forked from boostorg/smart_ptr
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207 lines
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<title>shared_ptr</title>
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<h1><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" align="center" width="277" height="86">Class
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<a name="shared_ptr">shared_ptr</a></h1>
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<p>Class <strong>shared_ptr</strong> stores a pointer to a dynamically allocated
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object. (Dynamically allocated objects are allocated with the C++ <tt>new</tt>
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expression.) The object pointed to is guaranteed to be deleted when
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the last <strong>shared_ptr</strong> pointing to it is deleted or reset.
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See <a href="#shared_ptr_example">example</a>.</p>
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<p>Class<strong> shared_ptr</strong> meets the <strong>CopyConstuctible</strong>
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and <strong>Assignable</strong> requirements of the C++ Standard Library, and so
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can be used in C++ Standard Library containers. A specialization of std::
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less< > for boost::shared_ptr<Y> is supplied so that <strong>
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shared_ptr</strong> works by default for Standard Library's Associative
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Container Compare template parameter. For compilers not supporting partial
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specialization, the user must explicitly pass the less<> functor.</p>
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<p>Class<strong> shared_ptr</strong> cannot correctly hold a pointer to a
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dynamically allocated array. See <a href="shared_array.htm"><strong>shared_array</strong></a>
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for that usage.</p>
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<p>Class<strong> shared_ptr</strong> will not work correctly with cyclic data
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structures. For example, if main() holds a shared_ptr to object A, which
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directly or indirectly holds a shared_ptr back to object A, then object A's
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use_count() will be 2, and destruction of the main() shared_ptr will leave
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object A dangling with a use_count() of 1.</p>
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<p>The class is a template parameterized on <tt>T</tt>, the type of the object
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pointed to. <tt>T</tt> must meet the smart pointer <a href="smart_ptr.htm#Common requirements">Common
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requirements</a>.</p>
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<h2>Class shared_ptr Synopsis</h2>
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<pre>#include <boost/smart_ptr.hpp>
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namespace boost {
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template<typename T> class shared_ptr {
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public:
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typedef T <a href="#shared_ptr_element_type">element_type</a>;
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explicit <a href="#shared_ptr_ctor">shared_ptr</a>( T* p=0 );
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<strong> </strong><a href="#shared_ptr_~shared_ptr">~shared_ptr</a>();
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<a href="#shared_ptr_ctor">shared_ptr</a>( const shared_ptr& );
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template<typename Y>
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<a href="#shared_ptr_ctor">shared_ptr</a>(const shared_ptr<Y>& r); // never throws
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template<typename Y>
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<a href="#shared_ptr_ctor">shared_ptr</a>(std::auto_ptr<Y>& r);
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shared_ptr& <a href="#shared_ptr_operator=">operator=</a>( const shared_ptr& ); // never throws
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template<typename Y>
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shared_ptr& <a href="#shared_ptr_operator=">operator=</a>(const shared_ptr<Y>& r); // never throws
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template<typename Y>
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shared_ptr& <a href="#shared_ptr_operator=">operator=</a>(std::auto_ptr<Y>& r);
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void <a href="#shared_ptr_reset">reset</a>( T* p=0 );
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T& <a href="#shared_ptr_operator*">operator*</a>() const; // never throws
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T* <a href="#shared_ptr_operator->">operator-></a>() const; // never throws
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T* <a href="#shared_ptr_get">get</a>() const; // never throws
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long <a href="#shared_ptr_use_count">use_count</a>() const; // never throws
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bool <a href="#shared_ptr_unique">unique</a>() const; // never throws
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void <a href="#shared_ptr_swap">swap</a>( shared_ptr<T>& other ) throw()
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};
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template<typename T, typename U>
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inline bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b)
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{ return a.get() == b.get(); }
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template<typename T, typename U>
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inline bool operator!=(const shared_ptr<T>& a, const shared_ptr<U>& b)
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{ return a.get() != b.get(); }
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}</pre>
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<pre>namespace std {
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template<typename T>
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inline void swap(boost::shared_ptr<T>& a, boost::shared_ptr<T>& b)
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{ a.swap(b); }
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template<typename T>
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struct less< boost::shared_ptr<T> >
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: binary_function<boost::shared_ptr<T>, boost::shared_ptr<T>, bool>
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{
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bool operator()(const boost::shared_ptr<T>& a,
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const boost::shared_ptr<T>& b) const
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{ return less<T*>()(a.get(),b.get()); }
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};
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} // namespace std </pre>
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<p>Specialization of std::swap uses the fast, non-throwing swap that's provided
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as a member function instead of using the default algorithm which creates a
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temporary and uses assignment.<br>
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<br>
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Specialization of std::less allows use of shared pointers as keys in C++
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Standard Library associative collections.<br>
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<br>
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The std::less specializations use std::less<T*> to perform the
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comparison. This insures that pointers are 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 std::less<> on pointers is well-defined (20.3.3 [lib.comparisons]
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paragraph 8).<br>
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<br>
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It's still a controversial question whether supplying only std::less is better
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than supplying a full range of comparison operators (<, >, <=, >=).</p>
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<p>The current implementation does not supply the specializations if the macro
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name BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION is defined.</p>
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<p>The current implementation does not supply the member template functions if
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the macro name BOOST_NO_MEMBER_TEMPLATES is defined.</p>
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<h2>Class shared_ptr Members</h2>
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<h3>shared_ptr <a name="shared_ptr_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="shared_ptr_ctor">shared_ptr constructors</a></h3>
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<pre>explicit shared_ptr( T* p=0 );</pre>
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<p>Constructs a <strong>shared_ptr</strong>, storing a copy of <tt>p</tt>, which
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must have been allocated via a C++ <tt>new</tt> expression or be 0. Afterwards, <strong>use_count()</strong>
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is 1 (even if p==0; see <a href="#shared_ptr_~shared_ptr">~shared_ptr</a>).</p>
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<p>The only exception which may be thrown by this constructor is <tt>std::bad_alloc</tt>.
