std::owner_less

From Cppreference

Defined in header `<memory>`

template< class T > struct owner_less; /* undefined */	(1)	(C++11 feature)

template< class T > struct owner_less<std::shared_ptr<T>>;	(2)	(C++11 feature)

template< class T > struct owner_less<std::weak_ptr<T>>;	(3)	(C++11 feature)

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 both manage the same object, 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)

[edit] Member types


Member type	Definition

result_type	bool

first_argument_type	1) T 2) std::shared_ptr<T> 3) std::weak_ptr<T>

second_argument_type	1) T 2) std::shared_ptr<T> 3) std::weak_ptr<T>

[edit] Member functions

operator()

compares its arguments using owner-based semantics
(function)

[edit] See also

owner_before

provides owner-based ordering of shared pointers
(public member function)