Using shared_from_this() with derived class

Question

When I try to run this code:

class A : public enable_shared_from_this<A> {
public:
    A() {
        cout << "A::A()" << endl;
    };
    
    virtual ~A() {
        cout << "A::~A()" << endl;
    };

    virtual void func() {
        cout << "A::func" << endl;
        auto ptr = shared_from_this();
        ptr->dosomething();
    }

    virtual void dosomething() {
        cout << "A::dosomething" << endl;
    }
};

class B : public A {
public:
    B() {
        cout << "B::B()" << endl;
    };
    
    ~B() {
        cout << "B::~B()" << endl;
    };

    void func() override {
        cout << "B::func" << endl;
        A::func();
    }

    void dosomething() override {
        cout << "B::dosomething" << endl;
    }
};


int main() {
    shared_ptr<B> pb = make_shared<B>();
    //shared_ptr<A> pa = make_shared<A>();
    pb->func();
}

The result I get is:

A::A()
B::B()
B::func
A::func
B::dosomething
B::~B()
A::~A()

The call chain is B::func->A::func->ptr->dosomething, this indicates that the return value of shared_from_this() is a shared_ptr<B>. Why is the result of calling shared_from_this() not shared_ptr<A>?

For a more realistic test, use `shared_ptr pb = make_shared();` — Some programmer dude, Oct 26 '22 at 08:25
As for your problem, it's not really related to `shared_from_this` but rather how virtual functions work. Even in the `A::func` function, `this` (and the pointer returned by `shared_from_this()`) is really pointing to a `B` object. — Some programmer dude, Oct 26 '22 at 08:27
`shared_from_this` will never create a new object, just return a pointer to the existing object — Alan Birtles, Oct 26 '22 at 08:31
It works the same as `auto ptr = this; ptr->dosomething();` (and as just `dosomething();`) and it's your normal runtime polymorphism working as intended. — molbdnilo, Oct 26 '22 at 08:42

Using shared_from_this() with derived class

0 Answers0