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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="middle" width="277" height="86">shared_array
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class template</h1>
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<p>The <b>shared_array</b> class template stores a pointer to a dynamically
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allocated array. (Dynamically allocated array are allocated with the C++ <b>new[]</b>
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expression.) The object pointed to is guaranteed to be deleted when the last <b>shared_array</b>
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pointing to it is destroyed or reset.</p>
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<p>Every <b>shared_array</b> meets the <b>CopyConstructible</b> and <b>Assignable</b>
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requirements of the C++ Standard Library, and so can be used in standard
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library containers. Comparison operators are supplied so that <b>shared_array</b>
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works with the standard library's associative containers.</p>
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<p>Normally, a <b>shared_array</b> cannot correctly hold a pointer to an object
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that has been allocated with the non-array form of <STRONG>new</STRONG>. See <a href="shared_ptr.htm">
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<b>shared_ptr</b></a> for that usage.</p>
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<p>Because the implementation uses reference counting, cycles of <b>shared_array</b>
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instances will not be reclaimed. For example, if <b>main()</b> holds a <b>shared_array</b>
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to <b>A</b>, which directly or indirectly holds a <b>shared_array</b> back to <b>A</b>,
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<b>A</b>'s use count will be 2. Destruction of the original <b>shared_array</b>
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will leave <b>A</b> dangling with a use count of 1.</p>
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<p>A <b>shared_ptr</b> to a <b>std::vector</b> is an alternative to a <b>shared_array</b>
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that is a bit heavier duty but far more flexible.</p>
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<p>The class template is parameterized on <b>T</b>, the type of the object pointed
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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>Synopsis</h2>
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<pre>namespace boost {
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template<class T> class shared_array {
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public:
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typedef T <a href="#element_type">element_type</a>;
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explicit <a href="#constructors">shared_array</a>(T * p = 0);
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template<class D> <a href="#constructors">shared_array</a>(T * p, D d);
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<a href="#destructor">~shared_array</a>(); // never throws
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<a href="#constructors">shared_array</a>(shared_array const & r); // never throws
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shared_array & <a href="#assignment">operator=</a>(shared_array const & r); // never throws
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void <a href="#reset">reset</a>(T * p = 0);
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template<class D> void <a href="#reset">reset</a>(T * p, D d);
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T & <a href="#indexing">operator[]</a>(std::ptrdiff_t i) const() const; // never throws
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T * <a href="#get">get</a>() const; // never throws
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bool <a href="#unique">unique</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>(shared_array<T> & b); // never throws
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};
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template<class T>
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bool <a href="#comparison">operator==</a>(shared_array<T> const & a, shared_array<T> const & b); // never throws
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template<class T>
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bool <a href="#comparison">operator!=</a>(shared_array<T> const & a, shared_array<T> const & b); // never throws
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template<class T>
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bool <a href="#comparison">operator<</a>(shared_array<T> const & a, shared_array<T> const & b); // never throws
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template<class T> void <a href="#free-swap">swap</a>(shared_array<T> & a, shared_array<T> & b); // never throws
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}</pre>
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<h2>Members</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 shared_array(T * p = 0);</pre>
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<p>Constructs a <b>shared_array</b>, storing a copy of <b>p</b>, which must be a
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pointer to an array that was allocated via a C++ <b>new[]</b> expression or be
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0. Afterwards, the <a href="#use_count">use count</a> is 1 (even if p == 0; see <a href="#destructor">
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~shared_array</a>). The only exception which may be thrown by this
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constructor is <b>std::bad_alloc</b>. If an exception is thrown, <b>delete[] p</b>
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is called.</p>
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<pre>template<class D> shared_array(T * p, D d);</pre>
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<p>Constructs a <b>shared_array</b>, storing a copy of <b>p</b> and of <b>d</b>.
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Afterwards, the <a href="#use_count">use count</a> is 1. <b>D</b>'s copy
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constructor and destructor must not throw. When the the time comes to delete
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the array pointed to by <b>p</b>, the object <b>d</b> is used in the statement <b>d(p)</b>.
