Re: perfomance of clear vs swap

"Aaron Graham" <>
29 Nov 2006 11:33:00 -0500

That's true for swap() alone, but what about the destructor of the
"temporary" variable (tmp in the OP's code) when it goes out of scoppe?
Doesn't it do the same things, including destruction and deallocation?

Hmmm... but when I try it that way, using clear() is actually quite a
bit faster than using temporary/swap:

#include <ext/hash_map>
__gnu_cxx::hash_map<int, int> m;
void func1() {
  __gnu_cxx::hash_map<int, int> t;
void func2() {
int main(int argc, char** argv) {
  for (int x = 0; x < 10000; x++) {
    for (int y = 0; y < 10000; y++) m[y] = y;
    if (argc == 1) func1();
    else func2();

g++ -O2
time ./a.out

20.413u 0.000s 0:20.40 100.0% 0+0k 0+0io 0pf+0w

time ./a.out 2

13.528u 0.016s 0:13.54 99.9% 0+0k 0+0io 0pf+0w

So I'm still not convinced that he's measuring what he thinks he's
measuring. I'm using gcc 4.1.1, by the way.


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