DOCUMENT:Q129783  23-JUL-2001  [visualc]
TITLE   :FIX: typedef Class Template Causes Compiler Errors C2440/C2561
PRODUCT :Microsoft C Compiler
PROD/VER:2.00 2.10
OPER/SYS:
KEYWORDS:kbcode

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

 - The C/C++ Compiler (CL.EXE), included with:
    - Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1 
-------------------------------------------------------------------------------

SYMPTOMS
========

Compiling a program that has all of the following in the code causes the
compiler to generate two errors:

 - Class A.

 - Class B derived from class A.

 - Template class C that has type T as its argument.

 - Class C has a member function that returns a type of A*.

 - Type T* value in the return statement of the function.

 - Function implementation inside the class C definition.

 - "typedef C<B> CB" before the class B definition.

Here are the two errors generated:

   

   error C2440 : 'return' : cannot convert from 'class B*' to
                            'class A*'

-and-

   

   error C2561 : 'f' : function must return a value

CAUSE
=====

The class template was instantiated within the typedef statement where the class
B definition (the derivation) has not been seen by the compiler yet. Therefore
no conversion is defined between the derived class and the base class.

RESOLUTION
==========

Use any one of the following three workarounds:

 - Do not create a typedef based on class B until class B is defined.

-or-

 - Due to the definition of the member function, the templated class C can only
   be instantiated with class T if a conversion from T* to A* exists. This may
   have been unintentional, and the design of class C could be re-thought.

-or-

 - Do not define the class template member function inside the class definition.
   The following sample code demonstrates this workaround:

      /* Compile options needed: none
      */ 

      class A {};

      template < class T >
      class C
      {
        public:
         A* f( T *t );
      };

      template < class T >
      inline A* C::f( T *t )
      {
        return t;
      }

      class B;
      typedef C<B> CB;

      class B : public A {};

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This problem was fixed in Microsoft Visual C++,
32-bit Edition, version 4.0.

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

Sample Code to Reproduce Problem
--------------------------------

   /* Compile options needed: none
   */ 
   class A {};
   template < class T >
   class C
   {
     public :
      A* f( T *t ) { return t; }
   };
   class B;
   typedef C<B> CB;
   // Work around #1: Move the following line, the definition of class B,
   // before the typedef which depends on class B
   class B : public A {};

Additional query words: 2.00 2.10 9.00 9.10

======================================================================
Keywords          : kbcode 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : 2.00 2.10
Issue type        : kbbug
Solution Type     : kbfix

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

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