C++11: weak_ptr

A weak_ptr is a kind of pointer that can share access to a pointer, but does not have a say in when the pointed to object is deleted. Here is an example:

#include <iostream>
#include <memory>

int main()
{
  std::shared_ptr<int> sptr1(new int());
  std::shared_ptr<int> sptr2;
  std::weak_ptr<int>   wptr1;

////
  *sptr1 = 4;
  sptr2  = sptr1;
  std::cout << (sptr1?*sptr1:0) << " " << (sptr2?*sptr2:0) << " ";

  wptr1  = sptr1;
  auto sptr3 = wptr1.lock();
  std::cout << (sptr3?*sptr3:0) << std::endl;
  wptr1.reset();
////
  sptr1.reset();
  std::cout << (sptr1?*sptr1:0) << " " << (sptr2?*sptr2:0) << " ";

  wptr1 = sptr2;  
  auto sptr4 = wptr1.lock();
  std::cout << (sptr4?*sptr4:0) << std::endl;
  wptr1.reset();
////
  sptr2.reset();
  std::cout << (sptr1?*sptr1:0) << " " << (sptr2?*sptr2:0) << " ";

  wptr1 = sptr2;
  auto sptr5 = wptr1.lock();
  std::cout << (sptr5?*sptr5:0) << std::endl;
  wptr1.reset();
////
  return 0;
}
/* Output:
4 4 4
0 4 4
0 0 0
*******/

Reference: https://isocpp.org/wiki/faq/cpp11-library#weak-ptr