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Invoking the object <b>d</b> with parameter <b>p</b> in this way must not
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throw. 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, <b>d(p)</b> is called.</p>
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<pre>shared_array(shared_array const & r); // never throws</pre>
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<p>Constructs a <b>shared_array</b>, as if by storing a copy of the pointer stored
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in <b>r</b>. Afterwards, the <a href="#use_count">use count</a> for all copies
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is 1 more than the initial use count.</p>
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<h3><a name="destructor">destructor</a></h3>
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<pre>~shared_array(); // never throws</pre>
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<p>Decrements the <a href="#use_count">use count</a>. Then, if the use count is 0,
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deletes the array pointed to by the stored pointer. Note that <b>delete[]</b> on
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a pointer with a value of 0 is harmless. <b>T</b> need not be a complete type.
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The guarantee that this does not throw exceptions depends on the requirement
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that the deleted object's destructor does not throw exceptions. See the smart
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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>shared_array & <a href="#assignment">operator=</a>(shared_array const & r); // never throws</pre>
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<p>Constructs a new <b>shared_array</b> as described <a href="#constructors">above</a>,
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then replaces this <b>shared_array</b> with the new one, destroying the
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replaced object.</p>
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<h3><a name="reset">reset</a></h3>
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<pre>void reset(T * p = 0);</pre>
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<p>Constructs a new <b>shared_array</b> as described <a href="#constructors">above</a>,
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then replaces this <b>shared_array</b> with the new one, destroying the
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replaced object. The only exception which may be thrown is <b>std::bad_alloc</b>.
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If an exception is thrown, <b>delete[] p</b> is called.</p>
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<pre>template<class D> void reset(T * p, D d);</pre>
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<p>Constructs a new <b>shared_array</b> as described <a href="#constructors">above</a>,
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then replaces this <b>shared_array</b> with the new one, destroying the
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replaced object. <b>D</b>'s copy constructor must not throw. The only exception
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which may be thrown is <b>std::bad_alloc</b>. If an exception is thrown, <b>d(p)</b>
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is called.</p>
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<h3><a name="indirection">indexing</a></h3>
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<pre>T & operator[](std::ptrdiff_t i) const; // never throws</pre>
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<p>Returns a reference to element <b>i</b> of the array pointed to by the stored
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pointer. Behavior is undefined and almost certainly undesirable if the stored
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pointer is 0, or if <b>i</b> is less than 0 or is greater than or equal to the
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number of elements in the array.</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. <b>T</b> need not be a complete type. See the smart
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pointer <a href="smart_ptr.htm#Common requirements">common requirements</a>.</p>
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<h3><a name="unique">unique</a></h3>
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<pre>bool unique() const; // never throws</pre>
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<p>Returns true if no other <b>shared_array</b> is sharing ownership of the stored
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pointer, false otherwise. <b>T</b> need not be a complete type. See the smart
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pointer <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_array</b> objects sharing ownership of the
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stored pointer. <b>T</b> need not be a complete type. See the smart pointer <a href="smart_ptr.htm#Common requirements">
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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>shared_array</b> that do not use an explicit reference
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count, it might be removed from some future version. Thus it should be used for
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debugging purposes only, and not production code.</p>
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<h3><a name="swap">swap</a></h3>
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<pre>void swap(shared_ptr & b); // never throws</pre>
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<p>Exchanges the contents of the two smart pointers. <b>T</b> need not be a
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complete type. See the smart pointer <a href="smart_ptr.htm#Common requirements">common
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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<class T>
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bool operator==(shared_array<T> const & a, shared_array<T> const & b); // never throws
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template<class T>
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bool operator!=(shared_array<T> const & a, shared_array<T> const & b); // never throws
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template<class T>
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bool operator<(shared_array<T> const & a, shared_array<T> const & b); // never throws</pre>
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<p>Compares the stored pointers of the two smart pointers. <b>T</b> need not be a
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complete type. See the smart pointer <a href="smart_ptr.htm#Common requirements">common
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requirements</a>.</p>
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<p>The <b>operator<</b> overload is provided to define an ordering so that <b>shared_array</b>
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objects can be used in associative containers such as <b>std::map</b>. The
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implementation uses <b>std::less<T *></b> to perform the comparison. This
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ensures that the comparison is handled correctly, since the standard mandates
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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
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[lib.comparisons] paragraph 8).</p>
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<h3><a name="free-swap">swap</a></h3>
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<pre>template<class T>
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void swap(shared_array<T> & a, shared_array<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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<hr>
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<p>Revised <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B %Y" startspan -->09 January 2003<!--webbot bot="Timestamp" i-checksum="32310" 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. This document is provided
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"as is" without express or implied warranty, and with no claim as to its
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suitability for any purpose.</p>
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</body>
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</html>
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