What is qobject_cast?

Question

Could someone explain in as simple terms as possible (or as simple as you would like) what qobject_cast is, what it does and why we would need to cast one class type to another?

Like, I get typecasting in the sense of casting an int as a char or QString maybe to use QMessageBox, but why cast into different classes?

Answer 1

Before you start learning what qobject_cast is, you would need to know what C++'s dynamic_cast is. Dynamic cast is all about polymorphism.

C++'s dynamic cast uses RTTI (Run Time Type Information) to cast an object. But qobject_cast does this without RTTI.

What is dynamic cast?

For example suppose we've got a car factory function. Like this:

Car* make_car(string brand){
    if(brand == "BMW"){
        return new BmwCar;
    }
    if(brand == "Audi"){
        return new AudiCar;
    }
    return nullptr;
}

Note that BmwCar and AudiCar classes inherit Car class. Using this function we can make different cars only using one function. For example:

string brand;
cin >> brand;
Car *car = make_car(brand);

BmwCar *bmw = dynamic_cast<BmwCar*>(car);
if (bmw != nullptr) {
    cout << "You've got a BMW!";
}

AudiCar *audi = dynamic_cast<AudiCar*>(car);
if (audi != nullptr) {
    cout << "You've got a Audi!";
}

Without dynamic_cast you won't be able to determine if car is a BmwCar or an AudiCar.

What is different between dynamic_cast and qobject_cast?

• qobject_cast can only be used with QObject derived classes having Q_OBJECT macro.

• qobject_cast doesn't use RTTI.