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## Class template weak_ptr

boost::interprocess::weak_ptr

## Synopsis

    // In header: <boost/interprocess/smart_ptr/weak_ptr.hpp>

    template<typename T, typename A, typename D>
    class weak_ptr {
    public:
      // types
      typedef T element_type;
      typedef T value_type;

      // construct/copy/destruct
      weak_ptr();
      template<typename Y> weak_ptr(weak_ptr< Y, A, D > const &);
      template<typename Y> weak_ptr(shared_ptr< Y, A, D > const &);
      template<typename Y> weak_ptr & operator=(weak_ptr< Y, A, D > const &);
      template<typename Y> weak_ptr & operator=(shared_ptr< Y, A, D > const &);

      // public member functions
      shared_ptr< T, A, D > lock() const;
      long use_count() const;
      bool expired() const;
      void reset();
      void swap(this_type &);
    };

## Description

The weak_ptr class template stores a "weak reference" to an object that's already managed by a shared_ptr. To access the object, a weak_ptr can be converted to a shared_ptr using the shared_ptr constructor or the member function lock. When the last shared_ptr to the object goes away and the object is deleted, the attempt to obtain a shared_ptr from the weak_ptr instances that refer to the deleted object will fail: the constructor will throw an exception of type bad_weak_ptr, and weak_ptr::lock will return an empty shared_ptr.

Every weak_ptr meets the CopyConstructible and Assignable requirements of the C++ Standard Library, and so can be used in standard library containers. Comparison operators are supplied so that weak_ptr works with the standard library's associative containers.

weak_ptr operations never throw exceptions.

The class template is parameterized on T, the type of the object pointed to.

### `weak_ptr` public construct/copy/destruct

1.     weak_ptr();

   Effects: Constructs an empty `weak_ptr`. Postconditions: use_count() == 0.

2.     template<typename Y> weak_ptr(weak_ptr< Y, A, D > const & r);

   Effects: If r is empty, constructs an empty `weak_ptr`; otherwise, constructs a `weak_ptr` that shares ownership with r as if by storing a copy of the pointer stored in r.

   Postconditions: use_count() == r.use_count().

   Throws: nothing.

3.     template<typename Y> weak_ptr(shared_ptr< Y, A, D > const & r);

   Effects: If r is empty, constructs an empty `weak_ptr`; otherwise, constructs a `weak_ptr` that shares ownership with r as if by storing a copy of the pointer stored in r.

   Postconditions: use_count() == r.use_count().

   Throws: nothing.

4.     template<typename Y> weak_ptr & operator=(weak_ptr< Y, A, D > const & r);

   Effects: Equivalent to weak_ptr(r).swap(*this).

   Throws: nothing.

   Notes: The implementation is free to meet the effects (and the implied guarantees) via different means, without creating a temporary.

5.     template<typename Y> weak_ptr & operator=(shared_ptr< Y, A, D > const & r);

   Effects: Equivalent to weak_ptr(r).swap(*this).

   Throws: nothing.

   Notes: The implementation is free to meet the effects (and the implied guarantees) via different means, without creating a temporary.

### `weak_ptr` public member functions

1.     shared_ptr< T, A, D > lock() const;

   Returns: expired()? shared_ptr<T>(): shared_ptr<T>(*this).

   Throws: nothing.

2.     long use_count() const;

   Returns: 0 if *this is empty; otherwise, the number of `shared_ptr` objects that share ownership with *this.

   Throws: nothing.

   Notes: use_count() is not necessarily efficient. Use only for debugging and testing purposes, not for production code.

3.     bool expired() const;

   Returns: Returns: use_count() == 0.

   Throws: nothing.

   Notes: expired() may be faster than use_count().

4.     void reset();

   Effects: Equivalent to: weak_ptr().swap(*this).

5.     void swap(this_type & other);

   Effects: Exchanges the contents of the two smart pointers.

   Throws: nothing.