////
Copyright 2002, 2003, 2015, 2017, 2019 Peter Dimov

Distributed under the Boost Software License, Version 1.0.

See accompanying file LICENSE_1_0.txt or copy at
http://www.boost.org/LICENSE_1_0.txt
////

[#enable_shared_from]
# enable_shared_from
:toc:
:toc-title:
:idprefix: enable_shared_from_

## Description

`enable_shared_from` is used as a base class that allows a `shared_ptr` or a
`weak_ptr` to be obtained given a raw pointer to the object, by using the
functions `shared_from` and `weak_from`.

`enable_shared_from` differs from `enable_shared_from_this<T>` by the fact
that it's not a template, and is its recommended replacement for new code.

## Example

```
#include <boost/smart_ptr/enable_shared_from.hpp>
#include <boost/shared_ptr.hpp>
#include <cassert>

class Y: public boost::enable_shared_from
{
public:

    boost::shared_ptr<Y> f()
    {
        return boost::shared_from( this );
    }
};

int main()
{
    boost::shared_ptr<Y> p(new Y);
    boost::shared_ptr<Y> q = p->f();
    assert(p == q);
    assert(!(p < q || q < p)); // p and q must share ownership
}
```

## Synopsis

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

```
namespace boost {

  class enable_shared_from: public enable_shared_from_this<enable_shared_from>
  {
  };

  template<class T> shared_ptr<T> shared_from( T * p );
  template<class T> weak_ptr<T> weak_from( T * p ) noexcept;
}
```

## Functions

```
template<class T> shared_ptr<T> shared_from( T * p );
```
[none]
* {blank}
+
Returns:: `shared_ptr<T>( p\->enable_shared_from::shared_from_this(), p )`.

NOTE: Throws `bad_weak_ptr` when `p` is not owned by a `shared_ptr`.

```
template<class T> weak_ptr<T> weak_from( T * p ) noexcept;
```
[none]
* {blank}
+
Returns:: `weak_ptr<T>( p\->enable_shared_from::weak_from_this(), p )`.

NOTE: Unlike `shared_from(this)`, `weak_from(this)` is valid in a destructor
      and returns a `weak_ptr` that is `expired()` but still shares ownership
      with other `weak_ptr` instances (if any) that refer to the object.