Compiler Error C2036

'class-key' : unknown size

The address of the specified undeclared identifier was used.

The size of an undeclared object cannot be used.

The following is an example of this error:

struct A* pA;
struct B { int i; };
B* pB;
void main()
{
   pA++;   // error, size of A not known
   pB++;   // OK, B has been declared
}