Question

I have a problem. There are two classes:

struct Base {
    Base* retain() {
    //retain here
        return this;
    }
};

struct Derived : Base {
};


Derived *d1 = new Derived();
Derived *d2 = d1->retain(); //error here: need to typecast to Derived*
Derived *d3 = (Derived*)d1->retain(); //OK

Is there any way to rewrite retain() function in a way that I don’t need to manually typecast result? In other words: retain() should return an object of the derived type.

Answer 1

template<typename T>
struct Base
{
    T* retain()
    {
        return (T*)this;
    }
};

struct Derived : Base<Derived>
{
};


Derived *d1 = new Derived();
Derived *d2 = d1->retain();

Alternatively:

struct Base
{
    template<typename T>
    void retain(T** ptr)
    {
        *ptr = (T*)this;
    }
};

struct Derived : Base
{
};

Derived *d1 = new Derived;
Derived *d2;
d1->retain(&d2);