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

From cppreference.com
< cpp‎ | memory‎ | shared ptr
 
 
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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

#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.

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

[edit] See also

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