Even if you create non-inline function definitions, you’ll usually want to put all declarations and definitions for a template into a header file. This may seem to violate the normal header file rule of “Don’t put in anything that allocates storage,” (which prevents multiple definition errors at link time), but template definitions are special. Anything preceded by template<...> means the compiler won’t allocate storage for it at that point, but will instead wait until it’s told to (by a template instantiation), and that somewhere in the compiler and linker there’s a mechanism for removing multiple definitions of an identical template. So you’ll almost always put the entire template declaration and definition in the header file, for ease of use.

There are times when you may need to place the template definitions in a separate cpp file to satisfy special needs (for example, forcing template instantiations to exist in only a single Windows dll file). Most compilers have some mechanism to allow this; you’ll have to investigate your particular compiler’s documentation to use it.

Some people feel that putting all of the source code for your implementation in a header file makes it possible for people to steal and modify your code if they buy a library from you. This might be an issue, but it probably depends on the way you look at the problem: Are they buying a product or a service? If it’s a product, then you have to do everything you can to protect it, and probably you don’t want to give source code, just compiled code. But many people see software as a service, and even more than that, a subscription service. The customer wants your expertise, they want you to continue maintaining this piece of reusable code so that they don’t have to – so they can focus on getting their job done. I personally think most customers will treat you as a valuable resource and will not want to jeopardize their relationship with you. As for the few who want to steal rather than buy or do original work, they probably can’t keep up with you anyway.