A simple C++11 command line argument parser
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Latest commit 9f955a5 Jun 18, 2018


Argument Aggregator

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This is yet another C++ command line argument/option parser. It was written as a simple and idiomatic alternative to other frameworks like getopt, Boost program options, TCLAP, and others. The goal is to achieve the majority of argument parsing needs in a simple manner with an easy to use API. It operates as a single pass over all arguments, recognizing flags prefixed by - (short) or -- (long) and aggregating them into easy to access structures with lots of convenience functions. It defers processing types until you access them, so the result structures end up just being pointers into the original command line argument C-strings.

argagg supports POSIX recommended argument syntax conventions:

  • Options (short) start with a hyphen (-) and long options start with two hyphens (--)
  • Multiple short options can be grouped following a single hyphen
    • -a -b -c can also be written -abc or -bac, etc.
  • Option names are alpha numeric but long options may include hyphens
    • -v is valid, --ftest-coverage is valid
    • -# is not valid, --bad$option is not valid
  • Short options can be provided arguments with or without whitespace delimiters
    • -I /usr/local/include and -I/usr/local/include are equally valid
  • Long options can be provided arguments with whitespace or equal sign delimiters
    • --output test.txt and --output=test.txt are equivalent
  • Options and positional arguments can be interleaved
  • -- can be specified to treat all following arguments as positional arguments (i.e. not options)

Help message formatting is provided via the fmt utility on {Li,U}nix systems.

The project has only one required header: argagg.hpp. Optional headers under include/argagg/convert contain extra argument conversion specializations.


To use just create an argagg::parser object. The struct doesn't provide any explicit methods for defining options. Instead we define the options using initialization lists.

argagg::parser argparser {{
    { "help", {"-h", "--help"},
      "shows this help message", 0},
    { "delim", {"-d", "--delim"},
      "delimiter (default: ,)", 1},
    { "num", {"-n", "--num"},
      "number", 1},

An option is specified by four things: the name of the option, the strings that activate the option (flags), the option's help message, and the number of arguments the option expects.

With the parser defined you actually parse the arguments by calling the argagg::parser::parse() method. If there are any problems an exception is thrown.

argagg::parser_results args;
try {
  args = argparser.parse(argc, argv);
} catch (const std::exception& e) {
  std::cerr << e.what() << std::endl;
  return EXIT_FAILURE;

You can check if an option shows up in the command line arguments by accessing the option by name from the parser results and using the implicit boolean conversion. You can write out a simplistic option help message by streaming the argagg::parser instance itself.

if (args["help"]) {
  std::cerr << argparser;
  //     -h, --help
  //         shows this help message
  //     -d, --delim
  //         delimiter (default: ,)
  //     -n, --num
  //         number
  return EXIT_SUCCESS;

That help message is only for the flags. If you want a usage message it's up to you to provide it.

if (args["help"]) {
  std::cerr << "Usage: program [options] ARG1 ARG2" << std::endl
            << argparser;
  // Usage: program [options] ARG1 ARG2
  //     -h, --help
  //         shows this help message
  //     -d, --delim
  //         delimiter (default: ,)
  //     -n, --num
  //         number
  return EXIT_SUCCESS;

A special output stream, argagg::fmt_ostream, is provided that will run the usage and help through fmt for nice word wrapping (see ./examples/joinargs.cpp for a better example).

if (args["help"]) {
  argagg::fmt_ostream fmt(std::cerr);
  fmt << "Usage: program [options] ARG1 ARG2" << std::endl
      << argparser;
  return EXIT_SUCCESS;

Generally argagg tries to do a minimal amount of work to leave most of the control with the user.

If you want to get an option argument but fallback on a default value if the option is not specified then you can use the argagg::option_results::as() API and provide a default value.

auto delim = args["delim"].as<std::string>(",");

If you don't mind being implicit an implicit conversion operator is provided allowing you to write simple assignments.

int x = 0;
if (args["num"]) {
  x = args["num"];

Finally, you can get all of the positional arguments as an std::vector using the argagg::parser_results::pos member. You can alternatively convert individual positional arguments using the same conversion functions as the option argument conversion methods.

auto y = 0.0;
if (args.pos.size() > 0) {
  y = args.as<double>(0);

One can also specify -- on the command line in order to treat all following arguments as not options.

