superbuild/modules/cxxopts/README.md

Release versions

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

Version 3 breaking changes

If you have used version 2, there are a couple of breaking changes in version 3 that you should be aware of. If you are new to cxxopts you can skip this section.

The parser no longer modifies its arguments, so you can pass a const argc and argv and expect them not to be changed.

The ParseResult object no longer depends on the parser. So it can be returned from a scope outside the parser and still work. Now that the inputs are not modified, ParseResult stores a list of the unmatched arguments. These are retrieved like follows:

auto result = options.parse(argc, argv);
result.unmatched(); // get the unmatched arguments

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") // 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.

Note that the result of options.parse should only be used as long as the options object that created it is in scope.

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::OptionException. Errors defining options derive from cxxopts::OptionSpecException and errors parsing arguments derive from cxxopts::OptionParseException.

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.

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".

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.

Default values are not counted by Options::count.

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.

std::vector<T> 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 definition CXXOPTS_VECTOR_DELIMITER is used, which is ',' 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

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>>())

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.

Example

Putting all together:

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;
}

Linking

This is a header only library.

Requirements

The only build requirement is a C++ compiler that supports C++11 features such as:

• regex
• constexpr
• default constructors

GCC >= 4.9 or clang >= 3.1 with libc++ are known to work.

The following compilers are known not to work:

• MSVC 2013

TODO list

• Allow unrecognised options.