2000-07-27 14:27:00 +00:00
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<title>shared_array</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_array">shared_array</a></h1>
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<p>Class <strong>shared_array</strong> stores a pointer to a dynamically
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allocated array. (Dynamically allocated arrays are allocated with the C++ <tt>new[]</tt>
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expression.) The array pointed to is guaranteed to be deleted,
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either on destruction of the <strong>shared_array</strong>, on <strong>shared_array::operator=()</strong>,
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or via an explicit <strong>shared_array::reset()</strong>. See <a href="#shared_array_example">example</a>.</p>
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<p>Class<strong> shared_array</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_array</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_array</strong> cannot correctly hold a pointer to a
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2000-12-06 14:46:44 +00:00
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single object. See <a href="shared_ptr.htm"><strong>shared_ptr</strong></a>
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2000-07-27 14:27:00 +00:00
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for that usage.</p>
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<p>Class<strong> shared_array</strong> will not work correctly with cyclic data
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structures. For example, if main() holds a shared_array pointing to array A,
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which directly or indirectly holds a shared_array pointing back to array A, then
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array A's use_count() will be 2, and destruction of the main() shared_array will
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leave array A dangling with a use_count() of 1.</p>
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<p>A heavier duty alternative to a <strong>shared_array</strong> is a <strong>shared_ptr</strong>
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to a C++ Standard Library <strong>vector</strong>.</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_array 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_array {
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public:
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typedef T <a href="#shared_array_element_type">element_type</a>;
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explicit <a href="#shared_array_ctor">shared_array</a>( T* p=0 );
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<a href="#shared_array_ctor">shared_array</a>( const shared_array& ); // never throws
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<strong> </strong><a href="#shared_array_~shared_array">~shared_array</a>();
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shared_array& <a href="#shared_array_operator=">operator=</a>( const shared_array& ); // never throws
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void <a href="#shared_array_reset">reset</a>( T* p=0 );
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T& <a href="#shared_array_operator[]">operator[]</a>(std::size_t i) const; // never throws
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T* <a href="#shared_array_get">get</a>() const; // never throws
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long <a href="#shared_array_use_count">use_count</a>() const; // never throws
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bool <a href="#shared_array_unique">unique</a>() const; // never throws
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void <a href="#shared_array_swap">swap</a>( shared_array<T>& other ) throw()
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};
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template<typename T>
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inline bool operator==(const shared_array<T>& a, const shared_array<T>& b)
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{ return a.get() == b.get(); }
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template<typename T>
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inline bool operator!=(const shared_array<T>& a, const shared_array<T>& 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_array<T>& a, boost::shared_array<T>& b)
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{ a.swap(b); }
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template<typename T>
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struct less< boost::shared_array<T> >
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: binary_function<boost::shared_array<T>, boost::shared_array<T>, bool>
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{
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bool operator()(const boost::shared_array<T>& a,
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const boost::shared_array<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 arrays 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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<h2>Class shared_array Members</h2>
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<h3>shared_array <a name="shared_array_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_array_ctor">shared_array constructors</a></h3>
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<pre>explicit shared_array( T* p=0 );</pre>
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<p>Constructs a <strong>shared_array</strong>, storing a copy of <tt>p</tt>,
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which must have been allocated via a C++ <tt>new</tt>[] expression or be 0.
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Afterwards, use_count() is 1 (even if p==0; see <a href="#shared_array_~shared_array">~shared_array</a>).</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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<pre>shared_array( const shared_array& r); // never throws</pre>
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<p>Constructs a <strong>shared_array</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>.</p>
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<h3><a name="shared_array_~shared_array">shared_array destructor</a></h3>
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<pre>~shared_array();</pre>
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<p>If <strong>use_count()</strong> == 1, deletes the array 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 with
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a value of 0 is harmless.</p>
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<p>Does not throw exceptions.</p>
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<h3>shared_array <a name="shared_array_operator=">operator=</a></h3>
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<pre>shared_array& operator=( const shared_array& r); // never throws</pre>
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<p>First, if <strong>use_count()</strong> == 1, deletes the array 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 a
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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>. </p>
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<h3>shared_array <a name="shared_array_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 array 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>[]
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on 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 <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_array_~shared_array">~shared_array</a>).</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_array <a name="shared_array_operator[]">operator[]</a></h3>
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<p><tt>T& operator[](std::size_t i) const; // never throws</tt></p>
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<p>Returns a reference to element <tt>i</tt> of the array pointed to by the
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stored pointer.</p>
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<p>Behavior is undefined (and almost certainly undesirable) if <tt>get()==0</tt>,
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or if <tt>i</tt> is less than 0 or is greater or equal to the number of elements
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in the array.</p>
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<h3>shared_array <a name="shared_array_get">get</a></h3>
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<pre>T* get() const; // never throws</pre>
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<p>Returns the stored pointer.</p>
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<h3>shared_array<a name="shared_array_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_arrays</strong> sharing ownership of the
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stored pointer.</p>
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<h3>shared_array <a name="shared_array_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_array_swap">shared_array swap</a></h3>
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<p><code>void swap( shared_array<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_array_example">shared_array 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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</html>
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