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

From cppreference.com

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)

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 std::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 output: 

(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.

DR Applied to Behavior as published Correct behavior
LWG 3427 C++20 operator<=>(shared_ptr, nullptr_t) was ill-formed definition fixed

See also

returns the stored pointer
(public member function) [edit]