structopt

It's this: https://github.com/p-ranav/structopt/blob/master/include/str... mashed into a single header with some other stuff by a build tool, I think. That looks like something that was proposed for inclusion in boost a long time ago. I think it eventually got accepted after some renaming :) The amount of preprocessor magic in that file makes my head hurt.

A nice way to declare command line options and arguments. I expected already templates, but some of the generated code looks strange: static VISIT_STRUCT_CONSTEXPR const int max_visitable_members = 69; https://github.com/p-ranav/structopt/blob/master/single_incl...

It requires some digging into the project, because at first glance it doesn't seem to be a simple header-only library. There's a conanfile.py, and a CMakeLists.txt and all sorts of "junk" in the project directory. But in the end it all seems to come down to this single header file which only depends on C++ stdlib headers and which can be dropped into your own project (which seems to be generated (aka "amalgamated") from multiple smaller header files): https://github.com/p-ranav/structopt/tree/master/single_incl... So for typical C++ libraries, this seems quite straightforward to integrate. Usually when looking at a project directory full of random files like this, it's a lot more complicated.