NAME¶

std::owner_equal - std::owner_equal

Synopsis¶

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.

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, (since C++26)

const std::shared_ptr<U>& rhs ) const noexcept;
template< class T, class U >

bool operator()( const std::shared_ptr<T>& lhs, (since C++26)

const std::weak_ptr<U>& rhs ) const noexcept;
template< class T, class U >

bool operator()( const std::weak_ptr<T>& lhs, (since C++26)

const std::shared_ptr<U>& rhs ) const noexcept;
template< class T, class U >

bool operator()( const std::weak_ptr<T>& lhs, (since C++26)

const std::weak_ptr<U>& rhs ) const noexcept;

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
Enabling the use of
__cpp_lib_smart_ptr_owner_equality 202306L (C++26) std::shared_ptr and std::weak_ptr
as keys in unordered associative
containers

See also¶

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