p-ranav / argparse

Argument Parser for Modern C++

Highlights

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

Table of Contents

• Quick Start
  • Positional Arguments
  • Optional Arguments
    • Requiring optional arguments
    • Accessing optional arguments without default values
    • Deciding if the value was given by the user
    • Joining values of repeated optional arguments
    • Repeating an argument to increase a value
    • Mutually Exclusive Group
  • Storing values into variables
  • Negative Numbers
  • Combining Positional and Optional Arguments
  • Printing Help
  • Adding a description and an epilog to help
  • List of Arguments
  • Compound Arguments
  • Converting to Numeric Types
  • Default Arguments
  • Gathering Remaining Arguments
  • Parent Parsers
  • Subcommands
  • Parse Known Args
  • Hidden argument and alias
  • ArgumentParser in bool Context
  • Custom Prefix Characters
  • Custom Assignment Characters
• Further Examples
  • Construct a JSON object from a filename argument
  • Positional Arguments with Compound Toggle Arguments
  • Restricting the set of values for an argument
  • Using option=value syntax
  • Advanced usage formatting
• Developer Notes
  • Copying and Moving
• CMake Integration
• Building, Installing, and Testing
• Supported Toolchains
• Contributing
• License

Quick Start

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

#include <argparse/argparse.hpp>

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

argparse::ArgumentParser program("program_name");

NOTE: There is an optional second argument to the ArgumentParser which is the program version. Example: argparse::ArgumentParser program("libfoo", "1.9.0");

NOTE: There are optional third and fourth arguments to the ArgumentParser which control default arguments. Example: argparse::ArgumentParser program("libfoo", "1.9.0", default_arguments::help, false); See Default Arguments, below.

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/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")
    .scan<'i', int>();

  try {
    program.parse_args(argc, argv);
  }
  catch (const std::exception& err) {
    std::cerr << err.what() << std::endl;
    std::cerr << program;
    return 1;
  }

  auto input = program.get<int>("square");
  std::cout << (input * input) << std::endl;

  return 0;
}

And running the code:

foo@bar:/home/dev/$ ./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. 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 .scan method 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::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

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

foo@bar:/home/dev/$ ./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.

Flag

When defining flag arguments, you can use the shorthand flag() which is the same as default_value(false).implicit_value(true).

argparse::ArgumentParser program("test");

program.add_argument("--verbose")
  .help("increase output verbosity")
  .flag();

try {
  program.parse_args(argc, argv);
}
catch (const std::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

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

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.");

Accessing optional arguments without default values

If you require an optional argument to be present but have no good default value for it, you can combine testing and accessing the argument as following:

if (auto fn = program.present("-o")) {
    do_something_with(*fn);
}

Similar to get, the present method also accepts a template argument. But rather than returning T, parser.present<T>(key) returns std::optional<T>, so that when the user does not provide a value to this parameter, the return value compares equal to std::nullopt.

Deciding if the value was given by the user

If you want to know whether the user supplied a value for an argument that has a .default_value, check whether the argument .is_used().

program.add_argument("--color")
  .default_value(std::string{"orange"})   // might otherwise be type const char* leading to an error when trying program.get<std::string>
  .help("specify the cat's fur color");

try {
  program.parse_args(argc, argv);    // Example: ./main --color orange
}
catch (const std::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

auto color = program.get<std::string>("--color");  // "orange"
auto explicit_color = program.is_used("--color");  // true, user provided orange

Joining values of repeated optional arguments

You may want to allow an optional argument to be repeated and gather all values in one place.

program.add_argument("--color")
  .default_value<std::vector<std::string>>({ "orange" })
  .append()
  .help("specify the cat's fur color");

try {
  program.parse_args(argc, argv);    // Example: ./main --color red --color green --color blue
}
catch (const std::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

auto colors = program.get<std::vector<std::string>>("--color");  // {"red", "green", "blue"}

Notice that .default_value is given an explicit template parameter to match the type you want to .get.