For a more detailed treatment take a look at the examples or test cases.

Custom argument conversion functions can also be defined by specializing either argagg::convert::arg<T>() or argagg::convert::converter<T>. See test_csv.cpp as well as TEST_CASE("custom conversion function") and TEST_CASE("parse_next_component() example") in test.cpp.

Mental Model

The parser just returns a structure of pointers to the C-strings in the original argv array. The parse() method returns a parser_results object which has two things: position arguments and option results. The position arguments are just a std::vector of const char*. The option results are a mapping from option name (std::string) to option_results objects. The option_results objects are just an std::vector of option_result objects. Each instance of an option_result represents the option showing up on the command line. If there was an argument associated with it then the option_result's arg member will not be nullptr.

Consider the following command:

gcc -g -I/usr/local/include -I. -o test main.o foo.o -L/usr/local/lib -lz bar.o -lpng

This would produce a structure like follows, written in psuedo-YAML, where each string is actually a const char* pointing to some part of a string in the original argv array:

  program: "gcc"
  pos: ["main.o", "foo.o", "bar.o"]
      - arg: null
      - arg: "/usr/local/include"
      - arg: "."
      - arg: "/usr/local/lib"
      - arg: "z"
      - arg: "png"
      - arg: "test"

Conversion to types occurs at the very end when the as<T>() API is used. Up to that point argagg is just dealing with C-strings.

API Reference

Doxygen documentation can be found here.

Quick Reference


  • option_result
    • const char* arg
  • option_results
    • std::vector<option_result> all
  • parser_results
    • const char* program
    • std::unordered_map<std::string, option_results> options
    • std::vector<const char*> pos
  • definition
    • const char* name
    • std::vector<std::string> flag
    • std::string help
    • unsigned int num_args
  • parser_map
    • std::array<const definition*, 256> short_map
    • std::unordered_map<std::string, const definition*> long_map
  • parser
    • std::vector<definition> definitions


  • unexpected_argument_error
  • unexpected_option_error
  • option_lacks_argument_error
  • invalid_flag


There is just a single required header file (argagg.hpp) so you can copy that whereever you want. If you want to properly install it you can use the CMake script. The CMake script exists primarily to build the tests and documentation, but an install target for the header is provided which will install all header files.

The standard installation dance using CMake and make is as follows:

mkdir build
cd build
cmake -DCMAKE_INSTALL_PREFIX=/usr/local ..
make install
ctest -V # optionally run tests

Override CMAKE_INSTALL_PREFIX to change the installation location. By default (on UNIX variants) it will install to /usr/local resulting in the header being copied to /usr/local/include/argagg/argagg.hpp.

If you have Doxygen it should build and install documentation as well.

There are no dependencies other than the standard library.

Edge Cases

There are some interesting edge cases that show up in option parsing. I used the behavior of gcc as my target reference in these cases.

Greedy Arguments

Remember that options that require arguments will greedily process arguments.

Say we have the following options: -a, -b, -c, and -o. They all don't accept arguments except -o. Below is a list of permutations for short flag grouping and the results:

  • -abco foo: -o's argument is foo
  • -aboc foo: -o's argument is c, foo is a positional argument
  • -aobc foo: -o's argument is bc, foo is a positional argument
  • -oabc foo: -o's argument is abc, foo is a positional argument

For whitespace delimited arguments the greedy processing means the next argument element (in argv) will be treated as an argument for the previous option, regardless of whether or not it looks like a flag or some other special entry. That means you get behavior like below:

  • --output=foo -- --bar: --output's argument is foo, --bar is a positional argument
  • --output -- --bar: --output's argument is --, --bar is treated as a flag
  • --output --bar: --output's argument is --bar

Adding Argument Conversions

Arguments are converted into types by defining template specializations for the function argagg::convert::arg<>(). Specializations for integer and floating point types are pre-defined in include/argagg/argagg.hpp. An extension for parsing an argument as a comma-separated list of strings is defined in include/argagg/convert/comma_separated_strings.hpp.