BUG: C2440 Expanding Template with a Pointer to a Nested Class

• The Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 4.2, 5.0

SYMPTOMS

At the point where a class template is expanded using a pointer to a nested class, the compiler reports the following errors (generated by the sample code listed in this article):

   [FileName](14) : error C2440: 'initializing' : cannot convert from
   'class A::B' to 'class A::B *' (new behavior; please see help)

   [FileName](14) : error C2439: 't' : member could not be initialized

   template <class T> C<T>::C() : t(T())  // error
   {

RESOLUTION

There are three possible workarounds to this problem. Each is stated here in terms of the sample code included in this article:

• Remove t(T()) from the initializer list and add t = T(); to the body of the constructor. This workaround assumes there is an operator= defined for class T.

  -or-

• Change C<A::B *> c; to C<A::B&> c; or C<A::B> c:

  -or-

• Change the definitions of class A and class B to that B is no longer nested in the definition of A.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

In this case, the problem is caused by the attempt to initialize the data member t.

Sample Code

   /* Compile options needed: None */
   template <class T> class C
   {
   public:
      C();
   private:
      T t;
   };

   template <class T> C<T>::C() : t(T())  // error
   {
   }

   class A
   {
   public:
      class B
      {
      };
   };

   main()
   {
      C<A::B *> c;
      return 0;
   }

Keywords          : CLIss vcbuglist400 vcbuglist500 kbprg
Version           : 4.0 4.1 4.2 5.0
Platform          : NT WINDOWS
Issue type        : kbbug