Repeating an argument to increase a value

A common pattern is to repeat an argument to indicate a greater value.

int verbosity = 0;
program.add_argument("-V", "--verbose")
  .action([&](const auto &) { ++verbosity; })
  .append()
  .default_value(false)
  .implicit_value(true)
  .nargs(0);

program.parse_args(argc, argv);    // Example: ./main -VVVV

std::cout << "verbose level: " << verbosity << std::endl;    // verbose level: 4

Mutually Exclusive Group

Create a mutually exclusive group using program.add_mutually_exclusive_group(required = false). `argparse`` will make sure that only one of the arguments in the mutually exclusive group was present on the command line:

auto &group = program.add_mutually_exclusive_group();
group.add_argument("--first");
group.add_argument("--second");

with the following usage will yield an error:

foo@bar:/home/dev/$ ./main --first 1 --second 2
Argument '--second VAR' not allowed with '--first VAR'

The add_mutually_exclusive_group() function also accepts a required argument, to indicate that at least one of the mutually exclusive arguments is required:

auto &group = program.add_mutually_exclusive_group(true);
group.add_argument("--first");
group.add_argument("--second");

with the following usage will yield an error:

foo@bar:/home/dev/$ ./main
One of the arguments '--first VAR' or '--second VAR' is required

Storing values into variables

It is possible to bind arguments to a variable storing their value, as an alternative to explicitly calling program.get<T>(arg_name) or program[arg_name]

This is currently implementeted for variables of type bool (this also implicitly calls flag()), int, double, std::string and std::vector<std::string>. If the argument is not specified in the command line, the default value (if set) is set into the variable.

bool flagvar = false;
program.add_argument("--flagvar").store_into(flagvar);

int intvar = 0;
program.add_argument("--intvar").store_into(intvar);

double doublevar = 0;
program.add_argument("--doublevar").store_into(doublevar);

std::string strvar;
program.add_argument("--strvar").store_into(strvar);

std::vector<std::string> strvar_repeated;
program.add_argument("--strvar-repeated").append().store_into(strvar_repeated);

std::vector<std::string> strvar_multi_valued;
program.add_argument("--strvar-multi-valued").nargs(2).store_into(strvar_multi_valued);

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")
  .scan<'i', int>();

program.add_argument("floats")
  .help("Vector of floats")
  .nargs(4)
  .scan<'g', float>();

try {
  program.parse_args(argc, argv);
}
catch (const std::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

// Some code to print arguments

foo@bar:/home/dev/$ ./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("main");

program.add_argument("square")
  .help("display the square of a given number")
  .scan<'i', int>();

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

try {
  program.parse_args(argc, argv);
}
catch (const std::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

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;
}

foo@bar:/home/dev/$ ./main 4
16

foo@bar:/home/dev/$ ./main 4 --verbose
The square of 4 is 16

foo@bar:/home/dev/$ ./main --verbose 4
The square of 4 is 16

Printing Help

std::cout << program prints 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:

foo@bar:/home/dev/$ ./main --help
Usage: main [-h] [--verbose] square

Positional arguments:
  square       	display the square of a given number

Optional arguments:
  -h, --help   	shows help message and exits
  -v, --version	prints version information and exits
  --verbose

You may also get the help message in string via program.help().str().

Adding a description and an epilog to help

ArgumentParser::add_description will add text before the detailed argument information. ArgumentParser::add_epilog will add text after all other help output.

#include <argparse/argparse.hpp>

int main(int argc, char *argv[]) {
  argparse::ArgumentParser program("main");
  program.add_argument("thing").help("Thing to use.").metavar("THING");
  program.add_argument("--member").help("The alias for the member to pass to.").metavar("ALIAS");
  program.add_argument("--verbose").default_value(false).implicit_value(true);

  program.add_description("Forward a thing to the next member.");
  program.add_epilog("Possible things include betingalw, chiz, and res.");

  program.parse_args(argc, argv);

  std::cout << program << std::endl;
}

Usage: main [-h] [--member ALIAS] [--verbose] THING

Forward a thing to the next member.

Positional arguments:
  THING         	Thing to use.

Optional arguments:
  -h, --help    	shows help message and exits
  -v, --version 	prints version information and exits
  --member ALIAS	The alias for the member to pass to.
  --verbose

Possible things include betingalw, chiz, and res.

