Using Command-Line Arguments in C++

In C++, programs can receive input directly from the command line. This is useful for utilities, file processors, scripts, automation, and more.

[!NOTE] You can reference the following videos:

• C++ Handling Program arguments (argc, argv, env)

1. The main Function Signature

To access command-line arguments, define main with parameters:

int main(int argc, char* argv[])

Meaning:

• argc — Argument Count. The number of command-line arguments, including the program name.
• argv — Argument Vector. An array of C-strings (char*) holding each argument.

Example command:

./myprogram file.txt 10

Values:

• argc == 3
• argv[0] → "./myprogram"
• argv[1] → "file.txt"
• argv[2] → "10"

2. Simple Example: Print All Arguments

#include <iostream>

int main(int argc, char* argv[]) {
    std::cout << "Argument count: " << argc << "\n";

    for (int i = 0; i < argc; ++i) {
        std::cout << "argv[" << i << "] = " << argv[i] << "\n";
    }

    return 0;
}

Compile and run:

g++ main.cpp -o demo
./demo hello world

Output:

Argument count: 3
argv[0] = ./demo
argv[1] = hello
argv[2] = world

3. Converting Arguments to Numbers

Arguments arrive as strings. To convert:

#include <iostream>
#include <cstdlib>   // for std::stoi, std::stof

int main(int argc, char* argv[]) {
    if (argc < 3) {
        std::cerr << "Usage: calc <a> <b>\n";
        return 1;
    }

    int a = std::stoi(argv[1]);
    int b = std::stoi(argv[2]);

    std::cout << "Sum = " << a + b << "\n";
}

Run:

./calc 5 7

4. Checking for Missing Arguments

Always protect your program:

if (argc < 2) {
    std::cerr << "Error: No filename provided.\n";
    return 1;
}

5. Optional Arguments and Flags

A common pattern:

if (std::string(argv[i]) == "--verbose") {
    verbose = true;
}

Example:

./tool input.txt --verbose --mode=fast