A way to require "optional" arguments to present

[zhihaoy]

Currently it's very inconvenient to make an option in the form of -o output required. It is not checked if it does not present, if you get("-o") you get a logic error, which requires lots of post processing. There should be a way to declare that this argument must present. If not presents, exception; if only -o presents, status quo (a different exception).

Also, the current situation makes -o equivalent to not presenting if there is a default_value. I think it's not a very good interface. I tried to simulate a "all or nothing" default value with .implicit_value(x).nargs(1), but no luck.

[wtdcode]

Same here.

Take gcc as an example.

/tmp $ echo "" > 1.c
/tmp $ gcc 1.c -o
gcc.exe: error: missing filename after '-o'

I will try to draft a PR if I am free.

[wtdcode]

See #30.

So you can write like

	program.add_argument("-o", "--output")
		.required()
		.help("specify the output file.");

or with a default value

	program.add_argument("-o", "--output")
		.default_value(std::string("-"))
                .required()
		.help("specify the output file.");

the program will throw an exception if no value is provided.

$ ./test.exe -o
error: -o: no value provided.

Give it a try. :)

[wtdcode]

To avoid break changes, I rewrite my PR.

See #33 for details.