1 minute read

I was implementing a template function inside a class. As usual, I wrote the prototype in .h file and the definition in a .cpp file. This templated method is then called in other part of the software. When I hit the compile button, the compiler complained about an undefined reference where I called the template function.

Since the function was called from a pointer of the class, my first try was to force the compilation of the the template function by defining a specialization of it in the .cpp file. I compiled again but the error remained.

After a quick search on web I found the solution: defining the template function immediately in the .h file because the Stackoverflow user’s told that template function declaration and definitions must stay in the same translation unit.

A translation unit is the result of the preprocessing stage and so the very first for the compiler. A translation unit consists of a source file where #include directives are literally included and macro are expanded. Now it is clear why it was not working. In my main.cpp I was including just the .h file of the class so the translation unit of main.cpp did not contain the definition of the function template.

However, I wanted a deeper insight about function templates and indeed isocpp FAQ has a huge section focused only for this kind of error. It says that templates are not functions or classes but just a pattern used by the compiler to generate a family of functions and classes. C++ compiler is based on the separate compilation model, that is while it is compiling one .cpp file, it is not required to remember the details of another .cpp file. This is why it complained when I tried to separate the template function from its definition.

Lesson learned. I conclude with these two useful rules:

  1. Define template functions immediately after their declaration.
  2. When C++ compiler is compiling a .cpp file, it is not required to use details of another .cpp file.

Resources

  1. Wikepedia definition of translation unit
  2. tutorialspoint brief definition of translation unit
  3. isocpp FAQ
  4. Official C++ reference for function templates

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