Q148808: FIX: C2670: Template Function Cannot Convert Parameter

Article: Q148808
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbcode kbCompiler kbCPPonly kbVC
Last Modified: 03-AUG-2001

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), included with:
   - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, version 4.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

The following error is generated:

  error C2670: 'AFunction' : the template function cannot convert parameter 1
  from type 'class Derived<int> *'

in code containing all of the following:

- A template class 'Derived' from another template class 'Base'.

- 'AFunction' taking a pointer to the 'Base' as a parameter.

- A call to the 'AFunction' passing a pointer to 'Derived' before instantiating
  any object of that type.

See the sample code in this article for an example.

RESOLUTION
==========

To work around this problem, either explicitly instantiate 'Derived' or create a
dummy object of type 'Derived' before calling the 'AFunction'.

See the Sample Code for an example.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This problem was corrected in Microsoft Visual C++,
32-bit Edition, version 4.1.

MORE INFORMATION
================

Sample Code
-----------

  /* Compile options needed: None
  */ 

  template <class T>
  class Base
  {
  public:
     T data;
     int i;
  };

  template <class T>
  class Derived : public Base<T>
  {
  public:
     int j;
  };

  template <class T>
  void AFunction(Base<T>* b)
  {
     return;
  }

  void main()
  {
     // Uncomment one of the following two statements to work around the
     // problem. If the template class definition is in a header file,
     // you may want to include one of the following two statements at
     // the end of that file.

     // template Derived<int>;
     // Derived<int> dummy;

     Derived<int> *j;

     AFunction(j);   // Gives C2670 here
  }

Additional query words: 4.00

======================================================================
Keywords          : kbcode kbCompiler kbCPPonly kbVC 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : winnt:
Issue type        : kbbug
Solution Type     : kbfix

=============================================================================

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