////
Copyright 2020 Peter Dimov
Distributed under the Boost Software License, Version 1.0.
https://www.boost.org/LICENSE_1_0.txt
////

[#owner_hash]
# owner_hash
:toc:
:toc-title:
:idprefix: owner_hash_to_

## Description

`owner_hash<T>` is a helper function object that takes a smart pointer `p`
and returns `p.owner_hash_value()`. It's useful for creating unordered
containers of `shared_ptr` that use ownership-based equality, instead of
the default pointer value equality. (It can be used with `weak_ptr` too,
but there's no need, because `boost::hash` and `std::hash` for `weak_ptr`
already use ownership-based equality.)

## Example

```
std::unordered_set< boost::shared_ptr<void>,
  boost::owner_hash< boost::shared_ptr<void> >,
  boost::owner_equal_to< boost::shared_ptr<void> > > set;
```

## Synopsis

`owner_hash` is defined in `<boost/smart_ptr/owner_hash.hpp>`.

```
namespace boost {

  template<class T> struct owner_hash
  {
    typedef std::size_t result_type;
    typedef T argument_type;

    std::size_t operator()( T const & p ) const noexcept;
  };
}
```

## Members

```
std::size_t operator()( T const & p ) const noexcept;
```
[none]
* {blank}
+
Returns::
  `p.owner_hash_value()`.