> Any better than the two-file version:
> #define LIB_KNOB 123
> #include "lib.c"
See, now you have 3 files, the .h/.c file pair of the library, and your own implementation source file with the configuration which includes a .c file. How is this better than having two files?
> Too vague. How is it better, concretely?
I can only assume from that question that you haven't had the "pleasure" to work much with C/C++ build systems yet, at least when it comes to integrating 3rd party libraries.
Since the implementation of the base library is in the same compilation unit as the extension library, the extension library has direct access to the private implementation details of the base library without having to add a separate 'private but actually public API'.
> no clear distinction between declarations and definitions
STB-style libraries have this clear distinction, C++ style header-only libraries don't.
> See, now you have 3 files, the .h/.c file pair of the library, and your own implementation source file with the configuration which includes a .c file.
The third source file is what the library authors suggested; I'd just create a seperate Makefile target for the implementation, which is then linked into the binary. (Yes, that's an extra target, but that costs very little and allows you to avoid unnecessary rebuilds.)
Why don't you give me a concrete counterexample where the single-header-setup makes things simpler? If it's so obviously beneficial it shouldn't be so hard to come up with a concrete example.
And you've saved a line (and removed the need for obscure preprocessor macros).
> ... without having to add a separate 'private but actually public API'.
Personally, I'd say this is exactly the right way to model this, but even if you disagree: it sounds like this simplifies things mainly for the library/extension authors, and not the library users, because if the extension author forward-declares the implementation details they depend on, there is no additional file to manage for the users either.
> Too vague. How is it better, concretely?
I can only assume from that question that you haven't had the "pleasure" to work much with C/C++ build systems yet, at least when it comes to integrating 3rd party libraries.
Re. extension headers:
Since the implementation of the base library is in the same compilation unit as the extension library, the extension library has direct access to the private implementation details of the base library without having to add a separate 'private but actually public API'.> no clear distinction between declarations and definitions
STB-style libraries have this clear distinction, C++ style header-only libraries don't.