CODEWITHSUNDEEP
c++c++11dynamic-caststatic-cast
NovoiceProgrammer
NovoiceProgrammer

Reputation: 35

Casting 1st Derived Class to 2nd Derived Class - Why does it work?

I simply created two derived class from a base class. Then created an object of a derived class and converted into base class. Then from the base class, i converted to derived class number two, I was hoping it won't work, but it did. Can someone please explain me how below code is working... And How do I prevent this from happening...

PS: Now edited the Program to make bit more sense. Also this is the situation I want to avoid if possible: 

class Animal
{
public:
    Animal() { std::cout << "I am an animal\r\n";  };
    virtual void makeSound() = 0;
    ~Animal() = default;
};

class Cat : public Animal
{
public:
    Cat() { std::cout << "I am a Cat\r\n"; };
    void makeSound() final { std::cout << "Meaow\r\n"; }
    ~Cat() = default;
};

class Dog : public Animal
{
public:
    Dog() { std::cout << "I am a Dog\r\n"; };
    void makeSound() final { std::cout << "Bark\r\n"; }
    ~Dog() = default;
};

template<typename baseType, typename derivedType>
std::unique_ptr<derivedType> dynamicConvert(std::unique_ptr<baseType> baseObj)
{
    auto tmp = dynamic_cast<derivedType*>(baseObj.get());
    std::unique_ptr<derivedType> derivedPointer;
    if (tmp != nullptr)
    {
        baseObj.release();
        derivedPointer.reset(tmp);
    }
    return derivedPointer;
}

int main()
{
    auto cat = std::make_unique<Cat>();
    auto base = dynamicConvert<Cat, Animal>(std::move(cat));
    base->makeSound();
    auto dog = dynamicConvert<Animal, Dog>(std::move(base));
    dog->makeSound();
    return 0;
}

Output: 

I am an animal
I am a Cat
Meaow
Bark

Upvotes: 1

Views: 55

Answers (1)

Quentin
Quentin

Reputation: 63114

You can't, that's part of the contract with static_cast. It is resolved at compile time, and thus will not check at runtime that you didn't make a mistake: casting to the wrong type just triggers UB.

Use dynamic_cast when you want runtime-checked conversions.

Upvotes: 3

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