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</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="interprocess.interprocess_smart_ptr"></a><a class="link" href="interprocess_smart_ptr.html" title="Ownership smart pointers">Ownership smart pointers</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="interprocess_smart_ptr.html#interprocess.interprocess_smart_ptr.intrusive_ptr">Intrusive
pointer</a></span></dt>
<dt><span class="section"><a href="interprocess_smart_ptr.html#interprocess.interprocess_smart_ptr.scoped_ptr">Scoped
pointer</a></span></dt>
<dt><span class="section"><a href="interprocess_smart_ptr.html#interprocess.interprocess_smart_ptr.shared_ptr">Shared
pointer and weak pointer</a></span></dt>
<dt><span class="section"><a href="interprocess_smart_ptr.html#interprocess.interprocess_smart_ptr.unique_ptr">Unique
pointer</a></span></dt>
</dl></div>
<p>
C++ users know the importance of ownership smart pointers when dealing with
resources. Boost offers a wide range of such type of pointers: <code class="computeroutput"><span class="identifier">intrusive_ptr</span><span class="special"><></span></code>,
<code class="computeroutput"><span class="identifier">scoped_ptr</span><span class="special"><></span></code>,
<code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special"><></span></code>...
</p>
<p>
When building complex shared memory/memory mapped files structures, programmers
would like to use also the advantages of these smart pointers. The problem
is that Boost and C++ TR1 smart pointers are not ready to be used for shared
memory. The cause is that those smart pointers contain raw pointers and they
use virtual functions, something that is not possible if you want to place
your data in shared memory. The virtual function limitation makes even impossible
to achieve the same level of functionality of Boost and TR1 with <span class="bold"><strong>Boost.Interprocess</strong></span> smart pointers.
</p>
<p>
Interprocess ownership smart pointers are mainly "smart pointers containing
smart pointers", so we can specify the pointer type they contain.
</p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.interprocess_smart_ptr.intrusive_ptr"></a><a class="link" href="interprocess_smart_ptr.html#interprocess.interprocess_smart_ptr.intrusive_ptr" title="Intrusive pointer">Intrusive
pointer</a>
</h3></div></div></div>
<p>
<code class="computeroutput"><a class="link" href="../boost/interprocess/intrusive_ptr.html" title="Class template intrusive_ptr">boost::interprocess::intrusive_ptr</a></code>
is the generalization of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive_ptr</span><span class="special"><></span></code> to allow non-raw pointers as intrusive
pointer members. As the well-known <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive_ptr</span></code>
we must specify the pointee type but we also must also specify the pointer
type to be stored in the intrusive_ptr:
</p>
<pre class="programlisting"><span class="comment">//!The intrusive_ptr class template stores a pointer to an object</span>
<span class="comment">//!with an embedded reference count. intrusive_ptr is parameterized on</span>
<span class="comment">//!T (the type of the object pointed to) and VoidPointer(a void pointer type</span>
<span class="comment">//!that defines the type of pointer that intrusive_ptr will store).</span>
<span class="comment">//!intrusive_ptr<T, void *> defines a class with a T* member whereas</span>
<span class="comment">//!intrusive_ptr<T, offset_ptr<void> > defines a class with a offset_ptr<T> member.</span>
<span class="comment">//!Relies on unqualified calls to:</span>
<span class="comment">//!</span>
<span class="comment">//!void intrusive_ptr_add_ref(T * p);</span>
<span class="comment">//!void intrusive_ptr_release(T * p);</span>
<span class="comment">//!</span>
<span class="comment">//!with (p != 0)</span>
<span class="comment">//!</span>
<span class="comment">//!The object is responsible for destroying itself.</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">VoidPointer</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">intrusive_ptr</span><span class="special">;</span>
</pre>
<p>
So <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">intrusive_ptr</span><span class="special"><</span><span class="identifier">MyClass</span><span class="special">,</span> <span class="keyword">void</span><span class="special">*></span></code>
is equivalent to <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive_ptr</span><span class="special"><</span><span class="identifier">MyClass</span><span class="special">></span></code>.
