2002-02-02 18:36:12 +00:00
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<title>weak_ptr</title>
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<h1><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" align="middle" width="277" height="86">weak_ptr class template</h1>
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<p>The <b>weak_ptr</b> class template stores a pointer to an
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object that's already managed by a <b>shared_ptr</b>. When the
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object last <b>shared_ptr</b> to the object goes away and the object
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is deleted, all <b>weak_ptr</b> objects have their stored pointers
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set to 0.</p>
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<p>Every <b>weak_ptr</b> meets the <b>CopyConstructible</b>
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and <b>Assignable</b> requirements of the C++ Standard Library, and so
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can be used in standard library containers. Comparison operators
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are supplied so that <b>weak_ptr</b> works with
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the standard library's associative containers.</p>
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<p>The class template is parameterized on <b>T</b>, the type of the object
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pointed to. <b>T</b> must meet the smart pointer
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<a href="smart_ptr.htm#Common requirements">common requirements</a>.</p>
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<h2><a name="Synopsis">Synopsis</a></h2>
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<pre>namespace boost {
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template<typename T> class weak_ptr {
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public:
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typedef T <a href="#element_type">element_type</a>;
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2002-02-09 01:18:00 +00:00
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<a href="#constructors">weak_ptr</a>();
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template<typename Y> <a href="#constructors">weak_ptr</a>(shared_ptr<Y> const & r); // never throws
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<a href="#destructor">~weak_ptr</a>(); // never throws
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<a href="#constructors">weak_ptr</a>(weak_ptr const & r); // never throws
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template<typename Y> <a href="#constructors">weak_ptr</a>(weak_ptr<Y> const & r); // never throws
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weak_ptr & <a href="#assignment">operator=</a>(weak_ptr const & r); // never throws
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template<typename Y> weak_ptr & <a href="#assignment">operator=</a>(weak_ptr<Y> const & r); // never throws
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template<typename Y> weak_ptr & <a href="#assignment">operator=</a>(shared_ptr<Y> const & r); // never throws
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2002-02-08 20:45:04 +00:00
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void <a href="#reset">reset</a>();
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T & <a href="#indirection">operator*</a>() const; // never throws
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T * <a href="#indirection">operator-></a>() const; // never throws
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T * <a href="#get">get</a>() const; // never throws
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long <a href="#use_count">use_count</a>() const; // never throws
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void <a href="#swap">swap</a>(weak_ptr<T> & b); // never throws
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};
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template<typename T, typename U>
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bool <a href="#comparison">operator==</a>(weak_ptr<T> const & a, weak_ptr<U> const & b); // never throws
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template<typename T, typename U>
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bool <a href="#comparison">operator!=</a>(weak_ptr<T> const & a, weak_ptr<U> const & b); // never throws
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template<typename T>
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bool <a href="#comparison">operator<</a>(weak_ptr<T> const & a, weak_ptr<T> const & b); // never throws
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template<typename T> void <a href="#free-swap">swap</a>(weak_ptr<T> & a, weak_ptr<T> & b); // never throws
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template<typename T, typename U>
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weak_ptr<T> <a href="#shared_static_cast">shared_static_cast</a>(weak_ptr<U> const & r); // never throws
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template<typename T, typename U>
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weak_ptr<T> <a href="#shared_dynamic_cast">shared_dynamic_cast</a>(weak_ptr<U> const & r);
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template<typename T, typename U>
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weak_ptr<T> <a href="#shared_polymorphic_cast">shared_polymorphic_cast</a>(weak_ptr<U> const & r);
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template<typename T, typename U>
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weak_ptr<T> <a href="#shared_polymorphic_downcast">shared_polymorphic_downcast</a>(weak_ptr<U> const & r); // never throws
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}</pre>
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<h2><a name="Members">Members</a></h2>
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<h3><a name="element_type">element_type</a></h3>
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<pre>typedef T element_type;</pre>
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<p>Provides the type of the stored pointer.</p>
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<h3><a name="constructors">constructors</a></h3>
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<pre>explicit weak_ptr();</pre>
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<p>Constructs a <b>weak_ptr</b>, with 0 as its stored pointer.
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The only exception which may be thrown by this constructor is <b>std::bad_alloc</b>.
