mastermann/argparse

Argument Parser for Modern C++

Highlights

• Single header file
• Requires C++17
• MIT License

Quick Start

Simply include argparse.hpp and you're good to go.

#include <argparse.hpp>

To start parsing command-line arguments, create an ArgumentParser.

argparse::ArgumentParser program("program name");

To add a new argument, simply call .add_argument(...). You can provide a variadic list of argument names that you want to group together, e.g., -v and --verbose

program.add_argument("foo");
program.add_argument("-v", "--verbose"); // parameter packing

Argparse supports a variety of argument types including positional, optional, and compound arguments. Below you can see how to configure each of these types:

Positional Arguments

Here's an example of a positional argument:

#include <argparse.hpp>

int main(int argc, char *argv[]) {
  argparse::ArgumentParser program("program name");

  program.add_argument("square")
    .help("display the square of a given integer")
    .action([](const std::string& value) { return std::stoi(value); });

  try {
    program.parse_args(argc, argv);
  }
  catch (const std::runtime_error& err) {
    std::cout << err.what() << std::endl;
    program.print_help();
    exit(0);
  }
  
  auto input = program.get<int>("square");
  std::cout << (input * input) << std::endl;

  return 0;
}

And running the code:

$ ./main 15
225

Here's what's happening:

• The add_argument() method is used to specify which command-line options the program is willing to accept. In this case, I’ve named it square so that it’s in line with its function.
• Command-line arguments are strings. Inorder to square the argument and print the result, we need to convert this argument to a number. In order to do this, we use the .action method and provide a lambda function that tries to convert user input into an integer.
• We can get the value stored by the parser for a given argument using parser.get<T>(key) method.

Optional Arguments

Now, let's look at optional arguments. Optional arguments start with - or --, e.g., --verbose or -a. Optional arguments can be placed anywhere in the input sequence.

argparse::ArgumentParser program("test");

program.add_argument("--verbose")
  .help("increase output verbosity")
  .default_value(false)
  .implicit_value(true);

try {
  program.parse_args(argc, argv);
}
catch (const std::runtime_error& err) {
  std::cout << err.what() << std::endl;
  program.print_help();
  exit(0);
}

if (program["--verbose"] == true) {
    std::cout << "Verbosity enabled" << std::endl;
}

$ ./main --verbose
Verbosity enabled

Here's what's happening:

• The program is written so as to display something when --verbose is specified and display nothing when not.
• Since the argument is actually optional, no error is thrown when running the program without --verbose. Note that by using .default_value(false), if the optional argument isn’t used, it's value is automatically set to false.
• By using .implicit_value(true), the user specifies that this option is more of a flag than something that requires a value. When the user provides the --verbose option, it's value is set to true.

Requiring optional arguments

There are scenarios where you would like to make an optional argument required. As discussed above, optional arguments either begin with - or --. You can make these types of arguments required like so:

	program.add_argument("-o", "--output")
		.required()
		.help("specify the output file.");

If the user does not provide a value for this parameter, an exception is thrown.

Alternatively, you could provide a default value like so:

	program.add_argument("-o", "--output")
		.default_value(std::string("-"))
                .required()
		.help("specify the output file.");

Negative Numbers

Optional arguments start with -. Can argparse handle negative numbers? The answer is yes!

argparse::ArgumentParser program;

program.add_argument("integer")
  .help("Input number")
  .action([](const std::string& value) { return std::stoi(value); });
  
program.add_argument("floats")
  .help("Vector of floats")
  .nargs(4)
  .action([](const std::string& value) { return std::stof(value); });

try {
  program.parse_args(argc, argv);
}
catch (const std::runtime_error& err) {
  std::cout << err.what() << std::endl;
  program.print_help();
  exit(0);
}

// Some code to print arguments

$ ./main -5 -1.1 -3.1415 -3.1e2 -4.51329E3
integer : -5
floats  : -1.1 -3.1415 -310 -4513.29

As you can see here, argparse supports negative integers, negative floats and scientific notation.

