6.3 Command-line Arguments

A program can receive arguments from the command line. argc is the argument count, and argv stores those arguments as strings.

#include <iostream>

int main(int argc, char* argv[]) {
    if (argc != 2) {
        std::cerr << "Usage: " << argv[0] << " file.txt" << std::endl;
        return 1;
    }

    std::cout << "processing " << argv[1] << std::endl;
    return 0;
}

argv[0] is usually the program name, argv[1] is the first user argument, and so on. Every argument is a string, so convert them with std::atoi() from <cstdlib> or std::stoi() from <string> when you need numbers.

int times = std::atoi(argv[1]);   // "3" -> 3

Write error messages to stderr and normal results to stdout. In C++, std::cerr writes to stderr, and std::cout writes to stdout. That way users can redirect normal output to a file while still seeing errors in the terminal.

./greet Ada > out.txt