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operator==, !=, <, <=, >, >=, <=> (std::shared_ptr)

Defined in header <memory>.

Compare two shared_ptr objects

// 1)
template < class T, class U >
bool operator==( const std::shared_ptr<T>& lhs,
                 const std::shared_ptr<U>& rhs ) noexcept;
// 7)
template< class T, class U >
std::strong_ordering operator<=>( const std::shared_ptr<T>& lhs,
                                  const std::shared_ptr<U>& rhs ) noexcept;

Compare a shared_ptr with a null pointer

// 8)
template< class T >
bool operator==( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
// 20)
template< class T >
std::strong_ordering operator<=>( const std::shared_ptr<T>& lhs,
                                  std::nullptr_t ) noexcept;

Compares two shared_ptr<T> objects or compares shared_ptr<T> with a null pointer.

Note that the comparison operators for shared_ptr simply compare pointer values; the actual objects pointed to are not compared. Having operator< defined for shared_ptr allows shared_ptrs to be used as keys in associative containers, like std::map and std::set.

The <, <=, >, >=, and != operators are synthesized from operator<=> and operator== respectively (since C++20).

Parameters​

lhs - the left-hand shared_ptr to compare rhs - the right-hand shared_ptr to compare

Return value​

  1. lhs.get() == rhs.get()
  2. !(lhs == rhs)
  3. std::less<V>()(lhs.get(), rhs.get()), where V is the composite pointer type of std::shared_ptr<T>::element_type* and std::shared_ptr<U>::element_type*
  4. rhs < lhs
  5. !(rhs < lhs)
  6. !(lhs < rhs)
  7. std::compare_three_way{}(x.get(), y.get())
  8. !lhs
  9. !rhs
  10. (bool)lhs
  11. (bool)rhs
  12. std::less<std::shared_ptr<T>::element_type*>()(lhs.get(), nullptr)
  13. std::less<std::shared_ptr<T>::element_type*>()(nullptr, rhs.get())
  14. nullptr < lhs
  15. rhs < nullptr
  16. !(nullptr < lhs)
  17. !(rhs < nullptr)
  18. !(lhs < nullptr)
  19. !(nullptr < rhs)
  20. std::compare_three_way{}(x.get(), static_cast<std::shared_ptr<T>::element_type*>(nullptr))

Notes​

In all cases, it is the stored pointer (the one returned by get()) that is compared, rather than the managed pointer (the one passed to the deleter when use_count goes to zero). The two pointers may differ in a shared_ptr created using the aliasing constructor.

Example​

#include <iostream>
#include <memory>
 
int main()
{
    std::shared_ptr<int> p1(new int(42));
    std::shared_ptr<int> p2(new int(42));
 
    std::cout << std::boolalpha
        << "(p1 == p1)       : " 
        << (p1 == p1) << '\n'
        << "(p1 <=> p1) == 0 : " 
        << ((p1 <=> p1) == 0) << '\n' // Since C++20
 
    // p1 and p2 point to different memory locations, so p1 != p2
        << "(p1 == p2)       : " 
        << (p1 == p2) << '\n'
        << "(p1 < p2)        : " 
        << (p1 < p2) << '\n'
        << "(p1 <=> p2) < 0  : " 
        << ((p1 <=> p2) < 0) << '\n'   // Since C++20
        << "(p1 <=> p2) == 0 : " 
        << ((p1 <=> p2) == 0) << '\n'; // Since C++20
}
Possible Result
(p1 == p1)       : true
(p1 <=> p1) == 0 : true
(p1 == p2)       : false
(p1 < p2)        : true
(p1 <=> p2) < 0  : true
(p1 <=> p2) == 0 : false

Defect reports​

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DRApplied toBehavior as publishedCorrect behavior
LWG 3427C++20operator<=>(shared_ptr, nullptr_t) was ill-formeddefinition fixed