When should I use -inl.h files?

Question

I just noted this item in the Google C++ Coding Style Guide - and I don't quite get it.

If I put an inline method or function in a file other than the header included by other files, it will not be a method of the class; and it will only be usable to bits of code which include it. So why even have such -inl.h files at all?

Also, why do we even want to inline long functions anyways? (i.e. other than in the case of templates, when we have to put the code in the header files for instantiation)

Answer 1

I just noted this item in the Google C++ Coding Style Guide - and I don't quite get it.

Do take that guide with a pinch of salt. Many of the guidelines are intended to help interact with Google's legacy codebase, and aren't particularly good advice for general C++ development.

So why even have such -inl.h files at all?

There's no particularly good reason; I don't do that myself. Some people like them because it minimises the amount of stuff in the main header file, which users of the header generally want to read, and separates out the implementation details, which they generally don't care about.

Also, why do we even want to inline long functions anyways?

Sometimes, we must: template definitions must be available in any translation unit that instantiates the template, so they (usually) need to be in headers.

Sometimes, we want to: by implementing a function inline in a header, we don't have to worry about building and linking a separate translation unit for it. This can make it more convenient to distribute a library; possibly at the cost of longer build times.