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::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

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 .scan, 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})
  .scan<'g', double>();

try {
  program.parse_args(argc, argv); // Example: ./main --query_point 3.5 4.7 9.2
}
catch (const std::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

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

You can also make a variable length list of arguments with the .nargs. Below are some examples.

program.add_argument("--input_files")
  .nargs(1, 3);  // This accepts 1 to 3 arguments.

Some useful patterns are defined like "?", "*", "+" of argparse in Python.

program.add_argument("--input_files")
  .nargs(argparse::nargs_pattern::any);  // "*" in Python. This accepts any number of arguments including 0.

program.add_argument("--input_files")
  .nargs(argparse::nargs_pattern::at_least_one);  // "+" in Python. This accepts one or more number of arguments.

program.add_argument("--input_files")
  .nargs(argparse::nargs_pattern::optional);  // "?" in Python. This accepts an argument optionally.

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})
  .scan<'g', float>();

try {
  program.parse_args(argc, argv);                  // Example: ./main -abc 1.95 2.47
}
catch (const std::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

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

foo@bar:/home/dev/$ ./main -ac 3.14 2.718
a = true
b = false
c = {3.14, 2.718}

foo@bar:/home/dev/$ ./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

Converting to Numeric Types

For inputs, users can express a primitive type for the value.

The .scan<Shape, T> method attempts to convert the incoming std::string to T following the Shape conversion specifier. An std::invalid_argument or std::range_error exception is thrown for errors.

program.add_argument("-x")
       .scan<'d', int>();

program.add_argument("scale")
       .scan<'g', double>();

Shape specifies what the input "looks like", and the type template argument specifies the return value of the predefined action. Acceptable types are floating point (i.e float, double, long double) and integral (i.e. signed char, short, int, long, long long).

The grammar follows std::from_chars, but does not exactly duplicate it. For example, hexadecimal numbers may begin with 0x or 0X and numbers with a leading zero may be handled as octal values.

Shape	interpretation
'a' or 'A'	hexadecimal floating point
'e' or 'E'	scientific notation (floating point)
'f' or 'F'	fixed notation (floating point)
'g' or 'G'	general form (either fixed or scientific)
'd'	decimal
'i'	std::from_chars grammar with base == 10
'o'	octal (unsigned)
'u'	decimal (unsigned)
'x' or 'X'	hexadecimal (unsigned)

Default Arguments

argparse provides predefined arguments and actions for -h/--help and -v/--version. By default, these actions will exit the program after displaying a help or version message, respectively. This exit does not call destructors, skipping clean-up of taken resources.

These default arguments can be disabled during ArgumentParser creation so that you can handle these arguments in your own way. (Note that a program name and version must be included when choosing default arguments.)

argparse::ArgumentParser program("test", "1.0", default_arguments::none);

program.add_argument("-h", "--help")
  .action([=](const std::string& s) {
    std::cout << help().str();
  })
  .default_value(false)
  .help("shows help message")
  .implicit_value(true)
  .nargs(0);

The above code snippet outputs a help message and continues to run. It does not support a --version argument.

The default is default_arguments::all for included arguments. No default arguments will be added with default_arguments::none. default_arguments::help and default_arguments::version will individually add --help and --version.

The default arguments can be used while disabling the default exit with these arguments. This forth argument to ArgumentParser (exit_on_default_arguments) is a bool flag with a default true value. The following call will retain --help and --version, but will not exit when those arguments are used.

argparse::ArgumentParser program("test", "1.0", default_arguments::all, false)

Gathering Remaining Arguments

argparse supports gathering "remaining" arguments at the end of the command, e.g., for use in a compiler:

foo@bar:/home/dev/$ compiler file1 file2 file3

To enable this, simply create an argument and mark it as remaining. All remaining arguments passed to argparse are gathered here.

argparse::ArgumentParser program("compiler");

program.add_argument("files")
  .remaining();

try {
  program.parse_args(argc, argv);
}
catch (const std::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

try {
  auto files = program.get<std::vector<std::string>>("files");
  std::cout << files.size() << " files provided" << std::endl;
  for (auto& file : files)
    std::cout << file << std::endl;
} catch (std::logic_error& e) {
  std::cout << "No files provided" << std::endl;
}

When no arguments are provided:

foo@bar:/home/dev/$ ./compiler
No files provided

and when multiple arguments are provided:

foo@bar:/home/dev/$ ./compiler foo.txt bar.txt baz.txt
3 files provided
foo.txt
bar.txt
baz.txt

