artpaul / cxxopts

Lightweight C++ command-line option parser

About

This is a lightweight C++ option parser library, supporting the standard GNU style syntax for options.

Features

Below are a few of the features which cxxopts supports:

• Flags / Switches (i.e. bool fields)
  • Both short and long versions supported (i.e. -f and --flag respectively)
  • Supports combining short versions (i.e. -fBgoZ is the same as -f -B -g -o -Z)
  • Supports multiple occurrences (i.e. -vvv or -v -v -v)
  • Supports -?
• Positional Arguments
  • Supports the Unix -- meaning, only positional arguments follow
  • Supports partial parsing (i.e. cmd <options> sub-command <options> <positional>)
• Option Arguments (i.e. those that take values)
  • Both short and long versions supported (i.e. -o value, -ovalue and --option value or --option=value respectively)
  • Supports multiple values (i.e. -o <val1> -o <val2>)
  • Supports delimited values (i.e. --option=val1,val2,val3)
  • Supports custom parsers
  • Supports vector of vector values (i.e --opt=1,2,3 --opt=4,5,6)
• Groups
  • Arguments can be made part of a group for the purposes of displaying help messages
• Default Values
  • Supports default value from ENV variable
• Requirements
  • No external dependencies and only C++ 11 compatible compiler is required

Quick start

Options can be given as:

--long
--long=<argument>
--long <argument>
-a
-ab
-abc <argument>
-c<argument>

where -c takes an argument, but -a and -b do not.

Additionally, anything after -- will be parsed as a positional argument.

Basics

#include <cxxopts.hpp>

Create a cxxopts::options instance.

cxxopts::options options("MyProgram", "One line description of MyProgram");

Then use add_options.

options.add_options()
  ("d,debug", "Enable debugging") // a bool parameter
  ("i,integer", "Int param", cxxopts::value<int>())
  ("f,file", "File name", cxxopts::value<std::string>())
  ("v,verbose", "Verbose output", cxxopts::value<bool>()->default_value("false"))
  ;

Options are declared with a long and an optional short option. A description must be provided. The third argument is the value, if omitted it is boolean. Any type can be given as long as it can be parsed, with operator>>.

To parse the command line do:

auto result = options.parse(argc, argv);

To retrieve an option use result.count("option") to get the number of times it appeared, and

result["opt"].as<type>()

to get its value. If "opt" doesn't exist, or isn't of the right type, then an exception will be thrown.

Boolean values

Boolean options have a default implicit value of "true", which can be overridden. The effect is that writing -o by itself will set option o to true. However, they can also be written with various strings using =value. There is no way to disambiguate positional arguments from the value following a boolean, so we have chosen that they will be positional arguments, and therefore, -o false does not work.

Vector values

Parsing of list of values in form of an std::vector<T> is also supported, as long as T can be parsed. To separate single values in a list the ',' is used by default. Ensure that you use no whitespaces between values because those would be interpreted as the next command-line option. Example for a command-line option that can be parsed as a std::vector<double>:

--my_list=1,-2.1,3,4.5

To set up custom separation character (semicolon, for example) use:

cxxopts::value<std::vector<std::string>>()->delimiter(';')

Value from ENV variable

When a parameter is not set, a value will be fetched from an environment variable (if such variable is defined).

cxxopts::value<int>()->env("MY_VAR")

Default and implicit values

An option can be declared with a default or an implicit value, or both.

A default value is the value that an option takes when it is not specified on the command line. The following specifies a default value for an option:

cxxopts::value<std::string>()->default_value("value")

An implicit value is the value that an option takes when it is given on the command line without an argument. The following specifies an implicit value:

cxxopts::value<std::string>()->implicit_value("implicit")

If an option had both, then not specifying it would give the value "value", writing it on the command line as --option would give the value "implicit", and writing --option=another would give it the value "another".

Note that the default and implicit value is always stored as a string, regardless of the type that you want to store it in. It will be parsed as though it was given on the command line.

