Why is vector of pointers not saved as a class member after using the constructor?

Question

I wrote a weird example where I instantiate an object, give it a bunch of like objects and I try to call a function on the main object using the like objects.

I want to save a vector of pointers as a class member as in

#include <iostream>
#include <vector>

class A {
  public: 
    virtual ~A() = default;
};

class B: public A {
  public:
    std::vector<B*> bees;

    B() {};

    B(std::vector<B*> bees) {
      bees = bees;
    }

    int h() {
      return 2;
    }

    void fly() {
      std::cout << bees.size() << std::endl;
      std::cout << bees[0]->h() << std::endl;
    }
};

int main() {
  std::cout << "hello world" << std::endl;

  B b1, b2, b3;

  std::vector<B*> bs {&b1, &b2, &b3};
  B(bs).fly();

  return 0;
}

but for some reason, I am getting

hello world
0
Segmentation fault (core dumped)

I am not sure if this has something to do with shadowing or something, but I can't figure it out.

I expect

hello world
3
2

Answer 1

bees = bees;

This does nothing. It assigns bees to itself. It's likely to get optimized to nothing.

this->bees = bees;

This will explicitly assign the local variable to the instance field, which is what you're looking to do.

B(std::vector<B*> bees) : bees(bees) {}

However, this is the best practice, as it uses the constructor initialization syntax to initialize the field at the same time as the class itself.

Answer 2

In the constructor body, bees = bees; is assigning the constructor parameter bees (by itself), but not the data member bees. When fly() is called the data member bees is still empty, and bees[0] is trying to access the 1st element which doesn't exist; this leads to UB.

You can change it to

B(std::vector<B*> bees) {
  this->bees = bees;
}

Or initialize the data member by member initializer list

B(std::vector<B*> bees) : bees(bees) {}