CODEWITHSUNDEEP
c++c++11casting
folibis
folibis

Reputation: 12874

Why static_cast works but dynamic_cast fails?

Suppose I have a base class and 2 classed derived from that. All the derived classes has its own logic that incompatible between them. The test application is as following:

#include <iostream>
#include <vector>
    
class Base
{
public:
    Base(int x) { m_x = x; }
    virtual int getResult() { return m_x; }
protected:
    int m_x;
};


class DerivedA : public Base
{
public:
    DerivedA(int x) : Base(x) {}
    int getResult() override { return m_x * 2; }
};

class DerivedB : public Base
{
public:
    DerivedB(int x) : Base(x) {}
    int getResult() override { return m_x * m_x; }
};


int main()
{
    std::vector<Base *> objects;
    objects.push_back(new DerivedA(1));
    objects.push_back(new DerivedB(2));
    objects.push_back(new DerivedA(3));
    objects.push_back(new DerivedB(4));

    for(Base *object: objects)
    {
        DerivedA *obj = static_cast<DerivedA *>(object);
        if(obj != nullptr)
        {
            std::cout << obj->getResult() << std::endl;
        }
    }

    return 0;
}

I expect to get only 2 result i.e. that only instances of type DerivedA can be casted to DerivedA but not DerivedB. But to my surprise this is not the case. static_cast casts DerivedB to DerivedA without problem. Why is this happening? I can understand that if I would cast to Base but not that. At the same time dynamic_cast works as expected, i.e. casting DerivedB to DerivedA fails.

Upvotes: 1

Views: 242

Answers (1)

Bathsheba
Bathsheba

Reputation: 234875

The behaviour on dereferencing a pointer that's the result of a static_cast of a DerivedB* to a DerivedA* is undefined. It's a strict aliasing violation.

Upvotes: 3

Related Questions