如何使用C ++智能指针复制变化的指针？

Question

struct A {
   int val;
};
struct B {
    int val2;
    A* ptr = nullptr;
};

int main() {
    A a1, a2;
    B b;
    if (...) { B.ptr = &a1; } else { B.ptr = &a2; }
    return 0;
}

How can ptr be represented with a smart ptr? It doesn't own but needs to access the objects a1 and a2. B 'uses' only one of a1 or a2, which later may toggle.

Changing ptr doesn't mean a1 or a2 should be deleted ofcourse. So, unique_ptr.reset() is not a solution.

Answer 1

In this particular case you don't need a smart pointer in order to delete a1 or a2.

So it would seem that a smart pointer is not needed at all...

Unless you wanted some special cleanup code to be executed on only one of the objects when b goes out of scope.

In which case it would look something like this:

struct A {
   int val = 0;
};

struct cleanup_A {
  void operator()(A* pa) const {
    if (pa) { my_cleanup_action(*pa); }
  }
};

struct B {
  using special_A_ptr = std::unique_ptr<A, cleanup_A>;
  int val2 = 0;
  special_A_ptr ptr = { nullptr, cleanup_A() };
};

int main() {
  A a1,a2;
  B b;
  if (...) { b.ptr.reset(&a1); } else { b.ptr.reset(&a2); }
  return 0;
}

Of course in such cases of non-standard use, all the usual warnings apply...