Q149327: FIX: C2440 Expanding Template with a Pointer to a Nested Class

Article: Q149327
Product(s): Microsoft C Compiler
Version(s): 
Operating System(s): 
Keyword(s): kbVC400bug kbVC410bug kbVC420bug kbVC500bug kbVC600fix
Last Modified: 30-JUL-2001

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), used with:
   - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
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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

Lines 13 and 14 in the sample code are:

  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.

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

- 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.

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;

  }

Additional query words:

======================================================================
Keywords          : kbVC400bug kbVC410bug kbVC420bug kbVC500bug kbVC600fix 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Issue type        : kbbug
Solution Type     : kbfix

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

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