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If an exception is thrown, the constructor has no effect.</p>
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<pre>template<typename Y> weak_ptr</a>(shared_ptr<Y> const & r); // never throws</pre>
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<p>Constructs a <b>weak_ptr</b>, as if by storing a copy of the pointer stored in <b>r</b>.
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Afterwards, the <a href="#use_count">use count</a> for all copies is unchanged.
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When the last <b>shared_ptr</b> is destroyed, the use count and stored pointer become 0.</p>
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<pre>weak_ptr(weak_ptr const & r); // never throws
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template<typename Y> weak_ptr(weak_ptr<Y> const & r); // never throws</pre>
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<p>Constructs a <b>weak_ptr</b>, as if by storing a copy of the
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pointer stored in <b>r</b>.</p>
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<h3><a name="destructor">destructor</a></h3>
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<pre>~weak_ptr(); // never throws</pre>
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<p>Destroys this <b>weak_ptr</b> but has no effect on the object its stored pointer points to.
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<b>T</b> need not be a complete type.
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See the smart pointer <a href="smart_ptr.htm#Common requirements">common requirements</a>.</p>
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<h3><a name="operator=">assignment</a></h3>
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<pre>weak_ptr & <a href="#assignment">operator=</a>(weak_ptr const & r); // never throws
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template<typename Y> weak_ptr & <a href="#assignment">operator=</a>(weak_ptr<Y> const & r); // never throws
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template<typename Y> weak_ptr & <a href="#assignment">operator=</a>(shared_ptr<Y> const & r); // never throws</pre>
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<p>Constructs a new <b>weak_ptr</b> as described <a href="#constructors">above</a>,
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then replaces this <b>weak_ptr</b> with the new one, destroying the replaced object.</p>
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<h3><a name="reset">reset</a></h3>
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<pre>void reset();</pre>
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<p>Constructs a new <b>weak_ptr</b> as described <a href="#constructors">above</a>,
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then replaces this <b>weak_ptr</b> with the new one, destroying the replaced object.
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The only exception which may be thrown is <b>std::bad_alloc</b>. If
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an exception is thrown, the <b>reset</b> has no effect.</p>
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<h3><a name="indirection">indirection</a></h3>
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<pre>T & operator*() const; // never throws</pre>
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<p>Returns a reference to the object pointed to by the stored pointer.
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Behavior is undefined if the stored pointer is 0.
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Note that the stored pointer becomes 0 if all <b>shared_ptr</b> objects for that
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pointer are destroyed.</p>
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<pre>T * operator->() const; // never throws</pre>
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<p>Returns the stored pointer.
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Behavior is undefined if the stored pointer is 0.
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Note that the stored pointer becomes 0 if all <b>shared_ptr</b> objects for that
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pointer are destroyed.</p>
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<h3><a name="get">get</a></h3>
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<pre>T * get() const; // never throws</pre>
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<p>Returns the stored pointer.
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Note that the stored pointer becomes 0 if all <b>shared_ptr</b> objects for that
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pointer are destroyed.
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<b>T</b> need not be a complete type.
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See the smart pointer
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<a href="smart_ptr.htm#Common requirements">common requirements</a>.</p>
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<h3><a name="use_count">use_count</a></h3>
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<pre>long use_count() const; // never throws</pre>
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<p>Returns the number of <b>shared_ptr</b> objects sharing ownership of the
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stored pointer.
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<b>T</b> need not be a complete type.
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See the smart pointer
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<a href="smart_ptr.htm#Common requirements">common requirements</a>.</p>
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<p>Because <b>use_count</b> is not necessarily efficient to implement for
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implementations of <b>weak_ptr</b> that do not use an explicit reference
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count, it might be removed from some future version. Thus it should
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be used for debugging purposes only, and <b>get</b> should be used for
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production code.</p>
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<h3><a name="swap">swap</a></h3>
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<pre>void swap(weak_ptr & b); // never throws</pre>
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<p>Exchanges the contents of the two smart pointers.
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<b>T</b> need not be a complete type.
