Q249045: BUG: C2964 On Pointer-To-Member As Template Argument

Article: Q249045
Product(s): Microsoft C Compiler
Version(s): winnt:6.0
Operating System(s): 
Keyword(s): kbtemplate kbCompiler kbCPPonly kbLangCPP kbVC600 kbVC600bug
Last Modified: 03-MAY-2001

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), included with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SYMPTOMS
========

The compiler generates an error such as the following when a pointer-to-member
is used as a class template parameter.

  error C2964: invalid expression as template parameter

CAUSE
=====

This C++ language feature has not yet been implemented in Visual C++.

RESOLUTION
==========

To not use the pointer-to-member as a template argument, pass it as a parameter
to the class constructor.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

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

Use the following sample code to reproduce the behavior:

  #ifdef BUG

  struct MyClass {
     char a,b;
     MyClass() : a('a'), b('b') {};
  };

  template<char MyClass::*member_ptr> 
  class AnotherClass {
  private:
     MyClass m_var;
  public:
     char value() { return m_var.*member_ptr; }
  };

  int main()
  {
     AnotherClass<&MyClass::a> var1;
     AnotherClass<&MyClass::b> var2;	//<<<< Causes compiler error.
     return 0;
  } 

  #else

  // WORKAROUND
  struct MyClass {
     char a,b;
     MyClass() : a('a'), b('b') {};
  };

  template<typename MbrType, typename ClassType> 
  class AnotherClass {
  private:
     MyClass m_var;
     MbrType ClassType::*member_ptr;
  public:
     AnotherClass(MbrType ClassType::*mem_ptr) : member_ptr(mem_ptr) {}
     char value() { return m_var.*member_ptr; }
  };

  int main()
  {
     AnotherClass<char,MyClass> var1(&MyClass::a);
     AnotherClass<char,MyClass> var2(&MyClass::b);
     return 0;
  } 

  #endif

Additional query words:

======================================================================
Keywords          : kbtemplate kbCompiler kbCPPonly kbLangCPP kbVC600 kbVC600bug 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : winnt:6.0
Issue type        : kbbug
Solution Type     : kbpending

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