FIX: typedef Class Template Causes Compiler Errors C2440/C2561 (129783)

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-
-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-

-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 {};