Combining Positional and Optional Arguments

argparse::ArgumentParser program("test");

program.add_argument("square")
  .help("display the square of a given number")
  .action([](const std::string& value) { return std::stoi(value); });

program.add_argument("--verbose")
  .default_value(false)
  .implicit_value(true);

try {
  program.parse_args(argc, argv);
}
catch (const std::runtime_error& err) {
  std::cout << err.what() << std::endl;
  program.print_help();
  exit(0);
}

int input = program.get<int>("square");

if (program["--verbose"] == true) {
  std::cout << "The square of " << input << " is " << (input * input) << std::endl;
}
else {
  std::cout << (input * input) << std::endl;
}

$ ./main 4
16

$ ./main 4 --verbose
The square of 4 is 16

$ ./main --verbose 4
The square of 4 is 16

Printing Help

ArgumentParser.print_help() print a help message, including the program usage and information about the arguments registered with the ArgumentParser. For the previous example, here's the default help message:

$ ./main --help
Usage: ./main [options] square

Positional arguments:
square         display a square of a given number

Optional arguments:
-h, --help     show this help message and exit
-v, --verbose  enable verbose logging

List of Arguments

ArgumentParser objects usually associate a single command-line argument with a single action to be taken. The .nargs associates a different number of command-line arguments with a single action. When using nargs(N), N arguments from the command line will be gathered together into a list.

argparse::ArgumentParser program("main");

program.add_argument("--input_files")
  .help("The list of input files")
  .nargs(2);

try {
  program.parse_args(argc, argv);   // Example: ./main --input_files config.yml System.xml
}
catch (const std::runtime_error& err) {
  std::cout << err.what() << std::endl;
  program.print_help();
  exit(0);
}

auto files = program.get<std::vector<std::string>>("--input_files");  // {"config.yml", "System.xml"}

ArgumentParser.get<T>() has specializations for std::vector and std::list. So, the following variant, .get<std::list>, will also work.

auto files = program.get<std::list<std::string>>("--input_files");  // {"config.yml", "System.xml"}

Using .action, one can quickly build a list of desired value types from command line arguments. Here's an example:

argparse::ArgumentParser program("main");

program.add_argument("--query_point")
  .help("3D query point")
  .nargs(3)
  .default_value(std::vector<double>{0.0, 0.0, 0.0})
  .action([](const std::string& value) { return std::stod(value); });

try {
  program.parse_args(argc, argv); // Example: ./main --query_point 3.5 4.7 9.2
}
catch (const std::runtime_error& err) {
  std::cout << err.what() << std::endl;
  program.print_help();
  exit(0);
}

auto query_point = program.get<std::vector<double>>("--query_point");  // {3.5, 4.7, 9.2}

Compound Arguments

Compound arguments are optional arguments that are combined and provided as a single argument. Example: ps -aux

argparse::ArgumentParser program("test");

program.add_argument("-a")
  .default_value(false)
  .implicit_value(true);

program.add_argument("-b")
  .default_value(false)
  .implicit_value(true);

program.add_argument("-c")
  .nargs(2)
  .default_value(std::vector<float>{0.0f, 0.0f})
  .action([](const std::string& value) { return std::stof(value); });

try {
  program.parse_args(argc, argv);                  // Example: ./main -abc 1.95 2.47
}
catch (const std::runtime_error& err) {
  std::cout << err.what() << std::endl;
  program.print_help();
  exit(0);
}

auto a = program.get<bool>("-a");                  // true
auto b = program.get<bool>("-b");                  // true
auto c = program.get<std::vector<float>>("-c");    // {1.95, 2.47}

/// Some code that prints parsed arguments

$ ./main -ac 3.14 2.718
a = true
b = false
c = {3.14, 2.718}

$ ./main -cb
a = false
b = true
c = {0.0, 0.0}

Here's what's happening:

• We have three optional arguments -a, -b and -c.
• -a and -b are toggle arguments.
• -c requires 2 floating point numbers from the command-line.
• argparse can handle compound arguments, e.g., -abc or -bac or -cab. This only works with short single-character argument names.
  • -a and -b become true.
  • argv is further parsed to identify the inputs mapped to -c.
  • If argparse cannot find any arguments to map to c, then c defaults to {0.0, 0.0} as defined by .default_value