The process of gathering remaining arguments plays nicely with optional arguments too:

argparse::ArgumentParser program("compiler");

program.add_arguments("-o")
  .default_value(std::string("a.out"));

program.add_argument("files")
  .remaining();

try {
  program.parse_args(argc, argv);
}
catch (const std::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

auto output_filename = program.get<std::string>("-o");
std::cout << "Output filename: " << output_filename << std::endl;

try {
  auto files = program.get<std::vector<std::string>>("files");
  std::cout << files.size() << " files provided" << std::endl;
  for (auto& file : files)
    std::cout << file << std::endl;
} catch (std::logic_error& e) {
  std::cout << "No files provided" << std::endl;
}

foo@bar:/home/dev/$ ./compiler -o main foo.cpp bar.cpp baz.cpp
Output filename: main
3 files provided
foo.cpp
bar.cpp
baz.cpp

NOTE: Remember to place all optional arguments BEFORE the remaining argument. If the optional argument is placed after the remaining arguments, it too will be deemed remaining:

foo@bar:/home/dev/$ ./compiler foo.cpp bar.cpp baz.cpp -o main
5 arguments provided
foo.cpp
bar.cpp
baz.cpp
-o
main

Parent Parsers

A parser may use arguments that could be used by other parsers.

These shared arguments can be added to a parser which is then used as a "parent" for parsers which also need those arguments. One or more parent parsers may be added to a parser with .add_parents. The positional and optional arguments in each parent is added to the child parser.

argparse::ArgumentParser surface_parser("surface", 1.0, argparse::default_arguments::none);
parent_parser.add_argument("--area")
  .default_value(0)
  .scan<'i', int>();

argparse::ArgumentParser floor_parser("floor");
floor_parser.add_argument("tile_size").scan<'i', int>();
floor_parser.add_parents(surface_parser);
floor_parser.parse_args({ "./main", "--area", "200", "12" });  // --area = 200, tile_size = 12

argparse::ArgumentParser ceiling_parser("ceiling");
ceiling_parser.add_argument("--color");
ceiling_parser.add_parents(surface_parser);
ceiling_parser.parse_args({ "./main", "--color", "gray" });  // --area = 0, --color = "gray"

Changes made to parents after they are added to a parser are not reflected in any child parsers. Completely initialize parent parsers before adding them to a parser.

Each parser will have the standard set of default arguments. Disable the default arguments in parent parsers to avoid duplicate help output.

Subcommands

Many programs split up their functionality into a number of sub-commands, for example, the git program can invoke sub-commands like git checkout, git add, and git commit. Splitting up functionality this way can be a particularly good idea when a program performs several different functions which require different kinds of command-line arguments. ArgumentParser supports the creation of such sub-commands with the add_subparser() member function.

#include <argparse/argparse.hpp>

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

  // git add subparser
  argparse::ArgumentParser add_command("add");
  add_command.add_description("Add file contents to the index");
  add_command.add_argument("files")
    .help("Files to add content from. Fileglobs (e.g.  *.c) can be given to add all matching files.")
    .remaining();

  // git commit subparser
  argparse::ArgumentParser commit_command("commit");
  commit_command.add_description("Record changes to the repository");
  commit_command.add_argument("-a", "--all")
    .help("Tell the command to automatically stage files that have been modified and deleted.")
    .default_value(false)
    .implicit_value(true);

  commit_command.add_argument("-m", "--message")
    .help("Use the given <msg> as the commit message.");

  // git cat-file subparser
  argparse::ArgumentParser catfile_command("cat-file");
  catfile_command.add_description("Provide content or type and size information for repository objects");
  catfile_command.add_argument("-t")
    .help("Instead of the content, show the object type identified by <object>.");

  catfile_command.add_argument("-p")
    .help("Pretty-print the contents of <object> based on its type.");

