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 b

Answer 1

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.