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std::owner_less(3) C++ Standard Libary std::owner_less(3) 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 differ- ent (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 (un- til 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 own- ership. Parameters lhs, rhs - shared-ownership pointers to compare Return value true if lhs is less than rhs as determined by the owner-based order- ing. Defect reports The following behavior-changing defect reports were applied retroac- tively 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>) http://cppreference.com 2022.07.31 std::owner_less(3)
NAME | Synopsis | Specializations | Member types | Member functions | Parameters | Return value | See also
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