Drop const from named() and positional() (or have it both ways?)

[xparq]

const std::vector<std::string>& positional() const { return unnamed_params; }
//! Note: std::map[key] would want to create a key if not yet there,
//! so named()[...] would fail, if const! But constness it is... (for consistency).
//! Use the op() and op[] accessors for direct element access!
const std::map<std::string, std::vector<std::string>>& named() const { return named_params; }

It's pretty orthodox to have them always const. I mean it's yours, change it all you want! :) Especially for #54!...

I can imagine use cases where a pristine command-line, safe from tampering, is nice to have, but as soon as you have the modifying methods, the same object is no longer tamper-proof anyway: just having const accessors, too, won't help. ;)
So, just copy your command-line to a const object if you want a safe reference.

Going with non-const only... (You'll get your compilation errors all right when trying to actually use those methods on a const obj.)
-> Err... OK, but then... No... Other const accessors do call these, so I still need the const pairs after all, too... :-/