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If an exception is thrown, <tt>delete p</tt> is called.</p>
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<pre>shared_ptr( const shared_ptr& r); // never throws
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template<typename Y>
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shared_ptr(const shared_ptr<Y>& r); // never throws
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template<typename Y>
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shared_ptr(std::auto_ptr<Y>& r);</pre>
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<p>Constructs a <strong>shared_ptr</strong>, as if by storing a copy of the
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pointer stored in <strong>r</strong>. Afterwards, <strong>use_count()</strong>
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for all copies is 1 more than the initial <strong>r.use_count()</strong>, or 1
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in the <strong>auto_ptr</strong> case. In the <strong>auto_ptr</strong> case, <strong>r.release()</strong>
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is called.</p>
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<p>The only exception which may be thrown by the constructor from <strong>auto_ptr</strong>
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is <tt>std::bad_alloc</tt>. If an exception is thrown, that
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constructor has no effect.</p>
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<h3><a name="shared_ptr_~shared_ptr">shared_ptr destructor</a></h3>
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<pre>~shared_ptr();</pre>
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<p>If <strong>use_count()</strong> == 1, deletes the object pointed to by the
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stored pointer. Otherwise, <strong>use_count()</strong> for any remaining
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copies is decremented by 1. Note that in C++ <tt>delete</tt> on a pointer
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with a value of 0 is harmless.</p>
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<p>Does not throw exceptions.</p>
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<h3>shared_ptr <a name="shared_ptr_operator=">operator=</a></h3>
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<pre>shared_ptr& operator=( const shared_ptr& r);
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template<typename Y>
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shared_ptr& operator=(const shared_ptr<Y>& r);
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template<typename Y>
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shared_ptr& operator=(std::auto_ptr<Y>& r);</pre>
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<p>First, if <strong>use_count()</strong> == 1, deletes the object pointed to by
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the stored pointer. Otherwise, <strong>use_count()</strong> for any
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remaining copies is decremented by 1. Note that in C++ <tt>delete</tt> on
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a pointer with a value of 0 is harmless.</p>
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<p>Then replaces the contents of <strong>this</strong>, as if by storing a copy
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of the pointer stored in <strong>r</strong>. Afterwards, <strong>use_count()</strong>
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for all copies is 1 more than the initial <strong>r.use_count()</strong>, or 1
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in the <strong>auto_ptr</strong> case. In the <strong>auto_ptr</strong> case, <strong>r.release()</strong>
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is called.</p>
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<p>The first two forms of <tt>operator=</tt> above do not throw exceptions.</p>
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<p>The only exception which may be thrown by the <strong>auto_ptr</strong> form
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is <tt>std::bad_alloc</tt>. If an exception is thrown, the function
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has no effect.</p>
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<h3>shared_ptr <a name="shared_ptr_reset">reset</a></h3>
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<pre>void reset( T* p=0 );</pre>
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<p>First, if <strong>use_count()</strong> == 1, deletes the object pointed to by
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the stored pointer. Otherwise, <strong>use_count()</strong> for any
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remaining copies is decremented by 1. </p>
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<p>Then replaces the contents of <strong>this</strong>, as if by storing a copy
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of <strong>p</strong>, which must have been allocated via a C++ <tt>new</tt>
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expression or be 0. Afterwards, <strong>use_count()</strong> is 1 (even if p==0;
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see <a href="#shared_ptr_~shared_ptr">~shared_ptr</a>). Note that in C++ <tt>delete</tt>
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on a pointer with a value of 0 is harmless.</p>
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<p>The only exception which may be thrown is <tt>std::bad_alloc</tt>. If
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an exception is thrown, <tt>delete p</tt> is called.</p>
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<h3>shared_ptr <a name="shared_ptr_operator*">operator*</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.</p>
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<h3>shared_ptr <a name="shared_ptr_operator->">operator-></a> and <a name="shared_ptr_get">get</a></h3>
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<pre>T* operator->() const; // never throws
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T* get() const; // never throws</pre>
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<p>Both return the stored pointer.</p>
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<h3>shared_ptr<a name="shared_ptr_use_count"> use_count</a></h3>
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<p><tt>long use_count() const; // never throws</tt></p>
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<p>Returns the number of <strong>shared_ptrs</strong> sharing ownership of the
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stored pointer.</p>
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<h3>shared_ptr <a name="shared_ptr_unique">unique</a></h3>
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<p><tt>bool unique() const; // never throws</tt></p>
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<p>Returns <strong>use_count()</strong> == 1.</p>
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<h3><a name="shared_ptr_swap">shared_ptr swap</a></h3>
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<p><code>void swap( shared_ptr<T>& other ) throw()</code></p>
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<p>Swaps the two smart pointers, as if by std::swap.</p>
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<h2>Class <a name="shared_ptr_example">shared_ptr example</a></h2>
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<p>[To be supplied. In the meantime, see <a href="smart_ptr_test.cpp">smart_ptr_test.cpp</a>.]</p>
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<hr>
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<p>Revised December 8, 1999</p>
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<p><EFBFBD> Copyright Greg Colvin and Beman Dawes 1999. Permission to copy, use,
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modify, sell and distribute this document is granted provided this copyright
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notice appears in all copies. This document is provided "as is"
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without express or implied warranty, and with no claim as to its suitability for
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any purpose.</p>
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</body>
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