The weak_ptr class template stores a weak reference to an object that is already managed by a shared_ptr. To access the object, a weak_ptr can be converted to a shared_ptr using the member function lock.
namespace std {
template<class T> class weak_ptr {
public:
typedef T element_type;
// [util.smartptr.weak.const], constructors
constexpr weak_ptr() noexcept;
template<class Y> weak_ptr(shared_ptr<Y> const& r) noexcept;
weak_ptr(weak_ptr const& r) noexcept;
template<class Y> weak_ptr(weak_ptr<Y> const& r) noexcept;
// [util.smartptr.weak.dest], destructor
~weak_ptr();
// [util.smartptr.weak.assign], assignment
weak_ptr& operator=(weak_ptr const& r) noexcept;
template<class Y> weak_ptr& operator=(weak_ptr<Y> const& r) noexcept;
template<class Y> weak_ptr& operator=(shared_ptr<Y> const& r) noexcept;
// [util.smartptr.weak.mod], modifiers
void swap(weak_ptr& r) noexcept;
void reset() noexcept;
// [util.smartptr.weak.obs], observers
long use_count() const noexcept;
bool expired() const noexcept;
shared_ptr<T> lock() const noexcept;
template<class U> bool owner_before(shared_ptr<U> const& b);
template<class U> bool owner_before(weak_ptr<U> const& b);
};
// [util.smartptr.weak.spec], specialized algorithms
template<class T> void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;
} // namespace std
Specializations of weak_ptr shall be CopyConstructible and CopyAssignable, allowing their use in standard containers. The template parameter T of weak_ptr may be an incomplete type.
constexpr weak_ptr() noexcept;
Effects: Constructs an empty weak_ptr object.
Postconditions: use_count() == 0.
weak_ptr(const weak_ptr& r) noexcept;
template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept;
template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept;
Requires: The second and third constructors shall not participate in the overload resolution unless Y* is implicitly convertible to T*.
Effects: If r is empty, constructs an empty weak_ptr object; otherwise, constructs a weak_ptr object that shares ownership with r and stores a copy of the pointer stored in r.
Postconditions: use_count() == r.use_count().
Effects: Destroys this weak_ptr object but has no effect on the object its stored pointer points to.
weak_ptr& operator=(const weak_ptr& r) noexcept;
template<class Y> weak_ptr& operator=(const weak_ptr<Y>& r) noexcept;
template<class Y> weak_ptr& operator=(const shared_ptr<Y>& r) noexcept;
Effects: Equivalent to weak_ptr(r).swap(*this).
Remarks: The implementation may meet the effects (and the implied guarantees) via different means, without creating a temporary.
void swap(weak_ptr& r) noexcept;
Effects: Exchanges the contents of *this and r.
Effects: Equivalent to weak_ptr().swap(*this).
long use_count() const noexcept;
Returns: 0 if *this is empty; otherwise, the number of shared_ptr instances that share ownership with *this.
[ Note: use_count() is not necessarily efficient. — end note ]
bool expired() const noexcept;
Returns: use_count() == 0.
[ Note: expired() may be faster than use_count(). — end note ]
shared_ptr<T> lock() const noexcept;
Returns: expired() ? shared_ptr<T>() : shared_ptr<T>(*this).
template<class U> bool owner_before(shared_ptr<U> const& b);
template<class U> bool owner_before(weak_ptr<U> const& b);
Returns: An unspecified value such that
x.owner_before(y) defines a strict weak ordering as defined in [alg.sorting];
under the equivalence relation defined by owner_before, !a.owner_before(b) && !b.owner_before(a), two shared_ptr or weak_ptr instances are equivalent if and only if they share ownership or are both empty.
template<class T> void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept;
Effects: Equivalent to a.swap(b).
The class template owner_less allows ownership-based mixed comparisons of shared and weak pointers.
namespace std {
template<class T> struct owner_less;
template<class T> struct owner_less<shared_ptr<T> > {
typedef bool result_type;
typedef shared_ptr<T> first_argument_type;
typedef shared_ptr<T> second_argument_type;
bool operator()(shared_ptr<T> const&, shared_ptr<T> const&) const;
bool operator()(shared_ptr<T> const&, weak_ptr<T> const&) const;
bool operator()(weak_ptr<T> const&, shared_ptr<T> const&) const;
};
template<class T> struct owner_less<weak_ptr<T> > {
typedef bool result_type;
typedef weak_ptr<T> first_argument_type;
typedef weak_ptr<T> second_argument_type;
bool operator()(weak_ptr<T> const&, weak_ptr<T> const&) const;
bool operator()(shared_ptr<T> const&, weak_ptr<T> const&) const;
bool operator()(weak_ptr<T> const&, shared_ptr<T> const&) const;
};
}
operator()(x,y) shall return x.owner_before(y). [ Note: Note that
operator() defines a strict weak ordering as defined in [alg.sorting];
under the equivalence relation defined by operator(), !operator()(a, b) && !operator()(b, a), two shared_ptr or weak_ptr instances are equivalent if and only if they share ownership or are both empty.