  // git submodule subparser
  argparse::ArgumentParser submodule_command("submodule");
  submodule_command.add_description("Initialize, update or inspect submodules");
  argparse::ArgumentParser submodule_update_command("update");
  submodule_update_command.add_description("Update the registered submodules to match what the superproject expects");
  submodule_update_command.add_argument("--init")
    .default_value(false)
    .implicit_value(true);
  submodule_update_command.add_argument("--recursive")
    .default_value(false)
    .implicit_value(true);
  submodule_command.add_subparser(submodule_update_command);

  program.add_subparser(add_command);
  program.add_subparser(commit_command);
  program.add_subparser(catfile_command);
  program.add_subparser(submodule_command);

  try {
    program.parse_args(argc, argv);
  }
  catch (const std::exception& err) {
    std::cerr << err.what() << std::endl;
    std::cerr << program;
    return 1;
  }

  // Use arguments
}

foo@bar:/home/dev/$ ./git --help
Usage: git [-h] {add,cat-file,commit,submodule}

Optional arguments:
  -h, --help   	shows help message and exits
  -v, --version	prints version information and exits

Subcommands:
  add           Add file contents to the index
  cat-file      Provide content or type and size information for repository objects
  commit        Record changes to the repository
  submodule     Initialize, update or inspect submodules

foo@bar:/home/dev/$ ./git add --help
Usage: add [-h] files

Add file contents to the index

Positional arguments:
  files        	Files to add content from. Fileglobs (e.g.  *.c) can be given to add all matching files.

Optional arguments:
  -h, --help   	shows help message and exits
  -v, --version	prints version information and exits

foo@bar:/home/dev/$ ./git commit --help
Usage: commit [-h] [--all] [--message VAR]

Record changes to the repository

Optional arguments:
  -h, --help   	shows help message and exits
  -v, --version	prints version information and exits
  -a, --all    	Tell the command to automatically stage files that have been modified and deleted.
  -m, --message	Use the given <msg> as the commit message.

foo@bar:/home/dev/$ ./git submodule --help
Usage: submodule [-h] {update}

Initialize, update or inspect submodules

Optional arguments:
  -h, --help   	shows help message and exits
  -v, --version	prints version information and exits

Subcommands:
  update        Update the registered submodules to match what the superproject expects

When a help message is requested from a subparser, only the help for that particular parser will be printed. The help message will not include parent parser or sibling parser messages.

Additionally, every parser has the .is_subcommand_used("<command_name>") and .is_subcommand_used(subparser) member functions to check if a subcommand was used.

Sometimes there may be a need to hide part of the subcommands from the user by suppressing information about them in an help message. To do this, ArgumentParser contains the method .set_suppress(bool suppress):

argparse::ArgumentParser program("test");

argparse::ArgumentParser hidden_cmd("hidden");
hidden_cmd.add_argument("files").remaining();
hidden_cmd.set_suppress(true);

program.add_subparser(hidden_cmd);

foo@bar:/home/dev/$ ./main -h
Usage: test [--help] [--version] {}

Optional arguments:
  -h, --help    shows help message and exits
  -v, --version prints version information and exits

foo@bar:/home/dev/$ ./main hidden -h
Usage: hidden [--help] [--version] files

Positional arguments:
  files         [nargs: 0 or more]

Optional arguments:
  -h, --help    shows help message and exits
  -v, --version prints version information and exits

Getting Argument and Subparser Instances

Argument and ArgumentParser instances added to an ArgumentParser can be retrieved with .at<T>(). The default return type is Argument.

argparse::ArgumentParser program("test");

program.add_argument("--dir");
program.at("--dir").default_value(std::string("/home/user"));

program.add_subparser(argparse::ArgumentParser{"walk"});
program.at<argparse::ArgumentParser>("walk").add_argument("depth");

Parse Known Args

Sometimes a program may only parse a few of the command-line arguments, passing the remaining arguments on to another script or program. In these cases, the parse_known_args() function can be useful. It works much like parse_args() except that it does not produce an error when extra arguments are present. Instead, it returns a list of remaining argument strings.

