Initialize child class before parent class without composition / decorator

Once again I need to do something like this -
I need to initialize first the vector, so I pass its data() address to the parent class.

#include <vector>

struct A{
    A(int *a) : a(a){}
    
    int *a;
};

struct B : A{
    B() : A( v.data() ){}
    
    std::vector<int> v { 1024 };
};

#include <cstdio>

int main(){
    B b;
    b.v[55] = 5;

    printf("%d\n", b.a == b.v.data() );
    printf("%d\n", b.a[55]);
    printf("%d\n", b.v[55]);
}

Problem here is that the vector is initialized after parent class and v.data() contains junk. I am even surprised this compiles.

I know I can use composition / decorator for this or protected member setPtr(int *a), but I wonder if there is another way to do it.

Please do not comment about raw pointers. This is just an example for dependencies between parent and child classes.