Is it allowed to pass "this" of derived class to constructor of base class?

Here is the code I originally wanted to write:

class A {
public:
    A(someType someData) {
        this->init(someData);
    }
   
    virtual void init(someType) = 0;
}

This is not allowed, because at the point when base class constructor is called, instance of the derived class doesn't exist yet. So I changed it to this:

class A {
public:
    A(someType someData, A* derived) {
        derived->init(someData);
    }
   
    virtual void init(someType) = 0;
}

class B : public A {
public:
    B(someType someData)
    : A(someData, this) {}
    
}

The code compiles and runs without crashing but is it allowed or UB?

No, it's just as undefined as calling the virtual function. The instance of B isn't constructed until after As constructor completes so you can't call virtual functions on it.

One approach to fix this is to do a two stage construction, fully construct the class, and only then, call the virtual method:

class A {
public:
    virtual void init(someType) = 0;

    template <typename T>
    static T construct(someType someData)
    {
       T obj;
       obj.init(someData);
       return obj;
    }
}

class B : public A {
public:
    void init(someType) override
    {}
}

int main()
{
  B b = A::construct<B>();
}