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2014-01-29 05:42:42 -08:00
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<title>make_shared and allocate_shared for arrays</title>
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2012-11-06 14:17:32 +00:00
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2014-01-29 05:42:42 -08:00
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<h1><img height="86" alt="boost.png (6897 bytes)" src="../../boost.png"
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width="277" align="middle" border="0">make_shared and allocate_shared
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for arrays</h1>
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<p><a href="#Introduction">Introduction</a><br>
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<a href="#Synopsis">Synopsis</a><br>
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<a href="#functions">Free Functions</a><br>
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<a href="#example">Examples</a><br>
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<a href="#history">History</a><br>
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<a href="#references">References</a></p>
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<h2><a name="Introduction">Introduction</a></h2>
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<p>Originally the Boost function templates <code>make_shared</code> and
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<code>allocate_shared</code> were for efficient allocation of shared
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objects only. There was a need to have efficient allocation of
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shared arrays. One criticism of class template <code>shared_array</code>
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was always the lack of a <a href="make_shared.html">make_shared</a>
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utility which ensures only a single allocation.</p>
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<p>The header files <boost/smart_ptr/make_shared_array.hpp> and
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<boost/smart_ptr/allocate_shared_array.hpp> provide function
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templates, overloads of <code>make_shared</code> and
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<code>allocate_shared</code> for array types, to address this need.
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<code>make_shared</code> uses the global operator <code>new</code> to
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allocate memory, whereas <code>allocate_shared</code> uses an
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user-supplied allocator, allowing finer control.</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 U> // U is T[]
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shared_ptr<U> <a href="#functions">make_shared</a>(size_t size);
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template<typename U, typename A> // U is T[]
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shared_ptr<U> <a href="#functions">allocate_shared</a>(const A& allocator, size_t size);
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template<typename U> // U is T[N]
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shared_ptr<U> <a href="#functions">make_shared</a>();
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template<typename U, typename A> // U is T[N]
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shared_ptr<U> <a href="#functions">allocate_shared</a>(const A& allocator);
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template<typename U> // U is T[]
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shared_ptr<U> <a href="#functions">make_shared</a>(size_t size, const T& value);
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template<typename U, typename A> // U is T[]
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shared_ptr<U> <a href="#functions">allocate_shared</a>(const A& allocator, size_t size, const T& value);
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template<typename U> // U is T[N]
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shared_ptr<U> <a href="#functions">make_shared</a>(const T& value);
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template<typename U, typename A> // U is T[N]
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shared_ptr<U> <a href="#functions">allocate_shared</a>(const A& allocator, const T& value);
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template<typename U> // U is T[]
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shared_ptr<U> <a href="#functions">make_shared_noinit</a>(size_t size);
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template<typename U, typename A> // U is T[]
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shared_ptr<U> <a href="#functions">allocate_shared_noinit</a>(const A& allocator, size_t size);
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template<typename U> // U is T[N]
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shared_ptr<U> <a href="#functions">make_shared_noinit</a>();
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template<typename U, typename A> // U is T[N]
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shared_ptr<U> <a href="#functions">allocate_shared_noinit</a>(const A& allocator);
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}</pre>
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<h2><a name="functions">Free Functions</a></h2>
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<pre>template<typename U> // U is T[]
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shared_ptr<U> make_shared(size_t size);
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template<typename U, typename A> // U is T[]
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shared_ptr<U> allocate_shared(const A& allocator, size_t size);
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template<typename U> // U is T[N]
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shared_ptr<U> make_shared();
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template<typename U, typename A> // U is T[N]
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shared_ptr<U> allocate_shared(const A& allocator);</pre>
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<blockquote>
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<p><b>Requires:</b> The expression
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<code>new(pointer) T()</code>, where <code>pointer</code> is a
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<code>void*</code> pointing to storage suitable to hold an object of
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type <code>T</code>, shall be well-formed. <code>A</code> shall be an
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<em>Allocator</em>, as described in section 20.1.5 (<strong>Allocator
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requirements</strong>) of the C++ Standard. The copy constructor and
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destructor of <code>A</code> shall not throw.</p>
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<p><b>Effects:</b> Allocates memory suitable for an array of type
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<code>T</code> and size <code>size</code> and constructs an array of
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objects in it via the placement new expression <code>new(pointer)
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T()</code>. <code>allocate_shared</code> uses a copy of
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<code>allocator</code> to allocate memory. If an exception is thrown,
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has no effect.</p>
