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

From cppreference.com
< cpp‎ | memory‎ | shared ptr
C++
Memory management library
(exposition only*)
Uninitialized memory algorithms
(C++17)
(C++17)
(C++17)
(C++17)
(C++17)
(C++11)
(C++17)
(C++17)
(C++17)
(C++17)
(C++20)
Constrained uninitialized
memory algorithms
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
C Library
Allocators
(C++11)
(C++23)
(C++11)
(C++11)
(C++20)
(C++20)
(C++20)
(C++11)
(C++17)
Memory resources
(C++17)
(C++17)
(C++17)
(C++17)
(C++17)
(C++17)
(C++17)
(C++17)
(C++17)
Garbage collection support
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
Uninitialized storage
(until C++20*)
(until C++20*)
(until C++20*)
Explicit lifetime management
(C++23)(C++23)
std::shared_ptr
Member functions
Modifiers
Observers
(C++17)
(until C++20*)
(C++26)
(C++26)
Non-member functions
(C++20)
(C++20)
(C++17)
operator==operator!=operator<operator<=operator>operator>=operator<=>
(until C++20) (until C++20) (until C++20) (until C++20) (until C++20) (C++20)
functions (until C++26*)
Helper classes
(C++20)
Deduction guides(C++17)
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==

(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

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

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)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/memory/shared_ptr/operator_cmp&oldid=153804