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?

Saving code may be worthwhile (reduced complexity is easier to reason about)... but saving *lines* of code? Utterly useless, often counter-productive. — Ben Voigt, Dec 21 '12 at 14:42
No no no, you need to do `using std::iter_swap; iter_swap(i, k);` because `iter_swap` might have been specialized. (Just kidding. I think.) — Potatoswatter, Dec 21 '12 at 14:52
You have looked into *one* implementation of `iter_swap`, but why do you assume it cannot be implemented with `std::swap`? — R. Martinho Fernandes, Dec 21 '12 at 14:54
@R.MartinhoFernandes It's specified to use ADL properly. (17.6.3.2) — Potatoswatter, Dec 21 '12 at 15:01
Similar Q, more in-depth answers: http://stackoverflow.com/q/14024228/153285 — Potatoswatter, Jan 24 '13 at 11:42

score 41 · Accepted Answer · answered Dec 21 '12 at 14:28

41

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).

answered Dec 21 '12 at 14:28

sehe

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Mark B · Answer 2 · 2012-12-21T14:42:40.747

12

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.

edited Dec 21 '12 at 14:42

answered Dec 21 '12 at 14:37

Mark B

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Oh I neglected this because I'm absolutely convinced Fred knows about ADL and std::swap idioms :) – sehe Dec 21 '12 at 14:45
There is no `std::swap` invocation in my second question. My point was that `std::iter_swap` will also pick up user-defined swaps, without having to write a `using` statement. Right? – fredoverflow Dec 21 '12 at 14:52
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@FredOverflow: But not user-defined `iter_swap`s ;) – MSalters Dec 21 '12 at 16:17

What's the point of iter_swap?

2 Answers2

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