Parent Parsers

Sometimes, several parsers share a common set of arguments. Rather than repeating the definitions of these arguments, a single parser with all the shared arguments can be added as a parent to another ArgumentParser instance. The .add_parents method takes a list of ArgumentParser objects, collects all the positional and optional actions from them, and adds these actions to the ArgumentParser object being constructed:

argparse::ArgumentParser parent_parser("main");
parent_parser.add_argument("--parent")
  .default_value(0)
  .action([](const std::string& value) { return std::stoi(value); });

argparse::ArgumentParser foo_parser("foo");
foo_parser.add_argument("foo");
foo_parser.add_parents(parent_parser);
foo_parser.parse_args({ "./main", "--parent", "2", "XXX" });   // parent = 2, foo = XXX

argparse::ArgumentParser bar_parser("bar");
bar_parser.add_argument("--bar");
bar_parser.parse_args({ "./main", "--bar", "YYY" });           // bar = YYY

Note You must fully initialize the parsers before passing them via .add_parents. If you change the parent parsers after the child parser, those changes will not be reflected in the child.

Further Examples

Construct a JSON object from a filename argument

argparse::ArgumentParser program("json_test");

program.add_argument("config")
  .action([](const std::string& value) {
    // read a JSON file
    std::ifstream stream(value);
    nlohmann::json config_json;
    stream >> config_json;
    return config_json;
  });

try {
  program.parse_args({"./test", "config.json"});
}
catch (const std::runtime_error& err) {
  std::cout << err.what() << std::endl;
  program.print_help();
  exit(0);
}

nlohmann::json config = program.get<nlohmann::json>("config");

Positional Arguments with Compound Toggle Arguments

argparse::ArgumentParser program("test");

program.add_argument("numbers")
  .nargs(3)
  .action([](const std::string& value) { return std::stoi(value); });

program.add_argument("-a")
  .default_value(false)
  .implicit_value(true);

program.add_argument("-b")
  .default_value(false)
  .implicit_value(true);

program.add_argument("-c")
  .nargs(2)
  .action([](const std::string& value) { return std::stof(value); });

program.add_argument("--files")
  .nargs(3);

try {
  program.parse_args(argc, argv);
}
catch (const std::runtime_error& err) {
  std::cout << err.what() << std::endl;
  program.print_help();
  exit(0);
}

auto numbers = program.get<std::vector<int>>("numbers");        // {1, 2, 3}
auto a = program.get<bool>("-a");                               // true
auto b = program.get<bool>("-b");                               // true
auto c = program.get<std::vector<float>>("-c");                 // {3.14f, 2.718f}
auto files = program.get<std::vector<std::string>>("--files");  // {"a.txt", "b.txt", "c.txt"}

/// Some code that prints parsed arguments

$ ./main 1 2 3 -abc 3.14 2.718 --files a.txt b.txt c.txt
numbers = {1, 2, 3}
a = true
b = true
c = {3.14, 2.718}
files = {"a.txt", "b.txt", "c.txt"}

Restricting the set of values for an argument

argparse::ArgumentParser program("test");

program.add_argument("input")
  .default_value("baz")
  .action([](const std::string& value) {
    static const std::vector<std::string> choices = { "foo", "bar", "baz" };
    if (std::find(choices.begin(), choices.end(), value) != choices.end()) {
      return value;
    }
    return std::string{ "baz" };
  });

try {
  program.parse_args(argc, argv);
}
catch (const std::runtime_error& err) {
  std::cout << err.what() << std::endl;
  program.print_help();
  exit(0);
}

auto input = program.get("input");
std::cout << input << std::endl;

$ ./main fex
baz

Supported Compilers

• GCC >= 7.0.0
• Clang >= 4.0
• MSVC >= 2017

Contributing

Contributions are welcome, have a look at the CONTRIBUTING.md document for more information.

License

The project is available under the MIT license.