Question

when I use strtok to tokenize a c++ string, it happens a confusing problem, see the simple code below:

void a(string s){
    strtok((char*)s.c_str(), " ");
}
int main(){
    string s;
    s = "world hello";
    a(s);
    cout<<s<<endl;
    return 0;
}

the program outputs “world”.
Shouldn’t it output “world hello”? Because I pass the string as a value parameter to function a, the strtok shouldn’t modify the original s…
Can anyone explain this trick.
thank you.

Answer 1

The problem is (char*)s.c_str(), you are casting the constness away and modified the string contents in a way that you are not supposed to. While the original s should not be modified, I pressume you may have been hit by a smart optimization that expects you to play by the rules. For instance, a COW implementation of string would happen to show that behavior.