operator==, !=, <, <=, >, >=, <=> (std::shared_ptr)
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< cpp | memory | shared ptr
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std::shared_ptr
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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*)
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(C++20)
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Defined in header
<memory>
Compare two
shared_ptr objects.template< class T, class U >
(1)
(since C++11)
bool operator==( const std::shared_ptr <T>& lhs,
template< class T, class U >
(2)
(since C++11) bool operator!=( const std::shared_ptr <T>& lhs,
(until C++20)
template< class T, class U >
(3)
(since C++11) bool operator<( const std::shared_ptr <T>& lhs,
(until C++20)
template< class T, class U >
(4)
(since C++11) bool operator>( const std::shared_ptr <T>& lhs,
(until C++20)
template< class T, class U >
(5)
(since C++11) bool operator<=( const std::shared_ptr <T>& lhs,
(until C++20)
template< class T, class U >
(6)
(since C++11) bool operator>=( const std::shared_ptr <T>& lhs,
(until C++20)
template< class T, class U >
(7)
(since C++20)
std::strong_ordering operator<=>( const std::shared_ptr <T>& lhs,
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)
bool operator==( const std::shared_ptr <T>& lhs, std::nullptr_t ) noexcept;
template< class T >
bool operator==( std::nullptr_t, const std::shared_ptr <T>& rhs ) noexcept;
(9)
(since C++11) bool operator==( std::nullptr_t, const std::shared_ptr <T>& rhs ) noexcept;
(until C++20)
template< class T >
bool operator!=( const std::shared_ptr <T>& lhs, std::nullptr_t ) noexcept;
(10)
(since C++11) bool operator!=( const std::shared_ptr <T>& lhs, std::nullptr_t ) noexcept;
(until C++20)
template< class T >
bool operator!=( std::nullptr_t, const std::shared_ptr <T>& rhs ) noexcept;
(11)
(since C++11) bool operator!=( std::nullptr_t, const std::shared_ptr <T>& rhs ) noexcept;
(until C++20)
template< class T >
bool operator<( const std::shared_ptr <T>& lhs, std::nullptr_t ) noexcept;
(12)
(since C++11) bool operator<( const std::shared_ptr <T>& lhs, std::nullptr_t ) noexcept;
(until C++20)
template< class T >
bool operator<( std::nullptr_t, const std::shared_ptr <T>& rhs ) noexcept;
(13)
(since C++11) bool operator<( std::nullptr_t, const std::shared_ptr <T>& rhs ) noexcept;
(until C++20)
template< class T >
bool operator>( const std::shared_ptr <T>& lhs, std::nullptr_t ) noexcept;
(14)
(since C++11) bool operator>( const std::shared_ptr <T>& lhs, std::nullptr_t ) noexcept;
(until C++20)
template< class T >
bool operator>( std::nullptr_t, const std::shared_ptr <T>& rhs ) noexcept;
(15)
(since C++11) bool operator>( std::nullptr_t, const std::shared_ptr <T>& rhs ) noexcept;
(until C++20)
template< class T >
bool operator<=( const std::shared_ptr <T>& lhs, std::nullptr_t ) noexcept;
(16)
(since C++11) bool operator<=( const std::shared_ptr <T>& lhs, std::nullptr_t ) noexcept;
(until C++20)
template< class T >
bool operator<=( std::nullptr_t, const std::shared_ptr <T>& rhs ) noexcept;
(17)
(since C++11) bool operator<=( std::nullptr_t, const std::shared_ptr <T>& rhs ) noexcept;
(until C++20)
template< class T >
bool operator>=( const std::shared_ptr <T>& lhs, std::nullptr_t ) noexcept;
(18)
(since C++11) bool operator>=( const std::shared_ptr <T>& lhs, std::nullptr_t ) noexcept;
(until C++20)
template< class T >
bool operator>=( std::nullptr_t, const std::shared_ptr <T>& rhs ) noexcept;
(19)
(since C++11) bool operator>=( std::nullptr_t, const std::shared_ptr <T>& rhs ) noexcept;
(until C++20)
template< class T >
(20)
(since C++20)
std::strong_ordering operator<=>( const std::shared_ptr <T>& lhs,
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.
[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.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3427 | C++20 | operator<=>(shared_ptr, nullptr_t) was ill-formed
|
definition fixed |