Question

Normally when I create a class, I create a header and a source for that class. I have heard that with a template class, you have to put the function implementation in the header. I tried doing it both ways, and got compilation errors the first way. The second way worked fine. However, I like to organize my code into headers and source files, so is it possible to put the function implementations into a source file? (Maybe it requires special compilation flags or syntax?) Or should I just keep em in the header?

Thanks!

Answer 1

Generally, all template code must be in a header file since the compiler needs to know the complete type at the point of instantiation.

As Aaron says below it is possible to put the implementation details in a .cpp-file in the specific case where you know on before hand all possible types the template will be instantiated with and explicitly instantiate it with those types. You’ll then get a linker error if the template gets instantiated with another type somewhere in your code.

A quite common general solution to at least visually separate interface from implementation is to put all implementation in a .inc (or .tcc or .ipp)-file and include it at the end of the header file.

Note that the syntax for putting template class members outside the class-definition (whether you use the specific solution or the general) is slightly cumbersome. You’ll need to write:

// in test.h
template <class A>
class Test
{
public:
  void testFunction();
};

#include "test.inc"

// in test.inc    
template <class A>
void Test<A>::testFunction()
{
  // do something
}