Class template weak

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

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

```
shared_ptr< T, A, D > lock() const;
```
Returns: expired()? shared_ptr<T>(): shared_ptr<T>(*this).

Throws: nothing.
```
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.
```
bool expired() const;
```
Returns: Returns: use_count() == 0.

Throws: nothing.

Notes: expired() may be faster than use_count().
```
void reset();
```
Effects: Equivalent to: weak_ptr().swap(*this).
```
void swap(this_type & other);
```
Effects: Exchanges the contents of the two smart pointers.

Throws: nothing.