#include <argparse/argparse.hpp>
#include <cassert>

int main(int argc, char *argv[]) {
  argparse::ArgumentParser program("test");
  program.add_argument("--foo").implicit_value(true).default_value(false);
  program.add_argument("bar");

  auto unknown_args =
    program.parse_known_args({"test", "--foo", "--badger", "BAR", "spam"});

  assert(program.get<bool>("--foo") == true);
  assert(program.get<std::string>("bar") == std::string{"BAR"});
  assert((unknown_args == std::vector<std::string>{"--badger", "spam"}));
}

Hidden argument and alias

It is sometimes desirable to offer an alias for an argument, but without it appearing it in the usage. For example, to phase out a deprecated wording of an argument while not breaking backwards compatible. This can be done with the ``ArgumentParser::add_hidden_alias_for()` method.

argparse::ArgumentParser program("test");

auto &arg = program.add_argument("--suppress").flag();
program.add_hidden_alias_for(arg, "--supress"); // old misspelled alias

The Argument::hidden() method can also be used to prevent a (generally optional) argument from appearing in the usage or help.

argparse::ArgumentParser program("test");

program.add_argument("--non-documented").flag().hidden();

This can also be used on positional arguments, but in that later case it only makes sense in practice for the last ones.

ArgumentParser in bool Context

An ArgumentParser is false until it (or one of its subparsers) have extracted known value(s) with .parse_args or .parse_known_args. When using .parse_known_args, unknown arguments will not make a parser true.

Custom Prefix Characters

Most command-line options will use - as the prefix, e.g. -f/--foo. Parsers that need to support different or additional prefix characters, e.g. for options like +f or /foo, may specify them using the set_prefix_chars().

The default prefix character is -.

#include <argparse/argparse.hpp>
#include <cassert>

int main(int argc, char *argv[]) {
  argparse::ArgumentParser program("test");
  program.set_prefix_chars("-+/");

  program.add_argument("+f");
  program.add_argument("--bar");
  program.add_argument("/foo");

  try {
    program.parse_args(argc, argv);
  }
  catch (const std::exception& err) {
    std::cerr << err.what() << std::endl;
    std::cerr << program;
    return 1;
  }

  if (program.is_used("+f")) {
    std::cout << "+f    : " << program.get("+f") << "\n";
  }

  if (program.is_used("--bar")) {
    std::cout << "--bar : " << program.get("--bar") << "\n";
  }

  if (program.is_used("/foo")) {
    std::cout << "/foo  : " << program.get("/foo") << "\n";
  }  
}

foo@bar:/home/dev/$ ./main +f 5 --bar 3.14f /foo "Hello"
+f    : 5
--bar : 3.14f
/foo  : Hello

Custom Assignment Characters

In addition to prefix characters, custom 'assign' characters can be set. This setting is used to allow invocations like ./test --foo=Foo /B:Bar.

The default assign character is =.

#include <argparse/argparse.hpp>
#include <cassert>

int main(int argc, char *argv[]) {
  argparse::ArgumentParser program("test");
  program.set_prefix_chars("-+/");
  program.set_assign_chars("=:");

  program.add_argument("--foo");
  program.add_argument("/B");

  try {
    program.parse_args(argc, argv);
  }
  catch (const std::exception& err) {
    std::cerr << err.what() << std::endl;
    std::cerr << program;
    return 1;
  }

  if (program.is_used("--foo")) {
    std::cout << "--foo : " << program.get("--foo") << "\n";
  }

  if (program.is_used("/B")) {
    std::cout << "/B    : " << program.get("/B") << "\n";
  }
}

foo@bar:/home/dev/$ ./main --foo=Foo /B:Bar
--foo : Foo
/B    : Bar

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::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

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

Positional Arguments with Compound Toggle Arguments

argparse::ArgumentParser program("test");

program.add_argument("numbers")
  .nargs(3)
  .scan<'i', int>();

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)
  .scan<'g', float>();

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

try {
  program.parse_args(argc, argv);
}
catch (const std::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

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

foo@bar:/home/dev/$ ./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(std::string{"baz"})
  .choices("foo", "bar", "baz");

try {
  program.parse_args(argc, argv);
}
catch (const std::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

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

foo@bar:/home/dev/$ ./main fex
Invalid argument "fex" - allowed options: {foo, bar, baz}

Using choices also works with integer types, e.g.,

argparse::ArgumentParser program("test");

program.add_argument("input")
  .default_value(0)
  .choices(0, 1, 2, 3, 4, 5);

try {
  program.parse_args(argc, argv);
}
catch (const std::exception& err) {
  std::cerr << err.what() << std::endl;
  std::cerr << program;
  std::exit(1);
}

