std::owner_equal

 

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Defined in header <memory>
struct owner_equal;		(since C++26)

This function object provides owner-based (as opposed to value-based) mixed-type equal comparison of both std::weak_ptr and std::shared_ptr. The comparison is such that two smart pointers compare equivalent only if they are both empty or if they share ownership, even if the values of the raw pointers obtained by get() are different (e.g. because they point at different subobjects within the same object).

1) Owner-based mixed-type equal comparison is not provided for types other than std::shared_ptr and std::weak_ptr.

2) The owner-based mixed-type equal comparison of std::shared_ptr and std::weak_ptr.

It is the preferred comparison predicate when building unordered associative containers with std::shared_ptr and std::weak_ptr as keys together with std::owner_hash, that is, std::unordered_map<std::shared_ptr<T>, U, std::owner_hash, std::owner_equal> or std::unordered_map<std::weak_ptr<T>, U, std::owner_hash, std::owner_equal>.

3) std::owner_equal deduces the parameter types from the arguments.

Nested types

Nested type	Definition
is_transparent	unspecified

Member functions

operator()

compares its arguments using owner-based semantics (function)

std::owner_equal::operator()

template< class T, class U > bool operator()( const std::shared_ptr<T>& lhs,                  const std::shared_ptr<U>& rhs ) const noexcept;		(since C++26)
template< class T, class U > bool operator()( const std::shared_ptr<T>& lhs,                  const std::weak_ptr<U>& rhs ) const noexcept;		(since C++26)
template< class T, class U > bool operator()( const std::weak_ptr<T>& lhs,                  const std::shared_ptr<U>& rhs ) const noexcept;		(since C++26)
template< class T, class U > bool operator()( const std::weak_ptr<T>& lhs,                  const std::weak_ptr<U>& rhs ) const noexcept;		(since C++26)

Compares lhs and rhs using owner-based semantics. Effectively calls lhs.owner_equal(rhs).

The equal comparison is an equivalence relation.

lhs and rhs are equivalent only if they are both empty or share ownership.

Parameters

lhs, rhs

-

shared-ownership pointers to compare

Return value

true if lhs and rhs are both empty or share ownership as determined by the owner-based equal comparison, false otherwise.

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_smart_ptr_owner_equality	202306L	(C++26)	Enabling the use of std::shared_ptr and std::weak_ptr as keys in unordered associative containers

See also

owner_equal (C++26)	provides owner-based equal comparison of shared pointers (public member function of std::shared_ptr<T>)
owner_equal (C++26)	provides owner-based equal comparison of weak pointers (public member function of std::weak_ptr<T>)