Elegant solution to implementing c++ templates

Question

Inspired by this 2009 question

Background: I'm currently working on a small c++ project and decided to try my hand at creating my own templated classes. I immediately ran into a dozen linker errors.

As it stands, my understanding is that template specializations aren't generated until they absolutely need to be, and this implies that the implementation of a templated class must be either inlined, or accompanied by an explicit instantiation at the bottom. (Why one implies the other I'm not so sure)

Question: Why is that so? Is there something special about the order of compilation that makes it impossible for a compiler to instantiate the template on-demand if it is implemented in a separate .cpp file? In my mind the header and the implementation were simply appended together.

Additionally, the question I linked above was initially posted more than ten years ago, and some comments note that the c++-faq quote mentioned is out of date, so I was wondering if newer standards support solutions that enable both separate header/implementation files and implicit instantiation.

Answer 1

Why is it so? As templates compiles through two phases in first phase compiler checks mostly for syntactical errors. If there is no error found in your template is legal to be used, but at this stage compiler do not generate any code for it. And in the second phase compiler will generate the code for all the class members function, of templated functions you used.

Because templates are evaluated at compile time. So what happens when compiler compiles it? For example if you defined a template in templated.hpp file and its implementation in implementation.cpp file. Compiler compiles each file separately into an object and then linker link them together. As templates are evaluated at compile time so compiler need its implementation at compile time, which is not available if you are having it in different implementation file. So linkers complains to you that I could not find implementation for type T for your this template. This all happens at compile time.

So far until C++20 or even C++23 templates are still needed to be evaluated at compile time albeit C++ has added new concept modules, I am not sure it can be used this way, but you can read about it here.