boost shared_ptr operator =

Question

Here is code example

class A{
  int i;
public:
  A(int i) : i(i) {}
  void f() { prn(i); }
};

int main()
{
  A* pi = new A(9);
  A* pi2= new A(87);
  boost::shared_ptr<A> spi(pi);
  boost::shared_ptr<A> spi2(pi2);
  spi=spi2;
  spi->f();
  spi2->f();
  pi->f();
  pi2->f();
}

output:

87
87
0
87

The question is why is 0 in the output?

Note from documentation: Effects: Equivalent to shared_ptr(r).swap(*this).

But if shared_ptr objects just swapped, the result should be 9. And if the first object is deleted, there should be Segmentation fault.

So why 0?

Answer 1

Pay careful attention to what is being swapped:

shared_ptr(r).swap(*this)
// ^^^^^^^^^^

That's a temporary object constructed from r . The temporary goes out of scope immediately and dies, with whichever effects this has on the owned resource. Since in your code spi was the only owner of *spi , the object is deleted, and your subsequent access of pi->f() is simply undefined behaviour.

Answer 2

pi has been deleted. Accessing a member of a deleted object is undefined behavior; you could get 0, some other value, or a segmentation fault.

Answer 3

So why 0?

So anything!

Assigning a new value to spi will delete the previous object it is pointing to. The makes pi a dangling pointer that cannot be used anymore. Calling pi->f() is undefined behaviour and can have any result.

Answer 4

pi object is deleted after spi = spi2 and who know what your runtime does for freed memory. For me it prints -572662307 in Debug and 1315904 in Release.