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# Quick start

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

Options can be given as:

    --long
    --long=argument
    --long argument
    -a
    -ab
    -abc 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")
      ("f,file", "File name", cxxopts::value<std::string>())
      ;

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:

    options.parse(argc, argv);

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

    options["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.

## 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.

## Positional Arguments

Positional arguments can be optionally parsed into one or more options.
To set up positional arguments, call

    options.parse_positional({"first", "second", "last"})

where "last" should be the name of an option with a container type, and the
others should have a single value.

## 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"`.

# Linking

This is a header only library.

# Requirements

The only build requirement is a C++ compiler that supports C++11 regular
expressions. For example GCC >= 4.9 or clang with libc++.


# TODO list

* Allow unrecognised options.
* Various help strings.
* Unicode aware for help strings.