Realize virtual constructor through clone() function.
Textblock *a=previousObject.clone();
class NLComponent { public:
// declaration of virtual copy constructor
virtual NLComponent * clone() const = 0;
...
};
class TextBlock: public NLComponent {
public:
virtual TextBlock * clone() const // virtual copy
{ return new TextBlock(*this); } // constructor
...
};
class Graphic: public NLComponent {
public:
virtual Graphic * clone() const // virtual copy
{ return new Graphic(*this); } // constructor
...
};
Notice that the above implementation takes advantage of a relaxation in the rules for virtual function return types that was adopted relatively recently. No longer must a derived class's redefinition of a base class's virtual function declare the same return type. Instead, if the function's return type is a pointer (or a reference) to a base class, the derived class's function may return a pointer (or reference) to a class derived from that base class. This opens no holes in C++'s type system, and it makes it possible to accurately declare functions such as virtual copy constructors. That's why TextBlock's clone can return a TextBlock* and Graphic's clone can return a Graphic*, even though the return type of NLComponent's clone is NLComponent*.