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See the smart pointer
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<a href="smart_ptr.htm#Common requirements">common requirements</a>.</p>
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<h2><a name="functions">Free Functions</a></h2>
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<h3><a name="comparison">comparison</a></h3>
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<pre>template<typename T, typename U>
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bool operator==(weak_ptr<T> const & a, weak_ptr<U> const & b); // never throws
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template<typename T, typename U>
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bool operator!=(weak_ptr<T> const & a, weak_ptr<U> const & b); // never throws
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template<typename T>
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bool operator<(weak_ptr<T> const & a, weak_ptr<T> const & b); // never throws</pre>
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<p>Compares the stored pointers of the two smart pointers.
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<b>T</b> need not be a complete type.
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See the smart pointer
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<a href="smart_ptr.htm#Common requirements">common requirements</a>.</p>
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<p>The <b>operator<</b> overload is provided to define an ordering so that <b>weak_ptr</b>
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objects can be used in associative containers such as <b>std::map</b>.
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2002-02-04 22:52:48 +00:00
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The implementation uses <b>std::less<T *></b> to perform the
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comparison. This ensures that the comparison is handled correctly, since the
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standard mandates that relational operations on pointers are unspecified (5.9 [expr.rel]
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paragraph 2) but <b>std::less<></b> on pointers is well-defined (20.3.3 [lib.comparisons]
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paragraph 8).</p>
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<h3><a name="free-swap">swap</a></h3>
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<pre>template<typename T>
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void swap(weak_ptr<T> & a, weak_ptr<T> & b) // never throws</pre>
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<p>Equivalent to <b>a.swap(b)</b>. Matches the interface of <b>std::swap</b>.
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Provided as an aid to generic programming.</p>
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<h3><a name="shared_static_cast">shared_static_cast</a></h3>
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<pre>template<typename T, typename U>
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weak_ptr<T> shared_static_cast(weak_ptr<U> const & r); // never throws</pre>
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<p>Perform a <b>static_cast</b> on the stored pointer, returning another <b>weak_ptr</b>.
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The resulting smart pointer will share its use count with the original pointer.</p>
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<h3><a name="shared_dynamic_cast">shared_dynamic_cast</a></h3>
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<pre>template<typename T, typename U>
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weak_ptr<T> shared_dynamic_cast(weak_ptr<U> const & r);</pre>
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<p>Perform a <b>dynamic_cast</b> on the stored pointer, returning another <b>weak_ptr</b>.
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The resulting smart pointer will share its use count with the original pointer unless the result of the
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cast is 0. The only exception which may be thrown is <b>std::bad_alloc</b>, which may be thrown during the
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construction of the new <b>weak_ptr</b> if the result of the cast is 0. If an exception is thrown, the
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cast has no effect.</p>
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2002-02-06 19:42:04 +00:00
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<h3><a name="shared_polymorphic_cast">shared_polymorphic_cast</a></h3>
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<pre>template<typename T, typename U>
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weak_ptr<T> shared_polymorphic_cast(weak_ptr<U> const & r);</pre>
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<p>Perform a <a href="../conversion/cast.htm#Polymorphic_cast"><b>polymorphic_cast</b><a> on the stored pointer,
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returning another <b>weak_ptr</b>.
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The resulting smart pointer will share its use count with the original pointer.
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The only exception which may be thrown is <b>std::bad_cast</b>, if the pointer type can not be converted.
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If an exception is thrown, the cast has no effect.</p>
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<h3><a name="shared_polymorphic_downcast">shared_polymorphic_downcast</a></h3>
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<pre>template<typename T, typename U>
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weak_ptr<T> shared_polymorphic_downcast(weak_ptr<U> const & r); // never throws</pre>
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<p>Perform a <a href="../conversion/cast.htm#Polymorphic_cast"><b>polymorphic_downcast</b><a> on the stored pointer,
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returning another <b>weak_ptr</b>.
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The resulting smart pointer will share its use count with the original pointer.</p>
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2002-02-02 18:36:12 +00:00
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<hr>
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2002-02-09 01:18:00 +00:00
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<p>Revised <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B %Y" startspan -->8 February 2002<!--webbot bot="Timestamp" i-checksum="38439" endspan --></p>
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<p>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler.
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Permission to copy, use, modify, sell and distribute this document is granted
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provided this copyright notice appears in all copies.
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This document is provided "as is" without express or implied warranty,
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and with no claim as to its suitability for any purpose.</p>
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</body>
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</html>
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