But if we want to place the intrusive_ptr in shared memory we must specify
a relative pointer type like <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">intrusive_ptr</span><span class="special"><</span><span class="identifier">MyClass</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span>
<span class="special">></span></code>
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">managed_shared_memory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">smart_ptr</span><span class="special">/</span><span class="identifier">intrusive_ptr</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">;</span>
<span class="keyword">namespace</span> <span class="identifier">N</span> <span class="special">{</span>
<span class="comment">//A class that has an internal reference count</span>
<span class="keyword">class</span> <span class="identifier">reference_counted_class</span>
<span class="special">{</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="comment">//Non-copyable</span>
<span class="identifier">reference_counted_class</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">reference_counted_class</span> <span class="special">&);</span>
<span class="comment">//Non-assignable</span>
<span class="identifier">reference_counted_class</span> <span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">reference_counted_class</span> <span class="special">&);</span>
<span class="comment">//A typedef to save typing</span>
<span class="keyword">typedef</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">segment_manager</span> <span class="identifier">segment_manager</span><span class="special">;</span>
<span class="comment">//This is the reference count</span>
<span class="keyword">unsigned</span> <span class="keyword">int</span> <span class="identifier">m_use_count</span><span class="special">;</span>
<span class="comment">//The segment manager allows deletion from shared memory segment</span>
<span class="identifier">offset_ptr</span><span class="special"><</span><span class="identifier">segment_manager</span><span class="special">></span> <span class="identifier">mp_segment_manager</span><span class="special">;</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="comment">//Constructor</span>
<span class="identifier">reference_counted_class</span><span class="special">(</span><span class="identifier">segment_manager</span> <span class="special">*</span><span class="identifier">s_mngr</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">m_use_count</span><span class="special">(</span><span class="number">0</span><span class="special">),</span> <span class="identifier">mp_segment_manager</span><span class="special">(</span><span class="identifier">s_mngr</span><span class="special">){}</span>
<span class="comment">//Destructor</span>
<span class="special">~</span><span class="identifier">reference_counted_class</span><span class="special">(){}</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="comment">//Returns the reference count</span>
<span class="keyword">unsigned</span> <span class="keyword">int</span> <span class="identifier">use_count</span><span class="special">()</span> <span class="keyword">const</span>
<span class="special">{</span> <span class="keyword">return</span> <span class="identifier">m_use_count</span><span class="special">;</span> <span class="special">}</span>
<span class="comment">//Adds a reference</span>
<span class="keyword">inline</span> <span class="keyword">friend</span> <span class="keyword">void</span> <span class="identifier">intrusive_ptr_add_ref</span><span class="special">(</span><span class="identifier">reference_counted_class</span> <span class="special">*</span> <span class="identifier">p</span><span class="special">)</span>
<span class="special">{</span> <span class="special">++</span><span class="identifier">p</span><span class="special">-></span><span class="identifier">m_use_count</span><span class="special">;</span> <span class="special">}</span>
<span class="comment">//Releases a reference</span>
<span class="keyword">inline</span> <span class="keyword">friend</span> <span class="keyword">void</span> <span class="identifier">intrusive_ptr_release</span><span class="special">(</span><span class="identifier">reference_counted_class</span> <span class="special">*</span> <span class="identifier">p</span><span class="special">)</span>
<span class="special">{</span> <span class="keyword">if</span><span class="special">(--</span><span class="identifier">p</span><span class="special">-></span><span class="identifier">m_use_count</span> <span class="special">==</span> <span class="number">0</span><span class="special">)</span> <span class="identifier">p</span><span class="special">-></span><span class="identifier">mp_segment_manager</span><span class="special">-></span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">p</span><span class="special">);</span> <span class="special">}</span>
<span class="special">};</span>
<span class="special">}</span> <span class="comment">//namespace N {</span>
<span class="comment">//A class that has an intrusive pointer to reference_counted_class</span>
<span class="keyword">class</span> <span class="identifier">intrusive_ptr_owner</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">intrusive_ptr</span><span class="special"><</span><span class="identifier">N</span><span class="special">::</span><span class="identifier">reference_counted_class</span><span class="special">,</span>
<span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span> <span class="special">></span> <span class="identifier">intrusive_ptr_t</span><span class="special">;</span>
<span class="identifier">intrusive_ptr_t</span> <span class="identifier">m_intrusive_ptr</span><span class="special">;</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="comment">//Takes a pointer to the reference counted class</span>
<span class="identifier">intrusive_ptr_owner</span><span class="special">(</span><span class="identifier">N</span><span class="special">::</span><span class="identifier">reference_counted_class</span> <span class="special">*</span><span class="identifier">ptr</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">m_intrusive_ptr</span><span class="special">(</span><span class="identifier">ptr</span><span class="special">){}</span>
<span class="special">};</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">//Remove shared memory on construction and destruction</span>
<span class="keyword">struct</span> <span class="identifier">shm_remove</span>
<span class="special">{</span>
<span class="identifier">shm_remove</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">~</span><span class="identifier">shm_remove</span><span class="special">(){</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">}</span> <span class="identifier">remover</span><span class="special">;</span>
<span class="comment">//Create shared memory</span>
