[noobermin]

Hijacking your comment to ask my question below. Apart from the possibility you can't do header-only for everything (like something that requires a singleton for example) and the other known issues (build times, ballooning of object files and thus executables) is the only other reason you don't see as many header-only libraries in C (like you do in C++ where they are ubiquitous) is that people merely don't do it in C? That is, a social landscape reason, not a technical reason.

[Fronzie]

It's partly a historical reason as well. There are many projects with very complex Makefiles. Those are hard to integrate into your own project.

Header-only is the other extreme: No makefile at all. With package managers like Conan and vcpkg, using a build-system like e.g. CMake, it's possible to have very simple and short project files which are easy to integrate.

In this regard, C and C++ are a bit behind the times compared to other languages.

[tonyedgecombe]

Isn't C++ supposed to be getting proper namespaces soon?

[throwaway8941]

I am not an expert in C++, but there exists an opinion that it will not help as much as you might think.

https://cor3ntin.github.io/posts/modules/

[flohofwoe]

The solution for the "singleton problem" is to put the implementation code into a separate part of the header behind an #ifdef XXX_IMPL, and only define this before including the header in one place in the project.

This is also the approach used by this library, see:

https://github.com/jeremycw/httpserver.h#example

This approach also doesn't have the 'build time problems', since the (expensive to compile) implementation code is only seen once by the compiler (unlike many C++ header-only libs, which rely on inline code)

[nestorD]

The fact that you cannot define templates out of headers also explains why C++ has a lot of header-only libraries. : if you use templates and have no global references, you are most likely already header-only.

[bluGill]

Yeah, templates forces many C++ libraries to be header only. I suspect this came first, and only after a c++ header-only (because they had to be) libraries existed did people realize there were advantages to header only and so people wrote them where they wouldn't have to.

[fulafel]

Yes, the social reasons are the only ones, besides the technical reasons :)

An additional technical downside in this case is the code complexity in the client code, from the required modal preprocessor flags.