CLI11

Language: CPP

Utilities

CLI11 was created by Henry Schreiner to modernize C++ command line parsing. It was designed to be lightweight, flexible, and integrate naturally with C++11 and newer standards. Today, it is widely used in scientific software, tools, and utilities where robust command-line interfaces are needed.

CLI11 is a C++11 header-only library for parsing command line arguments. It provides an expressive, modern C++ API for handling flags, options, subcommands, and configuration files with minimal boilerplate.

Installation

cmake: find_package(CLI11 CONFIG REQUIRED)

vcpkg: vcpkg install cli11

conan: conan install cli11

Usage

CLI11 makes it simple to define options, flags, and subcommands while handling validation, default values, and automatic help text generation.

Basic flag and option

#include <CLI/CLI.hpp>
#include <iostream>

int main(int argc, char** argv) {
    CLI::App app{"My Program"};

    int count;
    app.add_option("-c,--count", count, "Number of iterations");

    bool verbose = false;
    app.add_flag("-v,--verbose", verbose, "Enable verbose output");

    CLI11_PARSE(app, argc, argv);

    std::cout << "Count: " << count << " Verbose: " << verbose << std::endl;
    return 0;
}

Parses a numeric option `--count` and a boolean flag `--verbose`.

Subcommands

CLI::App app{"Main app"};
auto* sub = app.add_subcommand("convert", "Convert files");
std::string input;
sub->add_option("input", input, "Input file")->required();

CLI11_PARSE(app, argc, argv);

if (*sub) {
    std::cout << "Converting file: " << input << std::endl;
}

Defines a `convert` subcommand with required arguments.

Default values and validation

int threads = 4;
app.add_option("-t,--threads", threads, "Number of threads")->check(CLI::PositiveNumber);

Provides a default value and validates that threads must be positive.

Configuration file

std::string config;
app.set_config("config.ini", true);
app.add_option("--config", config, "Config file");

Supports configuration files for loading command line arguments.

Enums

enum class Mode { fast, slow };
Mode mode;
app.add_option("--mode", mode)
    ->transform(CLI::CheckedTransformer({{"fast", Mode::fast}, {"slow", Mode::slow}}));

Allows parsing enums with string-to-enum transformation.

Error Handling

Unrecognized flag or option: CLI11 automatically provides helpful error messages and usage text.

Required option missing: Mark options as `required()` only when absolutely necessary.

Invalid type conversion: Use `transform` or `check` validators to prevent invalid input.

Best Practices

Use descriptive names for flags and options to improve usability.

Leverage validation checks to enforce correct argument values.

Group related commands into subcommands for structured CLI design.

Provide default values where applicable for a smoother user experience.

Use `app.set_config()` to combine command-line and config file parsing.

Official Docs Github