Split cxxopts.hpp into multiple files

[eyalroz]

One can observe a decomposition of cxxopts.hpp into several parts:

• non-option-specific string-related code
• Exception-related code
• the values namespace and the Value class
• KeyValue, Option, OptionParser and related code
• ParseResult and some Option/OptionParser implementations which need ParseResult
• Options, OptionAddr, help

And the above is also the order of dependencies.

Or, actually, maybe we could take this another step and have one file per (non-trivial) class, so separate files for Option, Option Adder, Options, OptionParser etc.

So, how about we split the huge single file into a file for each of these? With cxxopts.hpp remaining as an "includer" of all the others?

This will:

• Improve source code readability
• Make it easier to merge changes
• Make the revision history easier to understand (when looking at the affected files)

What do you think, @jarro2783 ?

[jarro2783]

I always wanted this to be single header to just drop into a project, and it still doesn't seem huge yet. If there was a nice way to generate a single header then I think it could be broken up.

[eyalroz]

@jarro2783 : Why is it important to be able to drop a single file rather than a subfolder?

Anyway, to create a single header, you run:

quom cxxopts.hpp cxxopts_single.hpp

and you get quom with:

pip3 --user install quom

See also: https://stackoverflow.com/a/62218074/1593077

[jarro2783]

Yeah I guess having multiple files is not so bad. I'll look at this after some of the other changes because it will mess up PRs.

[eyalroz]

So, yeah, the different PRs tend to clash. When you decide which of them you like, I'll rebase them onto each other so that you can merge.