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<title>make_shared and allocate_shared for arrays</title>
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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="#common">Common Requirements</a><br>
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<a href="#functions">Free Functions</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<class 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<class U, class 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<class U> // U is T[N]
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shared_ptr<U> <a href="#functions">make_shared</a>();
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template<class U, class 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<class 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<class U, class 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<class 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<class U, class 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<class 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<class U, class 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<class U> // U is T[N]
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shared_ptr<U> <a href="#functions">make_shared_noinit</a>();
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template<class U, class 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="common">Common Requirements</a></h2>
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<pre>template<class U>
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shared_ptr<U> make_shared(<em>args</em>);
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template<class U, class A>
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shared_ptr<U> allocate_shared(const A& allocator, <em>args</em>);
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template<class U>
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shared_ptr<U> make_shared_noinit(<em>args</em>);
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template<class U, class A>
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shared_ptr<U> allocate_shared_noinit(const A& allocator, <em>args</em>);</pre>
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<blockquote>
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<p><b>Requires:</b> <code>U</code> is of the form <code>T[]</code> or
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<code>T[N]</code>. <code>A</code> shall be an <em>Allocator</em>, as
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described in section 17.6.3.5 [<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 exceptions.</p>
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<p><b>Effects:</b> Allocates memory for an object of type <code>U</code>
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(or <code>T[size]</code> when <code>U</code> is <code>T[]</code>,
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where <code>size</code> is determined from <code><em>args</em></code>
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as specified by the concrete overload). The object is initialized as
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specified by the concrete overload. The templates
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<code>allocate_shared</code> and <code>allocate_shared_noinit</code>
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use a copy of <code>allocator</code> to allocate memory. If an
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exception is thrown, the functions have 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 object.</p>
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<p><b>Postconditions:</b> <code>r.get() != 0 &&
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r.use_count() == 1</code>, where <code>r</code> is the return
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value.</p>
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<p><b>Throws:</b> <code>bad_alloc</code>, an exception thrown from
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<code>A::allocate</code>, or from the initialization of the
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object.</p>
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<p><b>Remarks:</b></p>
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<blockquote>
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<p>This implementation performs no more than one memory
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allocation. This provides efficiency to equivalent to an intrusive
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smart pointer.</p>
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<p>When an object of an array type <code>T</code> is specified to be
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initialized to a value of the same type <code>value</code>, this
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shall be interpreted to mean that each array element of the object
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is initialized to the corresponding element from
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<code>value</code>.</p>
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<p>When an object of an array type is specified to be
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value-initialized, this shall be interpreted to mean that each
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array element of the object is value-initialized.</p>
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<p>Array elements are initialized in ascending order of their
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addresses.</p>
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<p>When a subobject of a non-array type <code>T</code> is specified to
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be initialized to a value <code>value</code>,
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<code>make_shared</code> shall perform this initialization via the
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expression <code>::new(ptr) T(value)</code>, where <code>ptr</code>
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has type <code>void*</code> and points to storage suitable to hold
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an object of type <code>T</code>.</p>
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<p>When a subobject of non-array type <code>T</code> is specified to
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be initialized to a value <code>value</code>,
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<code>allocate_shared</code> shall perform this initialization via
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the expression <code>allocator_traits<A2>::construct(a2, ptr,
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value)</code>, where <code>ptr</code> points to storage suitable to
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hold an object of type <code>T</code> and <code>a2</code> of type A2
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is a rebound copy of the allocator <code>allocator</code> passed to
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<code>allocate_shared</code> such that its <code>value_type</code>
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is <code>T</code>.</p>
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<p>When a subobject of non-array type <code>T</code> is specified to
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be value-initialized, <code>make_shared</code> shall perform this
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initialization via the expression <code>::new(ptr) T()</code>, where
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<code>ptr</code> has type <code>void*</code> and points to storage
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suitable to hold an object of type <code>T</code>.</p>
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<p>When a subobject of non-array type <code>T</code> is specified to
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be value-initialized, <code>allocate_shared</code> shall perform
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this initialization via the expression
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<code>allocator_traits<A2>::construct(a2, ptr)</code>, where
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<code>ptr</code> points to storage suitable to hold an object
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of type <code>T</code> and <code>a2</code> of type A2 is a rebound
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copy of the allocator <code>allocator</code> passed to
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<code>allocate_shared</code> such that its <code>value_type</code>
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is <code>T</code>.</p>
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<p>When a subobject of non-array type <code>T</code> is specified to
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be default-initialized, <code>make_shared_noinit</code> and
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<code>allocate_shared_noinit</code> shall perform this
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initialization via the expression <code>::new(ptr) T</code>, where
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<code>ptr</code> has type <code>void*</code> and points to storage
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suitable to hold an object of type <code>T</code>.</p>
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<p>When the lifetime of the object managed by the return value ends,
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or when the initialization of an array element throws an exception,
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the initialized elements should be destroyed in the reverse order
