weak_ptr
class templateweak_ptr
class templateIntroduction
Motivation
Synopsis
Members
Free Functions
Frequently Asked Questions
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 function make_shared. 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 boost::bad_weak_ptr, and make_shared 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.
Compared to shared_ptr, weak_ptr provides a very limited subset of operations since accessing its stored pointer is often dangerous in multithreaded programs, and sometimes unsafe even within a single thread (that is, it may invoke undefined behavior.) Consider, for example, this innocent piece of code:
shared_ptr<int> p(new int(5));
weak_ptr<int> q(p);
// some time later
if(int * r = q.get())
{
// use *r
}
Imagine that after the if, but immediately before r
is used, another thread executes the statement p.reset(). Now r
is a dangling pointer.
The solution to this problem is to create a temporary shared_ptr from q:
shared_ptr<int> p(new int(5));
weak_ptr<int> q(p);
// some time later
if(shared_ptr<int> r = make_shared(q))
{
// use *r
}
Now r holds a reference to the object that was pointed by q.
Even if p.reset() is executed in another thread, the object will
stay alive until r goes out of scope (or is reset.)
[a mechanism to avoid dangling pointers]
[a way to break shared_ptr cycles]
[weak pointer to this - a technique to obtain a shared_ptr to this from within a member function]
[map<weak_ptr, ...> - a technique to associate arbitrary data with shared_ptr managed objects]
[gameobject/tank example]
[cache example]
[comparison: weak_ptr vs observer and other approaches]
[hard to reinvent, subtle implementation, with many pitfalls]
namespace boost {
template<class T> class weak_ptr {
public:
typedef T element_type;
weak_ptr();
template<class Y> weak_ptr(shared_ptr<Y> const & r);
weak_ptr(weak_ptr const & r);
template<class Y> weak_ptr(weak_ptr<Y> const & r);
~weak_ptr();
weak_ptr & operator=(weak_ptr const & r);
template<class Y> weak_ptr & operator=(weak_ptr<Y> const & r);
template<class Y> weak_ptr & operator=(shared_ptr<Y> const & r);
void reset();
long use_count() const;
bool expired() const;
void swap(weak_ptr<T> & b);
};
template<class T, class U>
bool operator<(weak_ptr<T> const & a, weak_ptr<U> const & b);
template<class T>
void swap(weak_ptr<T> & a, weak_ptr<T> & b);
template<class T>
shared_ptr<T> make_shared(weak_ptr<T> const & r);
}
typedef T element_type;
Provides the type of the template parameter T.
weak_ptr();
Effects: Constructs an empty weak_ptr.
Postconditions:
use count() == 0???.Throws: nothing.
template<class Y> weak_ptr(shared_ptr<Y> const & r); weak_ptr(weak_ptr const & r); template<class Y> weak_ptr(weak_ptr<Y> 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.
Throws: nothing.
~weak_ptr();
Effects: Destroys this weak_ptr but has no effect on the object its stored pointer points to.
Throws: nothing.
weak_ptr & operator=(weak_ptr const & r); template<class Y> weak_ptr & operator=(weak_ptr<Y> const & r); template<class Y> weak_ptr & operator=(shared_ptr<Y> 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.
void reset();
Effects: Equivalent to
weak_ptr().swap(*this).
long use_count() const;
Returns: if *this is empty, an unspecified nonnegative value; 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:
use_count() == 0.Throws: nothing.
Notes:
expired()may be faster thanuse_count().
void swap(weak_ptr & b);
Effects: Exchanges the contents of the two smart pointers.
Throws: nothing.
template<class T, class U> bool operator<(weak_ptr<T> const & a, weak_ptr<U> const & b);
Returns: an unspecified value such that
- operator< is a strict weak ordering as described in section 25.3
[lib.alg.sorting]of the C++ standard;- under the equivalence relation defined by operator<,
!(a < b) && !(b < a), two weak_ptr instances are equivalent if and only if they share ownership.Throws: nothing.
Notes: Allows weak_ptr objects to be used as keys in associative containers.
template<class T> void swap(weak_ptr<T> & a, weak_ptr<T> & b)
Effects: Equivalent to
a.swap(b).Throws: nothing.
Notes: Matches the interface of std::swap. Provided as an aid to generic programming.
template<class T> shared_ptr<T> make_shared(weak_ptr<T> & const r)
Returns:
r.expired()? shared_ptr<T>(): shared_ptr<T>(r).Throws: nothing.
Q. Can an object create a weak_ptr to itself in its constructor?
A. No. A weak_ptr can only be created from a shared_ptr, and at object construction time no shared_ptr to the object exists yet. Even if you could create a temporary shared_ptr to this, it would go out of scope at the end of the constructor, and all weak_ptr instances would instantly expire.
The solution is to make the constructor private, and supply a factory function
that returns a shared_ptr:
class X
{
private:
X();
public:
static shared_ptr<X> create()
{
shared_ptr<X> px(new X);
// create weak pointers from px here
return px;
}
};
$Date$
Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler. Copyright 2002, 2003 Peter Dimov. Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. This document is provided "as is" without express or implied warranty, and with no claim as to its suitability for any purpose.