Question

I have a question:

class B : public class A {
public:
  vector<int*> vec; 
};

class A  {
};

vector<A*> vec_a;
vector<B*> vec_b;

if I push back an object of class B into both vectors.

B* b = new B;
vec_a.push_back(b);
vec_b.push_back(b);

then after that, I change something inside the object of class B,

such as:

int* i = ....
vec_b[0].push_back(i);

Does the vec_a change?

I am confused with that since I have checked that when vector push_back, it will only create a copy. But when I checked with the above codes, it changes. Are the two vectors hold a shared memory of object b?

Thanks

Answer 1

Your vectors contain pointers to a common object. Therefore anything you change inside that object via a derefence of one of those pointers is reflected in the object that they are pointing to. Adding the pointers to the vectors creates a copy of the pointers themselves, not the object they point to. Had you added a common instance of class B to both vectors, each vector would contain a separate copy of that object. Adding pointers results copies of your original pointer being added to each vector – but both copies will have the same value and that is the memory address of the object they point to.