What's the point of iter_swap?

Question

I was just wondering, why would anybody write this:

std::iter_swap(i, k); 

instead of this?

std::swap(*i, *k);   // saved a few keystrokes! 

Then I looked into the implementation of iter_swap, and of course it only uses swap instead of std::swap since we're already in namespace std, anyway. That leads me to the next question:

Why would anybody write this:

using std::swap; swap(a, b); 

instead of this?

std::iter_swap(&a, &b);   // saved an entire line of code! 

Are there any important differences/issues I am overlooking here?

Answer 1

From the SGI docs (here):

[1] Strictly speaking, iter_swap is redundant. It exists only for technical reasons: in some circumstances, some compilers have difficulty performing the type deduction required to interpret swap(*a, *b).

Answer 2

To answer your second question, the using + swap allows the compiler to use user-defined swap functions that may be more efficient than the default implementation (by using ADL). Explicitly saying std::swap inhibits ADL and any custom swap methods it maybe have been able to find.

As for iter_swap it's presumably there to use in templates and clearly indicate intention rather than a swap that might indicate you expect a pointer.