Questions regarding performance of swap

Question

I have three questions regarding swapping, most likely pretty basic to you.

(1) With regards to two same type STL containers a and b , both operations below would work

swap(a,b);
a.swap(b);

I understand that the second is specialized for the container (eg, only involves a number of iterator swaps) while the first is a global algorithm meant to work with generic datatypes and performs an internal copy-construct.

My question is if I write the first, will the compiler use the second regardless, or do I have to be careful to check if an appropriate specialization exists?

(2) Would swap(a,b) result in the same performance with swap(b,a) ? If a copy-construct is involved and the objects are of considerably different size, I suspect it may matter?

(3) In cases where the overloaded operator == exists and is relatively fast, checking for a == b before swapping would make sense so as to avoid unnecessary operations. Does std::swap apply this check first, or does it perform the operation regardless?

Thanks for your time!

Answer 1

1. The global swap template is specialized for each of the standard library types that supports swap ; the specialization calls the member swap .

2. There should be no difference between swap(a, b) and swap(b, a) . (I can imagine perverse types where it would matter, but that would never occur in practice).

3. No, std::swap typically doesn't check for equality. In many cases that would be slower than just doing the swap.