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<title>make_shared and allocate_shared</title>
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<h1><A href="../../index.htm"><IMG height="86" alt="boost.png (6897 bytes)" src="../../boost.png" width="277" align="middle"
border="0"></A>make_shared and allocate_shared function templates</h1>
<p><A href="#Introduction">Introduction</A><br>
<A href="#Synopsis">Synopsis</A><br>
<A href="#functions">Free Functions</A><br>
<A href="#example">Example</A><br>
<h2><a name="Introduction">Introduction</a></h2>
<p>Consistent use of <a href="shared_ptr.htm"><code>shared_ptr</code></a>
can eliminate the need to use an explicit <code>delete</code>,
but alone it provides no support in avoiding explicit <code>new</code>.
There have been repeated requests from users for a factory function that creates
an object of a given type and returns a <code>shared_ptr</code> to it.
Besides convenience and style, such a function is also exception safe and
considerably faster because it can use a single allocation for both the object
and its corresponding control block, eliminating a significant portion of
<code>shared_ptr</code>'s construction overhead.
This eliminates one of the major efficiency complaints about <code>shared_ptr</code>.
</p>
<p>The header file <boost/make_shared.hpp> provides a family of overloaded function templates,
<code>make_shared</code> and <code>allocate_shared</code>, to address this need.
<code>make_shared</code> uses the global operator <code>new</code> to allocate memory,
whereas <code>allocate_shared</code> uses an user-supplied allocator, allowing finer control.</p>
<p>
The rationale for choosing the name <code>make_shared</code> is that the expression
<code>make_shared<Widget>()</code> can be read aloud and conveys the intended meaning.</p>
<h2><a name="Synopsis">Synopsis</a></h2>
<pre>namespace boost {
template<typename T> class shared_ptr;
template<typename T>
shared_ptr<T> <a href="#functions">make_shared</a>();
template<typename T, typename A>
shared_ptr<T> <a href="#functions">allocate_shared</a>( A const & );
#if defined( BOOST_HAS_VARIADIC_TMPL ) && defined( BOOST_HAS_RVALUE_REFS ) // C++0x prototypes
template<typename T, typename... Args>
shared_ptr<T> <a href="#functions">make_shared</a>( Args && ... args );
template<typename T, typename A, typename... Args>
shared_ptr<T> <a href="#functions">allocate_shared</a>( A const & a, Args && ... args );
#else // no C++0X support
template<typename T, typename Arg1 >
shared_ptr<T> <a href="#functions">make_shared</a>( Arg1 const & arg1 );
template<typename T, typename Arg1, typename Arg2 >
shared_ptr<T> <a href="#functions">make_shared</a>( Arg1 const & arg1, Arg2 const & arg2 );
// ...
template<typename T, typename Arg1, typename Arg2, ..., typename ArgN >
shared_ptr<T> <a href="#functions">make_shared</a>( Arg1 const & arg1, Arg2 const & arg2, ..., ArgN const & argN );
template<typename T, typename A, typename Arg1 >
shared_ptr<T> <a href="#functions">allocate_shared</a>( A const & a, Arg1 const & arg1 );
template<typename T, typename A, typename Arg1, typename Arg2 >
shared_ptr<T> <a href="#functions">allocate_shared</a>( Arg1 const & arg1, Arg2 const & arg2 );
// ...
template<typename T, typename A, typename Arg1, typename Arg2, ..., typename ArgN >
shared_ptr<T> <a href="#functions">allocate_shared</a>( A const & a, Arg1 const & arg1, Arg2 const & arg2, ..., ArgN const & argN );
#endif
}</pre>
<h2><a name="functions">Free Functions</a></h2>
<pre>template<class T, class... Args>
shared_ptr<T> make_shared( Args && ... args );
template<class T, class A, class... Args>
shared_ptr<T> allocate_shared( A const & a, Args && ... args );</pre>
<blockquote>
<p><b>Requires:</b> The expression <code>new( pv ) T( std::forward<Args>(args)... )</code>,
where <code>pv</code> is a <code>void*</code> pointing to storage suitable
to hold an object of type <code>T</code>,
shall be well-formed. <code>A</code> shall be an <em>Allocator</em>,
as described in section 20.1.5 (<stong>Allocator requirements</strong>) of the C++ Standard.
The copy constructor and destructor of <code>A</code> shall not throw.</p>
<p><b>Effects:</b> Allocates memory suitable for an object of type <code>T</code>
and constructs an object in it via the placement new expression <code>new( pv ) T()</code>
or <code>new( pv ) T( std::forward<Args>(args)... )</code>.
<code>allocate_shared</code> uses a copy of <code>a</code> to allocate memory.
If an exception is thrown, has no effect.</p>
<p><b>Returns:</b> A <code>shared_ptr</code> instance that stores and owns the address
of the newly constructed object of type <code>T</code>.</p>
<p><b>Postconditions:</b> <code>get() != 0 && use_count() == 1</code>.</p>
<p><b>Throws:</b> <code>bad_alloc</code>, or an exception thrown from <code>A::allocate</code>
or the constructor of <code>T</code>.</p>
<p><b>Notes:</b> This implementation allocates the memory required for the
returned <code>shared_ptr</code> and an object of type <code>T</code> in a single
allocation. This provides efficiency equivalent to an intrusive smart pointer.</p>
<p>The prototypes shown above are used if your compiler supports rvalue references
and variadic templates. They perfectly forward the <code>args</code> parameters to
the constructors of <code>T</code>.</p>
<p>Otherwise, the implementation will fall back on
forwarding the arguments to the constructors of <code>T</code> as const references.
If you need to pass a non-const reference to a constructor of <code>T</code>,
you may do so by wrapping the parameter in a call to <code>boost::ref</code>.
In addition, you will be
limited to a maximum of 9 arguments (not counting the allocator argument of
allocate_shared).</p>
</blockquote>
<h2><a name="example">Example</a></h2>
<pre>boost::shared_ptr<std::string> x = boost::make_shared<std::string>("hello, world!");
std::cout << *x;</pre>
<hr>
<p>
$Date: 2008-05-19 15:42:39 -0400 (Mon, 19 May 2008) $</p>
<p><small>Copyright 2008 Peter Dimov. Copyright 2008 Frank Mori Hess.
Distributed under the Boost Software License,
Version 1.0. See accompanying file <A href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</A>
or copy at <A href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>.</small></p>
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