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Lightweight C++ command line option parser

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# 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:

```cpp
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

```cpp
#include <cxxopts.hpp>
```

Create a `cxxopts::Options` instance.

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

Then use `add_options`.

```cpp
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:

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

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

```cpp
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.

## 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:

```cpp
options.allow_unrecognised_options();
```

and in the result object they are retrieved with:

```cpp
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::exceptions::exception`. Errors
defining options derive from `cxxopts::exceptions::specification` and errors
parsing arguments derive from `cxxopts::exceptions::parsing`.

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, `help` by default prints help for all groups. If you want to only print help for a specific group, pass the groups that you would like displayed as a vector to the `help` function.
## 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:

```cpp
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);
```

Note : `parse_positional` defaults to replacing the positionals with the arguments provided in the function call. To append to the existing list of positionals, please use `cxxopts::PositionalMode::Append`.

```cpp
options.parse_positional("another_option", cxxopts::PositionalMode::Append);
```

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"}` |

## 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:

```cpp
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:

```cpp
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 if `option` has an implicit value, specifying `--option another` will not work. You must use `=` syntax - `--option=another` or `-o=another`(assumming `o` is a short name for the option). This is because no argument is required. Hence, there is no good way to determine if the next string is an argument to your option or a positional argument.

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`.

### Implicit values with disabled arguments

You can specify an option for which an argument cannot be specified using below syntax.

```cpp
cxxopts::value<bool>()->implicit_value("true", cxxopts::ImplicitArgPolicy::Disabled)
```

In this case specifying option as `--option=<value>` is disallowed and will throw `specified_disabled_args` exception. Note that `--option value` is not supported for implicit values. It is treated as positional/unmatched.

## 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 a list of values into a `std::vector<T>` is also supported, as long as `T`
can be parsed. To separate single values in a list the define symbol `CXXOPTS_VECTOR_DELIMITER`
is used, which is ',' by default. Ensure that you use no whitespaces between values
(unless the entire list is double quoted) 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
~~~

If the list of values is quoted, then spaces can be included. For example:

~~~
--my_list="review,memory sanitize,build help,reformat"
~~~

This will be parsed into `review`, `memory sanitize`, `build help`, and `reformat`.

## 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:
```cpp
#include <iostream>
#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;
}
```

# 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

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