試してみた

#include <string>
#include <windows.h>
using namespace std;

int main(){
strings;
string::iterator ite;
const int count = 1000000;
const int size = 1000;
DWORD start;
int found = 0;

for(int i = 0; i < size; ++i){
s += "a";
}

// iterator書き込み
start = GetTickCount();
for(int i = 0; i < count; ++i){
for(ite = s.begin(); ite != s.end(); ++ite){
*ite = 0;
}
}
printf("iterator書き込み msec=%d\n", GetTickCount() - start);