why we need dynamic_cast

[agoswami]

Hi,
I got some understanding about dynamic_cast but could anyone provide me any pratcial scenario where we need dynamic_cast and we do not have other alternative except dynamic_cast

[quirkyusername]

I don't think there is a scenario where we MUST use dynamic_cast, it just gives us the ability to check the validity of the cast, whereas using static_cast we can't know for certain whether it will succeed or not.

[kbw]

dymamic_cast allows for a typesafe downcast.

Consider these classes:

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

class Derived1 : public Base
{
    char c;
public:
    virtual ~Derived1();
};

class Derived2 : public Base
{
    long l;
public:
    virtual ~Derived2();
};

You can always do this:

    Derrived1* d1 = new Derived1;
    Base* b = d1;  // upcast is ok because Derived1 is a Base. 

But you can't do this.

    Derrived1* d1 = new Derived1;
    Base* b = d1;  // upcast is ok because Derived1 is a Base.
    Derived1* x1 = b;  // downcast is unsafe and fails.
    Derived2* x2 = b;  // downcast is unsafe and fails. 

You can force it, but it's unsafe:

    Derrived1* d1 = new Derived1;
    Base* b = d1;  // upcast is ok because Derived1 is a Base.
    Derived1* x1 = static_cast<Derived1*>(b);  // downcast is unsafe, but happens to be ok
    Derived2* x2 = static_cast<Derived1*>(b);  // downcast is unsafe and is in fact incorrect 

You can do a cast safely with dynamic cast because it uses RuntTime Type Info (RTTI) to determine if the cast is valid.

    Derrived1* d1 = new Derived1;
    Base* b = d1;  // upcast is ok because Derived1 is a Base.
    Derived1* x1 = dynamic_cast<Derived1*>(b);  // x1 is d1
    Derived2* x2 = dynamic_cast<Derived1*>(b);  // x2 is null 

I can't think of a credible example, but suppose you have a function that draws shapes on a device, but there's some optimised method for printing squares...

bool draw(Device* pDevice, Shape* pShape)
{
    if (PrintDevice* pPrintDev = dynamic_cast<PrintDevice*>(pDevice))
        if (Square* pSquare = dyanamic_cast<Square*>(pShape))
            return pSquare->Print(pPrintDev);

    return pShape->Draw(pDevice);
}