FIX: C2670: Template Function Cannot Convert Parameter (148808)

SYMPTOMS

The following error is generated:

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

in code containing all of the following:

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.

See the sample code in this article for an example.

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
}