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)
This is often done for long function templates. The regular header my_functions.h only contains the declarations, and the implementation file my_functions-inl.h contain the implementations. The reason is that function templates cannot be put in .cpp files. Note that the X.h file includes the X-inl.h file, not the other way around.
Other libraries have different naming conventions: e.g. some of the Boost libraries use .hpp for template headers and .ipp for template implementation files.