Q129783: FIX: typedef Class Template Causes Compiler Errors C2440/C2561

Article: Q129783
Product(s): Microsoft C Compiler
Version(s): 2.00 2.10
Operating System(s): 
Keyword(s): kbcode
Last Modified: 23-JUL-2001

-------------------------------------------------------------------------------
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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