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of their construction.</p>
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</blockquote>
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<p><b>Notes:</b> These functions will typically allocate more memory
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than <code>sizeof(U)</code> to allow for internal bookkeeping
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structures such as the reference counts.</p>
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</blockquote>
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<h2><a name="functions">Free Functions</a></h2>
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<pre>template<class U>
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shared_ptr<U> make_shared(size_t size);
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template<class U, class A>
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shared_ptr<U> allocate_shared(const A& allocator, size_t size);</pre>
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<blockquote>
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<p><b>Returns:</b> A <code>shared_ptr</code> to a value-initialized
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object of type <code>T[size]</code>.</p>
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<p><b>Remarks:</b> These overloads shall only participate in overload
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resolution when <code>U</code> is of the form <code>T[]</code>.</p>
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<p><b>Examples:</b></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[][2]> a2 = boost::make_shared<int[][2]>(size);</pre>
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</blockquote>
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</blockquote>
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<pre>template<class U>
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shared_ptr<U> make_shared();
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template<class U, class A>
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shared_ptr<U> allocate_shared(const A& allocator);</pre>
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<blockquote>
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<p><b>Returns:</b> A <code>shared_ptr</code> to a value-initialized
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object of type <code>T[N]</code>.</p>
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<p><b>Remarks:</b> These overloads shall only participate in overload
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resolution when <code>U</code> is of the form <code>T[N]</code>.</p>
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<p><b>Examples:</b></p>
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<blockquote>
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<pre>boost::shared_ptr<int[8]> a1 = boost::make_shared<int[8]>();
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boost::shared_ptr<int[4][2]> a2 = boost::make_shared<int[4][2]>();</pre>
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</blockquote>
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</blockquote>
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<pre>template<class U>
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shared_ptr<U> make_shared(size_t size, const T& value);
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template<class U, class A>
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shared_ptr<U> allocate_shared(const A& allocator, size_t size, const T& value);</pre>
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<blockquote>
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<p><b>Returns:</b> A <code>shared_ptr</code> to an object of type
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<code>T[size]</code>, where each array element of type <code>T</code>
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is initialized to <code>value</code>.</p>
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<p><b>Remarks:</b> These overloads shall only participate in overload
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resolution when <code>U</code> is of the form <code>T[]</code>.</p>
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<p><b>Examples:</b></p>
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<blockquote>
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<pre>boost::shared_ptr<int[]> a1 = boost::make_shared<int[]>(size, 1);
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boost::shared_ptr<int[][2]> a2 = boost::make_shared<int[][2]>(size, {1, 2});</pre>
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</blockquote>
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</blockquote>
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<pre>template<class U>
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shared_ptr<U> make_shared(const T& value);
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template<class U, class A>
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shared_ptr<U> allocate_shared(const A& allocator, const T& value);</pre>
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<blockquote>
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<p><b>Returns:</b> A <code>shared_ptr</code> to an object of type
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<code>T[N]</code>, where each array element of type <code>T</code> is
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initialized to <code>value</code>.</p>
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<p><b>Remarks:</b> These overloads shall only participate in overload
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resolution when <code>U</code> is of the form <code>T[N]</code>.</p>
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<p><b>Examples:</b></p>
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<blockquote>
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<pre>boost::shared_ptr<int[8]> a1 = boost::make_shared<int[8]>(1);
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boost::shared_ptr<int[4][2]> a2 = boost::make_shared<int[4][2]>({1, 2});</pre>
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</blockquote>
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</blockquote>
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<pre>template<class U>
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shared_ptr<U> make_shared_noinit(size_t size);
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template<class U, class A>
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shared_ptr<U> allocate_shared_noinit(const A& allocator, size_t size);</pre>
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<blockquote>
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<p><b>Returns:</b> A <code>shared_ptr</code> to a default-initialized
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object of type <code>T[size]</code>.</p>
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<p><b>Remarks:</b> These overloads shall only participate in overload
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resolution when <code>U</code> is of the form <code>T[]</code>.</p>
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<p><b>Examples:</b></p>
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<blockquote>
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<pre>boost::shared_ptr<int[]> a1 = boost::make_shared_noinit<int[]>(size);
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boost::shared_ptr<int[][2]> a2 = boost::make_shared_noinit<int[][2]>(size);</pre>
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</blockquote>
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</blockquote>
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<pre>template<class U>
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shared_ptr<U> make_shared_noinit();
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template<class U, class A>
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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>Returns:</b> A <code>shared_ptr</code> to a default-initialized
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object of type <code>T[N]</code>.</p>
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<p><b>Remarks:</b> These overloads shall only participate in overload
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resolution when <code>U</code> is of the form <code>T[N]</code>.</p>
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<p><b>Examples:</b></p>
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<blockquote>
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<pre>boost::shared_ptr<int[8]> a1 = boost::make_shared_noinit<int[8]>();
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boost::shared_ptr<int[4][2]> a2 = boost::make_shared_noinit<int[4][2]>();</pre>
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</blockquote>
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</blockquote>
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<h2><a name="history">History</a></h2>
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<p>February 2014. Glen Fernandes updated overloads of make_shared and
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allocate_shared to conform to the specification in C++ standard paper
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<a href="#N3870">N3870</a>, including resolving C++ standard library
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defect report 2070, and reduced the spatial overhead of the internal
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bookkeeping structures.</p>
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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>$Date$</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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