# std::owner_less

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

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).

1) Owner-based mixed-type ordering is not provided for types other than std::shared_ptr and std::weak_ptr.
2) The owner-based mixed-type ordering of std::shared_ptr.
It is the preferred comparison predicate when building associative containers with std::shared_ptr as keys, that is, std::map < std::shared_ptr <T>, U, std::owner_less< std::shared_ptr <T>>>.
3) The owner-based mixed-type ordering of std::weak_ptr.
It is the preferred comparison predicate when building associative containers with std::weak_ptr as keys, that is, std::map < std::weak_ptr <T>, U, std::owner_less< std::weak_ptr <T>>>.
4) The void specialization deduces the parameter types from the arguments.

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

owner_less<void> | function object providing mixed-type owner-based ordering of shared and weak pointers, regardless of the type of the pointee (class template specialization)

(since C++17)

### Nested types

Nested type | Definition
`result_type` (deprecated in C++17) | (2,3) bool
`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>

(until C++20)

### Member functions

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

## std::owner_less::operator()

member only of specialization (2) |  |
bool operator()( const std::shared_ptr <T>& lhs, const std::shared_ptr <T>& rhs ) const noexcept; |  | (since C++11)
member only of specialization (3) |  |
bool operator()( const std::weak_ptr <T>& lhs, const std::weak_ptr <T>& rhs ) const noexcept; |  | (since C++11)
member of both template specializations |  |
bool operator()( const std::shared_ptr <T>& lhs, const std::weak_ptr <T>& rhs ) const noexcept; |  | (since C++11)
bool operator()( const std::weak_ptr <T>& lhs, const std::shared_ptr <T>& rhs ) const noexcept; |  | (since C++11)

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, false otherwise.

### 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 | operator() was not required to be noexcept | required to be 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>`)

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