Question

I was (and have been for a long time) under the impression that you had to fully define all template functions in your .h files to avoid multiple definition errors that occur due to the template compilation process (non C++11).

I was reading a co-worker’s code, and he had a non-template class that had a template function declared in it, and he separated the function declaration from the function definition (declared in H, defined in CPP). It compiles and works fine to my surprise.

Is there a difference between how a template function in a non template class is compiled, and how a function in a template class is compiled? Can someone explain what that difference is or where I might be confused?

Answer 1

The interesting bit is how and when the template gets instantiated. If the instantiations can be found at link time, the template definition doesn’t need to be visible in the header file.

Sometimes, explicit instantiations are cause like this:

• header :

  struct X { 
      // function template _declaration_
      template <typename T> void test(const T&);
  };
  

• cpp:

  #include "X.h"
  
  // function template _definition_:
  template <typename T> 
      void X::test(const T&)
  {
  }
  
  // explicit function template _instantiation(s)_:
  template X::test<int>(const int&);    
  template X::test<std::string>(const std::string&);
  

Using this sample, linking will succeed unless uninstantiated definitions of the template are used in other translation units