atoi()不使用std:::string::substr()

This is a snippet of my code:

这是我的代码片段:

#include <iostream>
#include <fstream>
#include <string>
#include <stdlib.h> // atoi()


int main() {

    std::string line;

    std::ifstream numbers("numbertest.txt");

    if (numbers.is_open()) {
        while (std::getline(numbers, line)) {
            for (int i = 0; i < line.length() - 4; i++) {
                for (int n = 0; n < 5; n++) {
                    std::cout << atoi((line.substr(i, 5)[n]).c_str());
                }
#i