Command-line arguments

A tool that others can use takes its input from the command line, not from paths written into the code. For example: ./ledger transactions.csv --month 2026-03. main(int argc, char* argv[]) receives these arguments.

argc is how many there are, and argv[0] holds the program name, so check argc before reading argv[1]. Convert them to std::string or a std::vector<std::string> straight away, and do all the parsing of options in one function. When arguments are missing or invalid, print a short usage message and return a non-zero exit code, such as 1.

Scripts and CI (automatic build systems) rely on exit codes to know that something failed. Libraries such as CLI11 help with many options, but for two or three arguments plain argv is enough and teaches you more.