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Microsoft KB Archive/249045

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Knowledge Base


Article ID: 249045

Article Last Modified on 7/5/2005



APPLIES TO

  • Microsoft Visual C++ 6.0 Enterprise Edition
  • Microsoft Visual C++ 6.0 Professional Edition
  • Microsoft Visual C++ 6.0 Standard Edition



This article was previously published under Q249045

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 that this is a bug in the Microsoft products that are 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
                

Keywords: kbbug kblangcpp kbtemplate kbcpponly kbpending kbcompiler KB249045