FIX: C2670: Template Function Cannot Convert Parameter

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

SYMPTOMS

The following error is generated:

error C2670: 'AFunction' : the template function cannot convert parameter 1 from type 'class Derived<int> *'

• A template class 'Derived' from another template class 'Base'.



• 'AFunction' taking a pointer to the 'Base' as a parameter.



• A call to the 'AFunction' passing a pointer to 'Derived' before instantiating any object of that type.

RESOLUTION

To work around this problem, either explicitly instantiate 'Derived' or create a dummy object of type 'Derived' before calling the 'AFunction'.

See the Sample Code for an example.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem was corrected in Microsoft Visual C++, 32-bit Edition, version 4.1.

MORE INFORMATION

Sample Code

/* Compile options needed: None
*/ 

template <class T>
class Base
{
public:
   T data;
   int i;
};

template <class T>
class Derived : public Base<T>
{
public:
   int j;
};

template <class T>
void AFunction(Base<T>* b)
{
   return;
}

void main()
{
   // Uncomment one of the following two statements to work around the
   // problem. If the template class definition is in a header file,
   // you may want to include one of the following two statements at
   // the end of that file.

   // template Derived<int>;
   // Derived<int> dummy;

   Derived<int> *j;

   AFunction(j);   // Gives C2670 here
} 

Additional query words: 4.00

Keywords : kbcode kbCompiler kbCPPonly kbVC
Version : winnt:
Platform : winnt
Issue type :