Hey Codeforces!

I've been wondering this for a long time, but could never find a straight answer.

According to String substr on cplusplus.com, the complexity of substring is mostly linear but is unspecified. I thought this was interesting, but never thought much of it. After all, a string is just an array of characters?

I thought this at first until I found another post that claimed that C++ string was implemented with the Rope data structure.

So i tried the following code (on my Linux system).

#include <bits/stdc++.h>
using namespace std;


int main() {
	int t; cin >> t;
	string s=string(int(2e5), 'a');

	while(t--) {
		string v = s.substr(0);
	}
}

Surprisingly, this code took less than a second after entering t as 200000.

Now, I tried a true N^2 approach to test my code against the one above.

#include <bits/stdc++.h>
using namespace std;


int main() {
	int t; cin >> t;
	

	while(t--) {
		for(int i = 0; i <= 1e5; i++) {
			continue;
		}
	}
}

After entering t as 200000, the program takes 15 seconds (without compiler optimizations).

So, which one is correct — Rope or Char Array?

Thanks in advance.