When to put C++ function in header file

Question

I've been looking at Boost and various other C++ libraries. The vast majority of Boost is implemented in header files.

My question is: under what conditions do you do a header-only implementation (like Boost) or also include a .cpp file?

Answer 1

If you want to use a template in another translation unit (i.e. another source file), you should (almost always) define it in the header file. (There are exceptions, like the comments below point out, but IMHO this is a good rule of thumb.)

Same applies if you want to use an inline function from another translation unit.

Otherwise you should put the implementation into a separate .cpp file to minimize dependencies.