Options specified multiple times

The same option can be specified several times, with different arguments, which will all be recorded in order of appearance. An example:

--use train --use bus --use ferry

this is supported through the use of a vector of value for the option:

options.add_options()
  ("use", "Usable means of transport", cxxopts::value<std::vector<std::string>>())

Positional Arguments

Positional arguments are those given without a preceding flag and can be used alongside non-positional arguments. There may be multiple positional arguments, and the final positional argument may be a container type to hold a list of all remaining positionals.

To set up positional arguments, first declare the options, then configure a set of those arguments as positional like:

options.add_options()
  ("script", "The script file to execute", cxxopts::value<std::string>())
  ("server", "The server to execute on", cxxopts::value<std::string>())
  ("filenames", "The filename(s) to process", cxxopts::value<std::vector<std::string>>());

options.parse_positional("script", "server", "filenames");

// Parse options the usual way
options.parse(argc, argv);

For example, parsing the following arguments:

my_script.py my_server.com file1.txt file2.txt file3.txt

will result in parsed arguments like the following table:

Field	Value
"script"	"my_script.py"
"server"	"my_server.com"
"filenames"	{"file1.txt", "file2.txt", "file3.txt"}

Unrecognised arguments

You can allow unrecognised arguments to be skipped. This applies to both positional arguments that are not parsed into another option, and -- arguments that do not match an argument that you specify. This is done by calling:

options.allow_unrecognised_options();

and in the result object they are retrieved with:

result.unmatched()

Exceptions

Exceptional situations throw C++ exceptions. There are two types of exceptions: errors defining the options and errors when parsing a list of arguments. All exceptions derive from cxxopts::option_error. Errors defining options derive from cxxopts::spec_error and errors parsing arguments derive from cxxopts::parse_error.

All exceptions define a what() function to get a printable string explaining the error.

Help groups

Options can be placed into groups for the purposes of displaying help messages. To place options in a group, pass the group as a string to add_options. Then, when displaying the help, pass the groups that you would like displayed as a vector to the help function.

Custom help

The string after the program name on the first line of the help can be completely replaced by calling options.custom_help. Note that you might also want to override the positional help by calling options.positional_help.

Custom parsers

template <>
struct cxxopts::value_parser<custom_type> {
  using value_type = custom_type;
  /// Is this a container type?
  static constexpr bool is_container = <true> | <false>;

  void parse(const cxxopts::parse_context& ctx, const std::string& text, custom_type& value) {
    // parse value from text here
  }
};

Command / subcommand pattern

In case then a program has some global options and specific sets of options for each subcommand, the following technique can be used.

Enable stop on the first positional feature:

options.stop_on_positional()

parse global options:

auto result = options.parse(argc, argv);

adjust values of argc and argv:

argc -= result.consumed();
argv += result.consumed();

In the end, argv will point to the name of the subcommand.

Example

Putting all together:

#include <cxxopts.hpp>

int main(int argc, char** argv) {
  cxxopts::options options("test", "A brief description");

  options.add_options()
    ("b,bar", "Param bar", cxxopts::value<std::string>())
    ("d,debug", "Enable debugging", cxxopts::value<bool>()->default_value("false"))
    ("f,foo", "Param foo", cxxopts::value<int>()->default_value("10"))
    ("h,help", "Print usage");

  auto result = options.parse(argc, argv);

  if (result.count("help")) {
    std::cout << options.help() << std::endl;
    exit(0);
  }
  bool debug = result["debug"].as<bool>();
  std::string bar;
  if (result.count("bar"))
    bar = result["bar"].as<std::string>();
  int foo = result["foo"].as<int>();

  return 0;
}

Solving clang-format issue

Currently, there is no way to setup clang-format to format code beautifully with chained call operator. Consider to use initializer_list syntax:

options.add_options("group name", {
  {"b,bar", "Param bar", cxxopts::value<std::string>()},
  {"d,debug", "Enable debugging", cxxopts::value<bool>()->default_value("false")},
  {"f,foo", "Param foo", cxxopts::value<int>()->default_value("10")},
  {"h,help", "Print usage"}
});

Linking

This is a header only library.

Release versions

Note that master is generally a work in progress, and you probably want to use a tagged release version.