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

foo@bar:/home/dev/$ ./main 6
Invalid argument "6" - allowed options: {0, 1, 2, 3, 4, 5}

Using option=value syntax

#include "argparse.hpp"
#include <cassert>

int main(int argc, char *argv[]) {
  argparse::ArgumentParser program("test");
  program.add_argument("--foo").implicit_value(true).default_value(false);
  program.add_argument("--bar");

  try {
    program.parse_args(argc, argv);
  }
  catch (const std::exception& err) {
    std::cerr << err.what() << std::endl;
    std::cerr << program;
    return 1;
  }

  if (program.is_used("--foo")) {
    std::cout << "--foo: " << std::boolalpha << program.get<bool>("--foo") << "\n";
  }

  if (program.is_used("--bar")) {
    std::cout << "--bar: " << program.get("--bar") << "\n";
  }  
}

foo@bar:/home/dev/$ ./test --bar=BAR --foo
--foo: true
--bar: BAR

Advanced usage formatting

By default usage is reported on a single line.

The ArgumentParser::set_usage_max_line_width(width) method can be used to display the usage() on multiple lines, by defining the maximum line width.

It can be combined with a call to ArgumentParser::set_usage_break_on_mutex() to ask grouped mutually exclusive arguments to be displayed on a separate line.

ArgumentParser::add_usage_newline() can also be used to force the next argument to be displayed on a new line in the usage output.

The following snippet

    argparse::ArgumentParser program("program");
    program.set_usage_max_line_width(80);
    program.set_usage_break_on_mutex();
    program.add_argument("--quite-long-option-name").flag();
    auto &group = program.add_mutually_exclusive_group();
    group.add_argument("-a").flag();
    group.add_argument("-b").flag();
    program.add_argument("-c").flag();
    program.add_argument("--another-one").flag();
    program.add_argument("-d").flag();
    program.add_argument("--yet-another-long-one").flag();
    program.add_argument("--will-go-on-new-line").flag();
    program.add_usage_newline();
    program.add_argument("--new-line").flag();
    std::cout << program.usage() << std::endl;

will display:

Usage: program [--help] [--version] [--quite-long-option-name]
               [[-a]|[-b]]
               [-c] [--another-one] [-d] [--yet-another-long-one]
               [--will-go-on-new-line]
               [--new-line]

Furthermore arguments can be separated into several groups by calling ArgumentParser::add_group(group_name). Only optional arguments should be specified after the first call to add_group().

    argparse::ArgumentParser program("program");
    program.set_usage_max_line_width(80);
    program.add_argument("-a").flag().help("help_a");
    program.add_group("Advanced options");
    program.add_argument("-b").flag().help("help_b");

will display:

Usage: program [--help] [--version] [-a]

Advanced options:
               [-b]

Developer Notes

Copying and Moving

argparse::ArgumentParser is intended to be used in a single function - setup everything and parse arguments in one place. Attempting to move or copy invalidates internal references (issue #260). Thus, starting with v3.0, argparse::ArgumentParser copy and move constructors are marked as delete.

CMake Integration

Use the latest argparse in your CMake project without copying any content.

cmake_minimum_required(VERSION 3.14)

PROJECT(myproject)

# fetch latest argparse
include(FetchContent)
FetchContent_Declare(
    argparse
    GIT_REPOSITORY https://github.com/p-ranav/argparse.git
)
FetchContent_MakeAvailable(argparse)

add_executable(myproject main.cpp)
target_link_libraries(myproject argparse)

Building, Installing, and Testing

# Clone the repository
git clone https://github.com/p-ranav/argparse
cd argparse

# Build the tests
mkdir build
cd build
cmake -DARGPARSE_BUILD_SAMPLES=on -DARGPARSE_BUILD_TESTS=on ..
make

# Run tests
./test/tests

# Install the library
sudo make install

Supported Toolchains

Compiler	Standard Library	Test Environment
GCC >= 8.3.0	libstdc++	Ubuntu 18.04
Clang >= 7.0.0	libc++	Xcode 10.2
MSVC >= 16.8	Microsoft STL	Visual Studio 2019

Contributing

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

License

The project is available under the MIT license.