Intermediate⏱️ 7 min📘 Topic 12 of 32

⌨️ Command Line Arguments in C++ — argc, argv and Parsing User Input

Read command line arguments in C++ with argc and argv. Learn how to parse flags, convert strings to numbers and build CLI tools — with practical examples.

Every C++ program can accept inputs from the command line. The trick: declare main with two parameters.

📜 The signature

int main(int argc, char* argv[]) {
  // argc = ARGument Count (number of args)
  // argv = ARGument Vector (array of C-strings)
}

🔑 Key facts

argv[0] = the program's own name (or path)
argv[1] = first actual argument
argv[argc] = nullptr (sentinel)
All arguments come in as C-strings — convert if you need numbers

🔢 String → number

int n = std::stoi(argv[1]);   // throws on invalid input
double d = std::stod(argv[2]);

💡 Modern style: collect args into a vector

#include <vector>
#include <string>
std::vector<std::string> args(argv + 1, argv + argc);

💻 Code Examples

Print all arguments

int main(int argc, char* argv[]) {
  std::cout << "Got " << argc << " args\n";
  for (int i = 0; i < argc; i++) {
    std::cout << i << ": " << argv[i] << '\n';
  }
}

Output:

$ ./app hello world
Got 3 args
0: ./app
1: hello
2: world

Simple calculator from CLI

int main(int argc, char* argv[]) {
  if (argc != 3) {
    std::cerr << "Usage: " << argv[0] << " <a> <b>\n";
    return 1;
  }
  int a = std::stoi(argv[1]);
  int b = std::stoi(argv[2]);
  std::cout << a + b;
}

Output:

$ ./sum 5 7
12

⚠️ Common Mistakes

Forgetting that `argv[0]` is the program name, not the first user arg.
Not validating `argc` before accessing `argv[1]` — crash if missing.
Using `atoi` instead of `std::stoi` — atoi silently returns 0 on bad input, hiding bugs.
Assuming args are typed — they're always strings; convert explicitly.

🎯 Interview Questions

Real questions asked at top product and service-based companies.

Q1.What do argc and argv mean in main()?Beginner

argc = Argument Count, the number of command-line arguments including the program name. argv = Argument Vector, an array of C-strings holding each argument. argv[0] is the program name; argv[1..argc-1] are user-provided args.

Q2.How do you convert a command-line argument to an integer?Beginner

Use std::stoi(argv[i]) — throws std::invalid_argument or std::out_of_range on bad input. The older atoi(argv[i]) silently returns 0 on failure, which is unsafe.

Q3.What's argv[argc]?Intermediate

It's always nullptr — a sentinel value marking the end of the argument list. Useful for loops that iterate while *p != nullptr.

Q4.How would you implement a flag parser like -v or --verbose?Intermediate

Loop through argv, compare each string with std::string(argv[i]) == "-v" || std::string(argv[i]) == "--verbose". For complex CLIs, use a library like CLI11 or boost::program_options.

🧠 Quick Summary

main(int argc, char* argv[]) receives CLI input.
argv[0] = program name; argv[1..] = user arguments.
All arguments are C-strings — convert with std::stoi/stod.
Always validate argc before accessing argv[i].
For complex CLIs, use a parsing library.