Difference between member functions for a template class defined inside and outside of the class

Question

Is there a difference between defining member functions for a template class inside the class declaration versus outside?

Defined inside:

template 

        

Answer 1

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defining a member function outside of its template

It is not ok to provide the definition of a member function of a template class like this:

 // This might be in a header file:
 template 
 class xyz {
    void foo();
  };

// ...

 // This might be later on in the header file:
  void xyz::foo() {
// generic definition of foo()
   }

This is wrong for a few reasons. So is this:

      void xyz::foo() {
         // generic definition of foo()
      }

The proper definition needs the template keyword and the same template arguments that the class template's definition was declared with. So that gives:

       template 
          void xyz::foo() {
           // generic definition of foo()
                 }

Note that there are other types of template directives, such as member templates, etc., and each takes on their own form. What's important is to know which you have so you know how to write each flavor. This is so especially since the error messages of some compilers may not be clear as to what is wrong. And of course, get a good and up to date book.

If you have a nested member template within a template:

    template 
      struct xyz {
      // ...
      template 
       struct abc;
        // ...
       };

How do you define abc outside of xyz? This does not work:

     template 
    struct xyz::abc { // Nope
      // ...
  };

nor does this:

 template 
 struct xyz::abc { // Nope
// ...
 };

You would have to do this:

     template 
       template 
           struct xyz::abc {
            // ...
           };

Note that it's ...abc not ...abc because abc is a "primary" template. IOWs, this is not good:

// not allowed here: template template struct xyz::abc { };