Forward Declarations

From time to time we need to declare a class earlier than its definition. Consider the following example. To be able to define Object class I need to first declare String, otherwise the compiler will complain about the method that returns String.

//I need forward declaration here
class String;

class Object
{
public:
  //toString return a String object
    String toString();
};

//Now the definition comes
class String : public Object
{

};

What about forward declaration of a template class?

template <typename T>
class SmartPtr;

class Object
{
 public:
   SmartPtr<Object> clone();
};

//Now the definition comes
template <typename T>
class SmartPtr : public Object
{

};