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::cout << std::boolalpha;

  std::shared_ptr<int> p1(new int(3));
  std::shared_ptr<int> p2(new int(3));

  bool r1 = p1 < p2;
  std::cout << r1 << std::endl;

  bool r2 = p1 < nullptr;
  std::cout << r2 << std::endl;

  bool r3 = nullptr < p1;
  std::cout << r3 << std::endl;
}