Pointer to the same instance used in a different class

How to solve this situation: I have three classes, to call them A, B and C. In C I have members of A and B. How do I set a pointer in B to have the same instance from C to A ?

#include <stdio.h>
class A {
public:
  int x; // no init, random to can test
  A() {
    printf("From A, x=%d\n", x);
  }
  void getP(A *ptr) {
    ptr = this;
  }
};
class B {
public:
  A *a;
  B() {
    a->getP(a);
    printf("From B, x=%d\n", a->x);
  }
  void test() {
    printf("From test(), x=%d\n", a->x);
  }
};
class C {
public:
  A a; B b;
  C() {
    a.x=55;
    printf("From C, x=%d\n", a.x);
    b.test();
  }
};
int main () {
  C c;
  return 0;
}

In this test, the values are different, is not the same instance, I want to have 55 result in From test() . How to make this?

Issues in you code:

Your current code has UB (undefined behavior) in this line in B constructor:

a->getP(a);

This is because a is not initialized at this point and therefore it is UB to dereference it (with ->).

In addition, A::getP(A *ptr) is getting ptr by value, and therefore it does not update the pointer from the caller.
In order to do so you need to get it by reference (void getP(A * & ptr)). However it is not needed at all - see below.

Solution:

In order to achieve what you want, you can add a A* parameter to the constructor of B, that will be used to initialized B::a member.
Then pass the address of C::a member, when B is constructed from C's constructor.

There is also no need for A::getP anymore.

Below is your complete example with this modification.
(the changes are marked with "VVVV").

#include <stdio.h>
class A {
public:
    int x{ 0 };
    A() {
        printf("From A, x=%d\n", x);
    }
};
class B {
public:
    A* a;
//----VVVVVV----VVVV---
    B(A * a_) : a(a_) {
        printf("From B, x=%d\n", a->x);
    }
    void test() {
        printf("From test(), x=%d\n", a->x);
    }
};
class C {
public:
    A a; B b;
//--------VVVVV--
    C() : b(&a) {
        a.x = 55;
        printf("From C, x=%d\n", a.x);
        b.test();
    }
};
int main() {
    C c;
    return 0;
}

Output:

From A, x=0
From B, x=0
From C, x=55
From test(), x=55

Note that it's not a good practice to have uninitialized data, and therefore I initialized A::x to 0.