指向类数据成员的指针

Question

In this little test program, can someone explain me:

• Why is every addresses printed in both loops the same?

• Why is it different from one loop to another?

   struct A { A(){ std::cout << &v << "\\n"; }; int v; }; int main() { std::vector<A> vec; int i = 10; while (i--) vec.push_back(A()); for (A b : vec) std::cout << &(bv) << "\\n"; while (true); return 0; } 

I actually expected to see ten different addresses repeated 2 times

Answer 1

In this range based for loop :

    for (A b : vec)
        std::cout << &(b.v) << "\n";

b is a copy of the element in the vector. Thats why you see 10 times the same adress. It is different for two different loops, because that copy happens to be in a different memory location. You probably wanted this:

    for (A& b : vec)
        std::cout << &(b.v) << "\n";

here b is a reference to the element in the vector.

Answer 2

If you want to see the same addresses you need to look at the addresses inside the vector and not the addresses of temporary copies.

With vector::emplace_back , you can see the addresses inside the vector during the construction (see emplace_back ). A reference access gives you the address inside the vector during the iterations.

int main()
{
    std::vector<A> vec;

    int i = 10;
    while (i--)
        vec.emplace_back();

    for (const A& b : vec)
        std::cout << &(b.v) << "\n";
    ...
}