Optimising assignment using std::swap

From: Paul Moore (gustav_at_[hidden])
Date: 2001-01-21 17:33:27

I have a small optimisation issue - in the rational class, the gcd 
operation (one of the key hotspots) repeatedly executes

    IntType r(n%m);
    n = m;
    m = r;

This could be replaced with

    IntType r(n%m);
    std::swap(n,m)
    std::swap(m,r);

For built-in types, this is notably less efficient, as it will do 6 
assignments as opposed to 2. However, for a user-defined type 
which may have a highly optimised swap, it could be far more 
efficient.

Is there any way of using the most efficient version in each case - 
or alternatively would it make sense to use the swap version in all 
cases? (I feel that this is a bad case of premature optimisation, as 
we don't yet have a common candidate for an unlimited-precision 
integer type to use with rational<>)

[[ As usual, this could be handled by explicit specialisation - but I 
don't feel that is appropriate here. ]]

This came to me when I was writing a section for the 
documentation covering performance issues. If I don't use swap(), I 
feel that it might be worth pointing this out in this section.

BTW, I suspect this means that rational<> should provide a swap() 
specialisation, to delegate to (potential) swap() specialisations for 
IntType. Can anyone remind me what the canonical form of swap() 
is?

Paul