std::shared_ptr::owner_before(3)

NAME
std::shared_ptr::owner_before - std::shared_ptr::owner_before

Synopsis
   template< class Y >
   bool owner_before( const shared_ptr<Y>& other ) const noexcept;
   template< class Y >
   bool owner_before( const std::weak_ptr<Y>& other ) const noexcept;
   Checks whether this shared_ptr precedes other in implementation defined owner-based
   (as opposed to value-based) order. The order is such that two smart pointers compare
   equivalent only if they are both empty or if they both own the same object, even if
   the values of the pointers obtained by get() are different (e.g. because they point
   at different subobjects within the same object)
   This ordering is used to make shared and weak pointers usable as keys in associative
   containers, typically through std::owner_less.

Parameters
   other - the std::shared_ptr or std::weak_ptr to be compared

Return value
   true if *this precedes other, false otherwise. Common implementations compare the
   addresses of the control blocks.

Example
// Run this code
 #include <iostream>
 #include <memory>
 struct Foo {
     int n1;
     int n2;
     Foo(int a, int b) : n1(a), n2(b) {}
 };
 int main()
 {
     auto p1 = std::make_shared<Foo>(1, 2);
     std::shared_ptr<int> p2(p1, &p1->n1);
     std::shared_ptr<int> p3(p1, &p1->n2);
     std::cout << std::boolalpha
               << "p2 < p3 " << (p2 < p3) << '\n'
               << "p3 < p2 " << (p3 < p2) << '\n'
               << "p2.owner_before(p3) " << p2.owner_before(p3) << '\n'
               << "p3.owner_before(p2) " << p3.owner_before(p2) << '\n';
     std::weak_ptr<int> w2(p2);
     std::weak_ptr<int> w3(p3);
     std::cout
 // << "w2 < w3 " << (w2 < w3) << '\n' // won't compile
 // << "w3 < w2 " << (w3 < w2) << '\n' // won't compile
               << "w2.owner_before(w3) " << w2.owner_before(w3) << '\n'
               << "w3.owner_before(w2) " << w3.owner_before(w2) << '\n';
 }

Output:
 p2 < p3 true
 p3 < p2 false
 p2.owner_before(p3) false
 p3.owner_before(p2) false
 w2.owner_before(w3) false
 w3.owner_before(w2) false
  Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 2873 C++11 owner_before might not be declared noexcept declared noexcept

See also
   owner_less provides mixed-type owner-based ordering of shared and weak pointers
   (C++11) (class template)