Question

Please,look at this simple class:

class TernaryPolynomial;
class IntegerPolynomial;

class DenseTernaryPolynomial:public IntegerPolynomial,public TernaryPolynomial
{
 public:
 DenseTernaryPolynomial();
static DenseTernaryPolynomial generateRandom(int,int,int);
};

Can you please explain me why does the compiler complains that TernaryPolynomial must be a previously defined class or struct? I thought it shouldn’t care at all since i put a forward declaration of that class.
Here’s the TernaryPolynomial class

class Polynomial;
class IntegerPolynomial;

class TernaryPolynomial:public Polynomial
{
 public:
 TernaryPolynomial();
 virtual ~TernaryPolynomial();
 virtual IntegerPolynomial toIntegerPolynomial() = 0;
 };

Answer 1

For derivation a forward declaration is not enough, you need to include the full class definition.

Forward declarations can only be used when you define a pointer or a reference to the class as a member or method parameter. As soon as you really begin using things (by dereferencing, accessing a member over a reference or deriving for example) you need to include the whole class definition.