.inl files are never mandatory and have no special significance to the compiler. It’s just a way of structuring your code that provides a hint to the humans that might read it.
I use .inl files in two cases:
- For definitions of inline functions.
- For definitions of function templates.
In both cases, I put the declarations of the functions in a header file, which is included by other files, then I #include the .inl file at the bottom of the header file.
I like it because it separates the interface from the implementation and makes the header file a little easier to read. If you care about the implementation details, you can open the .inl file and read it. If you don’t, you don’t have to.