std::operator==(C++11)

namespace std {
  template<class T, class U>
  bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; // (1)

  template <class T>
  bool operator==(const shared_ptr<T>& x, nullptr_t) noexcept;              // (2)

  template <class T>
  bool operator==(nullptr_t, const shared_ptr<T>& x) noexcept;              // (3)
}

#include <iostream>
#include <memory>

int main()
{
  std::shared_ptr<int> p1(new int(3));
  std::shared_ptr<int> p2 = p1;
  if (p1 == p2) {
    std::cout << "equal" << std::endl;
  }

  std::shared_ptr<int> p3;
  if (p3 == nullptr) {
    std::cout << "p3 is nullptr" << std::endl;
  }

  if (nullptr == p3) {
    std::cout << "p3 is nullptr" << std::endl;
  }
}