NAME¶

std::owner_less - std::owner_less

Synopsis¶

Defined in header <memory>
template< class T > (since C++11)
struct owner_less; /* undefined */ (until C++17)
template< class T = void > (since C++17)
struct owner_less; /* undefined */
template< class T > (1) (2) (since C++11)
struct owner_less<std::shared_ptr<T>>;
template< class T > (3) (since C++11)
struct owner_less<std::weak_ptr<T>>;
template<> (4) (since C++17)
struct owner_less<void>;

This function object provides owner-based (as opposed to value-based) mixed-type
ordering of both std::weak_ptr and std::shared_ptr. The order 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).

This class template is the preferred comparison predicate when building associative
containers with std::shared_ptr or std::weak_ptr as keys, that is,

std::map<std::shared_ptr<T>, U, std::owner_less<std::shared_ptr<T>>>

or

std::map<std::weak_ptr<T>, U, std::owner_less<std::weak_ptr<T>>>.

The default operator< is not defined for weak pointers, and may wrongly consider two
shared pointers for the same object non-equivalent (see shared_ptr::owner_before).

Specializations¶

The standard library provides a specialization of std::owner_less when
T is not specified. In this case, the parameter types are deduced from
the arguments (each of which must still be either a std::shared_ptr or
a std::weak_ptr).
(since C++17)
function object providing mixed-type owner-based
owner_less<void> ordering of shared and weak pointers, regardless of
the type of the pointee
(class template specialization)

Member types¶

Member type Definition
result_type(deprecated in c++17) 2-3) bool (until C++20)
first_argument_type(deprecated in c++17) 2) std::shared_ptr<T>
3) std::weak_ptr<T>
second_argument_type(deprecated in c++17) 2) std::shared_ptr<T>
3) std::weak_ptr<T>

Member functions¶

operator() compares its arguments using owner-based semantics
(function)

std::owner_less::operator()

member only of owner_less<shared_ptr<T>> template specialization
bool operator()( const std::shared_ptr<T>& lhs, (since C++11)
const std::shared_ptr<T>& rhs ) const noexcept;
member only of owner_less<weak_ptr<T>> template specialization
bool operator()( const std::weak_ptr<T>& lhs, (since C++11)
const std::weak_ptr<T>& rhs ) const noexcept;
member of both template specializations
bool operator()( const std::shared_ptr<T>& lhs, (since C++11)
const std::weak_ptr<T>& rhs ) const noexcept;
bool operator()( const std::weak_ptr<T>& lhs, (since C++11)
const std::shared_ptr<T>& rhs ) const noexcept;

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

The ordering is strict weak ordering 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 is less than rhs as determined by the owner-based ordering.

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 2873 C++11 the operator()'s might not be declared declared noexcept
noexcept

See also¶

owner_before provides owner-based ordering of shared pointers
(public member function of std::shared_ptr<T>)
owner_before provides owner-based ordering of weak pointers
(public member function of std::weak_ptr<T>)