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

      From cppreference.com
      < cpp‎ | memory‎ | shared ptr
       
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      std::shared_ptr
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      operator==operator!=operator<operator<=operator>operator>=operator<=>
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      Defined in header <memory>
      Compare two shared_ptr objects.
      template< class T, class U >

      bool operator==( const std::shared_ptr<T>& lhs,

                       const std::shared_ptr<U>& rhs ) noexcept;
      		(1)(since C++11)
      template< class T, class U >

      bool operator!=( const std::shared_ptr<T>& lhs,

                       const std::shared_ptr<U>& rhs ) noexcept;
      		(2)(since C++11)
      (until C++20)
      template< class T, class U >

      bool operator<( const std::shared_ptr<T>& lhs,

                      const std::shared_ptr<U>& rhs ) noexcept;
      		(3)(since C++11)
      (until C++20)
      template< class T, class U >

      bool operator>( const std::shared_ptr<T>& lhs,

                      const std::shared_ptr<U>& rhs ) noexcept;
      		(4)(since C++11)
      (until C++20)
      template< class T, class U >

      bool operator<=( const std::shared_ptr<T>& lhs,

                       const std::shared_ptr<U>& rhs ) noexcept;
      		(5)(since C++11)
      (until C++20)
      template< class T, class U >

      bool operator>=( const std::shared_ptr<T>& lhs,

                       const std::shared_ptr<U>& rhs ) noexcept;
      		(6)(since C++11)
      (until C++20)
      template< class T, class U >

      std::strong_ordering operator<=>( const std::shared_ptr<T>& lhs,

                                        const std::shared_ptr<U>& rhs ) noexcept;
      		(7)(since C++20)
      Compare a shared_ptr with a null pointer.
      template< class T >
      bool operator==( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
      		(8)(since C++11)
      template< class T >
      bool operator==( std::nullptr_t, const std::shared_ptr<T>& rhs ) noexcept;
      		(9)(since C++11)
      (until C++20)
      template< class T >
      bool operator!=( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
      		(10)(since C++11)
      (until C++20)
      template< class T >
      bool operator!=( std::nullptr_t, const std::shared_ptr<T>& rhs ) noexcept;
      		(11)(since C++11)
      (until C++20)
      template< class T >
      bool operator<( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
      		(12)(since C++11)
      (until C++20)
      template< class T >
      bool operator<( std::nullptr_t, const std::shared_ptr<T>& rhs ) noexcept;
      		(13)(since C++11)
      (until C++20)
      template< class T >
      bool operator>( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
      		(14)(since C++11)
      (until C++20)
      template< class T >
      bool operator>( std::nullptr_t, const std::shared_ptr<T>& rhs ) noexcept;
      		(15)(since C++11)
      (until C++20)
      template< class T >
      bool operator<=( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
      		(16)(since C++11)
      (until C++20)
      template< class T >
      bool operator<=( std::nullptr_t, const std::shared_ptr<T>& rhs ) noexcept;
      		(17)(since C++11)
      (until C++20)
      template< class T >
      bool operator>=( const std::shared_ptr<T>& lhs, std::nullptr_t ) noexcept;
      		(18)(since C++11)
      (until C++20)
      template< class T >
      bool operator>=( std::nullptr_t, const std::shared_ptr<T>& rhs ) noexcept;
      		(19)(since C++11)
      (until C++20)
      template< class T >

      std::strong_ordering operator<=>( const std::shared_ptr<T>& lhs,

                                        std::nullptr_t ) noexcept;
      		(20)(since C++20)

      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)

      Contents

      [edit] Parameters

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

      [edit] 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))

      [edit] 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.

      [edit] Example

      Run this code
      #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 output:

      (p1 == p1)       : true
(p1 <=> p1) == 0 : true
(p1 == p2)       : false
(p1 < p2)        : true
(p1 <=> p2) < 0  : true
(p1 <=> p2) == 0 : false

      [edit] 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

      [edit] See also

      		returns the stored pointer
      (public member function) [edit]
      Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/memory/shared_ptr/operator_cmp&oldid=153804"