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

namespace std {
  template <class T1, class D1, class T2, class D2>
  bool operator==(const unique_ptr<T1, D1>& a, const unique_ptr<T2, D2>& b); // (1)

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

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

#include <iostream>
#include <memory>

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

  std::unique_ptr<int> p2;
  if (p2 == nullptr) {
    std::cout << "p2 is nullptr" << std::endl;
  }

  if (nullptr == p2) {
    std::cout << "p2 is nullptr" << std::endl;
  }
}