Prepend line nums - c++

main.cpp

#include "stdafx.h"
#include <fstream>
#include <string>
using namespace std;

int main() {
	std::string srcline, nt;
	auto li = 0;

	std::ifstream tf("pocetni.txt");
while (std::getline(tf, srcline)) {		nt += "[" + std::to_string(li++) + "] "+ srcline + "\n";
	}
	tf.close();

	std::ofstream ntf("zavrsni.txt");
	ntf << nt;
	ntf.close();

	return 0;
}