Why is execution-time method resolution faster than compile-time resolution?
- by Felix
At school, we about virtual functions in C++, and how they are resolved (or found, or matched, I don't know what the terminology is -- we're not studying in English) at execution time instead of compile time. The teacher also told us that compile-time resolution is much faster than execution-time (and it would make sense for it to be so). However, a quick experiment would suggest otherwise. I've built this small program:
#include <iostream>
#include <limits.h>
using namespace std;
class A {
public:
void f() {
// do nothing
}
};
class B: public A {
public:
void f() {
// do nothing
}
};
int main() {
unsigned int i;
A *a = new B;
for (i=0; i < UINT_MAX; i++) a->f();
return 0;
}
Where I made A::f() once normal, once virtual. Here are my results:
[felix@the-machine C]$ time ./normal
real 0m25.834s
user 0m25.742s
sys 0m0.000s
[felix@the-machine C]$ time ./virtual
real 0m24.630s
user 0m24.472s
sys 0m0.003s
[felix@the-machine C]$ time ./normal
real 0m25.860s
user 0m25.735s
sys 0m0.007s
[felix@the-machine C]$ time ./virtual
real 0m24.514s
user 0m24.475s
sys 0m0.000s
[felix@the-machine C]$ time ./normal
real 0m26.022s
user 0m25.795s
sys 0m0.013s
[felix@the-machine C]$ time ./virtual
real 0m24.503s
user 0m24.468s
sys 0m0.000s
There seems to be a steady ~1 second difference in favor of the virtual version. Why is this?
Relevant or not: dual-core pentium @ 2.80Ghz, no extra applications running between two tests. Archlinux with gcc 4.5.0. Compiling normally, like:
$ g++ test.cpp -o normal
Also, -Wall doesn't spit out any warnings, either.
