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

We have

Code:

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class Derived : public Base{
};

int main(){

        Base* b = new Base;

        Derived * d = dynamic_cast< Derived *>(b);

return 1;
}

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Why the complie needs Base class has at least a virtual method?

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

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:

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

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

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

Quote:

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Originally Posted by meili100

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

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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;
}

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Example

Code:

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#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;
}

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

Quote:

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Originally Posted by Daved

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

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