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

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(C++11) (until C++23)
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std::shared_ptr
Member functions
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(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)
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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_ptr s 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

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)
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