Conference turris::c_plus_plus

3481.0. "Deriving nested class from template don't automatically instantiate it" by NETRIX::"[email protected]" (Goedtler Peter) Mon Mar 10 1997 09:56

Hi,

we are working on a C++ project at a customer and we are often using
templates.
Beside all other troubles when providing a class library with templates for 
other engineering teams, the newest 'feature' occurs with nested template
classes:
The template class Enclosed<T> contains the class Nested which inherits from 
another template class Super<T>. The super class defines a pure virtual 
function trouble() which has to be implemented by the subclasses (here:
Nested).
Unfortunately, it does not work implementing the function unless it is
real inline.


//Template superclass for the nested class below
template<class T>
class Super
{
 public:
  Super() {};

  virtual void trouble()=0;
};


//
// Template class with a nested class
//
template<class T>
class Enclose
{
  public:
    Enclose() {};

    class Nested : Super<T>
    {
      public:
        Nested() : Super<T>() {};

	// Does not work 
        void trouble();

	// This would work, because it's inline     
        // void trouble() { cout << "No trouble" <<endl;};
    };
};

//
// Does not work
//
template<class T>
void Enclose<T>::Nested::trouble()
{ cout << "Trouble" << endl; }

//
// Test class used for template instantiation
//
class Hello
{
 public:
  Hello(){};
};


main()
{
   Enclose<Hello> hi; 
}

We are using  DEC C++ 5.5-010 on Digital UNIX (Alpha) 4.0. I compiled and
linked with:

cxx -o test_nested test_nested.cxx

ld:
Unresolved:
 __vtbl_16Enclose__T5Hello6Nested  Make sure that the first noninlined virtual
member function of 'Nested' is defined.


Do we need to implement all methods of nested template classes inline??
I don't hope.


Thanks
  Peter

  
[Posted by WWW Notes gateway]

I've logged your problem.
				/AHM

fixed by v6.0