Multitoken options

[maickrau]

Implements multitoken options from Boost.Program_options. A multitoken option will take values until the next option is given. This makes it possible to switch to cxxopts without breaking backwards compatibility for multitoken options, and also enables a slightly easier syntax for the user for options with multiple values.

Example code without multitoken which just takes multiple parameters and prints them:

#include <iostream>
#include <vector>
#include <string>
#include "cxxopts/include/cxxopts.hpp"

int main(int argc, char** argv)
{
	cxxopts::Options options { "test" };
	options.add_options()
		("a", "Option a")
		("b", "Option b", cxxopts::value<std::vector<std::string>>())
		("c", "Option c", cxxopts::value<std::vector<std::string>>())
		;
	auto params = options.parse(argc, argv);
	bool a = params.count("a");
	std::cerr << "a: " << (a ? "yes" : "no") << std::endl;
	std::cerr << "b:";
	if (params.count("b") != 0)
	{
		for (std::string value : params["b"].as<std::vector<std::string>>())
		{
			std::cerr << " " << value;
		}
	}
	std::cerr << std::endl;
	std::cerr << "c:";
	if (params.count("c") != 0)
	{
		for (std::string value : params["c"].as<std::vector<std::string>>())
		{
			std::cerr << " " << value;
		}
	}
	std::cerr << std::endl;
	std::cerr << "unmatched:";
	for (std::string value : params.unmatched())
	{
		std::cerr << " " << value;
	}
	std::cerr << std::endl;
}

$ ./main -b 1 2 -c 3 4 -a 5
a: yes
b: 1
c: 3
unmatched: 2 4 5

Applying multitoken to option -c while keeping -b as is:

		("b", "Option b", cxxopts::value<std::vector<std::string>>())
		("c", "Option c", cxxopts::value<std::vector<std::string>>()->multitoken())

will now cause option -c to take values until the next option is given:

$ ./main -b 1 2 -c 3 4 -a 5
a: yes
b: 1
c: 3 4
unmatched: 2 5

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This change is 

[jarro2783]

I'll have to get back to you on this one, I haven't forgotten it, but I can't comment further right now.

[jarro2783]

I wonder if it is worth putting some help text in to let users know that something is a multitoken option. Other than that this looks good.

[patrikhuber]

Missing multitoken support is also the last bit preventing me from completely ditching boost program_options and switching to cxxopts fully. Would be really nice to have this.

I'm assuming this would also work with cxxopts::value<std::vector<float>>()->multitoken()?

[maickrau]

@jarro2783 I was just going by the Boost implementation which doesn't change the help text for the parameter, but if you would like to add a help text that's fine by me since it shouldn't affect backwards compatibility.

@patrikhuber Yes, float works as well. Multitoken options use the same parse_option method as other options so I think it should work for all types.

[jarro2783]

I'm not too fussed, I thought it might be good so that it is immediately obvious to the users of your program.
I'll start with this PR and we can always add to it later.

[jarro2783]

Can you write a unit test?