Question

I’ve learned that Xor operation can be used to implement effective swap function. like this:

template<class T>
void swap(T& a, T& b)
{
    a = a^b;
    b = a^b;
    a = a^b;
}

But the implementation of swap all i can found on the internet is essentially like this:

template<class T>
void swap(T& a, T& b)
{
    T temp(a);
    a = b;
    b = temp;
}

It seems that the compiler didn’t generate the same code for the two form above because I tested it on VC++ 2010 and the first one is done the job more quickly than std::swap. Is there portable or any other problem with first one? Feel free to correct any of my mistake cause i’m not an English native and not good at C++.

(Editor’s note: likely that test was done with a non-optimized debug build, not a release build where std::swap could inline. Benchmarking debug builds is meaningless. Compilers generally don’t optimize away xor-swap into something more efficient.)

Answer 1

I’ve learned that Xor operation can be used to implement effective swap function

You learned wrong, I’m afraid. XOR swap is outdated: if it was ever reliably faster than using a temporary value then it shouldn’t be on modern compilers and processors (where by “modern” I mean roughly the last 20 years or more). You say it was faster for you, possibly you should show your benchmark code and see whether others get the same results.

Aside from the fact that your code only works at all on integer types, it has a fundamental bug. Try this with your version of swap:

int a = 1;
swap(a,a);
std::cout << a << '\n';