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<p><b>Returns:</b> A <code>shared_ptr</code> instance that stores and
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owns the address of the newly constructed array of type <code>T</code>
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and size <code>size</code>.</p>
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<p><b>Postconditions:</b>
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<code>get() != 0 && use_count() == 1</code>.</p>
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<p><b>Throws:</b> <code>bad_alloc</code>, or an exception thrown from
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<code>A::allocate</code> or the constructor of <code>T</code>.</p>
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<p><b>Notes:</b> This implementation allocates the memory required for
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the returned <code>shared_ptr</code> and an array of type
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<code>T</code> of size <code>size</code> in a single allocation. This
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provides efficiency to equivalent to an intrusive smart pointer.</p>
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</blockquote>
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<pre>template<typename U> // U is T[]
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shared_ptr<U> make_shared(size_t size, const T& value);
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template<typename U, typename A> // U is T[]
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shared_ptr<U> allocate_shared(const A& allocator, size_t size, const T& value);
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template<typename U> // U is T[N]
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shared_ptr<U> make_shared(const T& value);
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template<typename U, typename A> // U is T[N]
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shared_ptr<U> allocate_shared(const A& allocator, const T& value);</pre>
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2012-12-04 06:06:23 +00:00
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<blockquote>
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<p><b>Notes:</b> These overloads initialize array objects with the given
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value.</p>
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</blockquote>
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<pre>template<typename U> // U is T[]
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shared_ptr<U> make_shared_noinit(size_t size);
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template<typename U, typename A> // U is T[]
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shared_ptr<U> allocate_shared_noinit(const A& allocator, size_t size);
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template<typename U> // U is T[N]
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shared_ptr<U> make_shared_noinit();
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template<typename U, typename A> // U is T[N]
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shared_ptr<U> allocate_shared_noinit(const A& allocator);</pre>
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<blockquote>
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<p><b>Notes:</b> These overloads do not perform value initialization of
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array objects.</p>
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</blockquote>
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<h2><a name="example">Examples</a></h2>
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<p>The following examples value-initialize objects.</p>
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<blockquote>
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<pre>boost::shared_ptr<int[]> a1 = boost::make_shared<int[]>(size);
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boost::shared_ptr<int[8]> a2 = boost::make_shared<int[8]>();
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boost::shared_ptr<int[][2]> a3 = boost::make_shared<int[][2]>(size);
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boost::shared_ptr<int[4][2]> a4 = boost::make_shared<int[4][2]>();</pre>
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</blockquote>
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<p>The following examples initialize objects with a given value.</p>
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<blockquote>
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<pre>boost::shared_ptr<int[]> a5 = boost::make_shared<int[]>(size, 1);
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boost::shared_ptr<int[8]> a6 = boost::make_shared<int[8]>(1);
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boost::shared_ptr<int[][2]> a7 = boost::make_shared<int[][2]>(size, {1, 2});
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boost::shared_ptr<int[4][2]> a8 = boost::make_shared<int[4][2]>({1, 2});</pre>
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</blockquote>
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<p>The following examples default-initialize objects.</p>
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<blockquote>
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<pre>boost::shared_ptr<int[]> a9 = boost::make_shared_noinit<int[]>(size);
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boost::shared_ptr<int[8]> a10 = boost::make_shared_noinit<int[8]>();
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boost::shared_ptr<int[][2]> a11 = boost::make_shared_noinit<int[][2]>(size);
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boost::shared_ptr<int[4][2]> a12 = boost::make_shared_noinit<int[4][2]>();</pre>
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</blockquote>
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2012-11-20 03:00:02 +00:00
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<h2><a name="history">History</a></h2>
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<p>January 2014. Glen Fernandes reduced the overloads of make_shared and
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allocate_shared according to <a href="#N3870">N3870</a>.</p>
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2012-11-20 03:00:02 +00:00
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<p>November 2012. Glen Fernandes contributed implementations of
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make_shared and allocate_shared for arrays.</p>
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<h2><a name="references">References</a></h2>
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<p><a name="N3870">N3870</a>,
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<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3870.html">
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Extending make_shared to Support Arrays, Revision 1</a>, Peter Dimov
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& Glen Fernandes, January, 2014.</p>
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<hr>
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<p>$Date: 2014-01-20 11:10:00 -0800 (Mon, 20 Jan 2014) $</p>
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<p><small>Copyright 2012-2014 Glen Fernandes. Distributed under the
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Boost Software License, Version 1.0. See accompanying file
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<a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or copy at
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<a href="http://www.boost.org/LICENSE_1_0.txt">
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http://www.boost.org/LICENSE_1_0.txt</a>.</small></p>
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