If you think about what templates are, it's not hard to understand why they must go completely in the header files. It's literally a source code template. It's

If you think about what templates are, it's not hard to understand why they must go completely in the header files. It's literally a source code template. It's a set of instructions to generate code at compile time, depending on the template parameters, hence 100% of the source code must be available at the point of instantiation. Just the interface is not enough.