 
187 ), which discusses how to be honest it's a bit of a mess.

I see this has been previously posted about but wasn't discussed in 
depth. However for performing STL algorithms in constructs of any kind 
of complexity I imagine this is causing some serious dents in performance.

One reason discussed in the standard about why not to use swap() is that 
iter_swap may work in some places where swap() doesn't (vector<bool> is 
mentioned). I'm not an expert on this, but we seem to in general require 
"normal assignment" in for example __unguarded_linear_insert, and other 
places.

I'm happy to continue investigating this, but I suspect someone else 
might be able to advise quickly what is going on. If it turns out that 
we can't or won't change iter_swap in general, then a large number of 
overloads may have to be defined as at the moment many algorithms are 
not using swap and are therefore much more expensive than they should be.

Chris