Thread: Why dynamic_cast<> needs the class to has virtual methods?

When a class has virtual functions, there are certain pieces of special runtime data created for objects of that class. When downcasting, dynamic_cast<> needs this data in order to do what it does.

Code:

Derived * d = new Derived;
Base* b = dynamic_cast<Base*>(d);
//no problem
//no virtual function needed in class Base

In this example, you're upcasting. This is equivilant to a static_cast<>, so no virtuals are needed.

Originally Posted by meili100

Could you give me an example that using dynamic_cast to downcast and return value is not NULL?

Code:

class Fruit
{
public:
    virtual ~Fruit() { }
};

class Banana : public Fruit
{
};

int main(int argc, char **argv)
{
    Fruit *fp = new Banana;
    Banana *bp = dynamic_cast<Banana*>(fp);

    if (bp != NULL)
    {
        cout << "Banana detected!\n";
    }

    return 0;
}

Example

Code:

#include <iostream>


class CBaseClass
{
public:
	virtual void SayHello(){
		std::cout<<"Hello From The Base"<<std::endl;
	}
};

class CChildClass : public CBaseClass
{
public:
	virtual void SayHello(){
		std::cout<<"Hello From The Child"<<std::endl;
	}
};

int main()
{
	CBaseClass *pBaseChild = new CChildClass;
	CChildClass *pChild = NULL;

	//This is a correct cast since pBaseChild is actally a child class
	pChild = dynamic_cast<CChildClass*>(pBaseChild);

	if(NULL != pChild){
		pChild->SayHello();
	}//if
	else{
		std::cout<<"Child Was NULL"<<std::endl;
	}//else	

	CBaseClass *pBaseBase = new CBaseClass;
	CChildClass *pChildTwo = NULL;
	
	//This is an invalid cast because pBaseBase is actually a base class
	pChildTwo = dynamic_cast<CChildClass*>(pBaseBase);
	
	if(NULL != pChildTwo){
		pChildTwo->SayHello();
	}//if
	else{
		std::cout<<"Child Two was NULL"<<std::endl;
	}//else

	//Clean up
	delete pBaseChild;
	delete pBaseBase;

	std::cin.get();

	return 0;
}

Originally Posted by Daved

Your example needs a virtual destrctor in the base class, prog-bman.

And mine needs a delete statement. However, I used the phrase "Banana detected!" in a valid context, so it all cancels out.