<span class="identifier">managed_shared_memory</span> <span class="identifier">shmem</span><span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span> <span class="string">"MySharedMemory"</span><span class="special">,</span> <span class="number">10000</span><span class="special">);</span>
<span class="comment">//Create the unique reference counted object in shared memory</span>
<span class="identifier">N</span><span class="special">::</span><span class="identifier">reference_counted_class</span> <span class="special">*</span><span class="identifier">ref_counted</span> <span class="special">=</span>
<span class="identifier">shmem</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">N</span><span class="special">::</span><span class="identifier">reference_counted_class</span><span class="special">></span>
<span class="special">(</span><span class="string">"ref_counted"</span><span class="special">)(</span><span class="identifier">shmem</span><span class="special">.</span><span class="identifier">get_segment_manager</span><span class="special">());</span>
<span class="comment">//Create an array of ten intrusive pointer owners in shared memory</span>
<span class="identifier">intrusive_ptr_owner</span> <span class="special">*</span><span class="identifier">intrusive_owner_array</span> <span class="special">=</span>
<span class="identifier">shmem</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">intrusive_ptr_owner</span><span class="special">></span>
<span class="special">(</span><span class="identifier">anonymous_instance</span><span class="special">)[</span><span class="number">10</span><span class="special">](</span><span class="identifier">ref_counted</span><span class="special">);</span>
<span class="comment">//Now test that reference count is ten</span>
<span class="keyword">if</span><span class="special">(</span><span class="identifier">ref_counted</span><span class="special">-></span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">!=</span> <span class="number">10</span><span class="special">)</span>
<span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
<span class="comment">//Now destroy the array of intrusive pointer owners</span>
<span class="comment">//This should destroy every intrusive_ptr and because of</span>
<span class="comment">//that reference_counted_class will be destroyed</span>
<span class="identifier">shmem</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">intrusive_owner_array</span><span class="special">);</span>
<span class="comment">//Now the reference counted object should have been destroyed</span>
<span class="keyword">if</span><span class="special">(</span><span class="identifier">shmem</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">intrusive_ptr_owner</span><span class="special">>(</span><span class="string">"ref_counted"</span><span class="special">).</span><span class="identifier">first</span><span class="special">)</span>
<span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
<span class="comment">//Success!</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.interprocess_smart_ptr.scoped_ptr"></a><a class="link" href="interprocess_smart_ptr.html#interprocess.interprocess_smart_ptr.scoped_ptr" title="Scoped pointer">Scoped
pointer</a>
</h3></div></div></div>
<p>
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">scoped_ptr</span><span class="special"><></span></code>
is the big brother of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">scoped_ptr</span><span class="special"><></span></code>, which adds a custom deleter to specify
how the pointer passed to the scoped_ptr must be destroyed. Also, the <code class="computeroutput"><span class="identifier">pointer</span></code> typedef of the deleter will specify
the pointer type stored by scoped_ptr.
</p>
<pre class="programlisting"><span class="comment">//!scoped_ptr stores a pointer to a dynamically allocated object.</span>
<span class="comment">//!The object pointed to is guaranteed to be deleted, either on destruction</span>
<span class="comment">//!of the scoped_ptr, or via an explicit reset. The user can avoid this</span>
<span class="comment">//!deletion using release().</span>
<span class="comment">//!scoped_ptr is parameterized on T (the type of the object pointed to) and</span>
<span class="comment">//!Deleter (the functor to be executed to delete the internal pointer).</span>
<span class="comment">//!The internal pointer will be of the same pointer type as typename</span>
<span class="comment">//!Deleter::pointer type (that is, if typename Deleter::pointer is</span>
<span class="comment">//!offset_ptr<void>, the internal pointer will be offset_ptr<T>).</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Deleter</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">scoped_ptr</span><span class="special">;</span>
</pre>
<p>
<code class="computeroutput"><span class="identifier">scoped_ptr</span><span class="special"><></span></code>
comes handy to implement <span class="bold"><strong>rollbacks</strong></span> with
exceptions: if an exception is thrown or we call <code class="computeroutput"><span class="keyword">return</span></code>
in the scope of <code class="computeroutput"><span class="identifier">scoped_ptr</span><span class="special"><></span></code> the deleter is automatically called
so that <span class="bold"><strong>the deleter can be considered as a rollback</strong></span>
function. If all goes well, we call <code class="computeroutput"><span class="identifier">release</span><span class="special">()</span></code> member function to avoid rollback when
the <code class="computeroutput"><span class="identifier">scoped_ptr</span></code> goes out of
scope.
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">managed_shared_memory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">smart_ptr</span><span class="special">/</span><span class="identifier">scoped_ptr</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">;</span>
<span class="keyword">class</span> <span class="identifier">my_class</span>
<span class="special">{};</span>
<span class="keyword">class</span> <span class="identifier">my_exception</span>
<span class="special">{};</span>
<span class="comment">//A functor that destroys the shared memory object</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">my_deleter</span>
<span class="special">{</span>
<span class="keyword">private</span><span class="special">:</span>
<span class="comment">//A typedef to save typing</span>
<span class="keyword">typedef</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">segment_manager</span> <span class="identifier">segment_manager</span><span class="special">;</span>
<span class="comment">//This my_deleter is created in the stack, not in shared memory,</span>
<span class="comment">//so we can use raw pointers</span>
<span class="identifier">segment_manager</span> <span class="special">*</span><span class="identifier">mp_segment_manager</span><span class="special">;</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="comment">//This typedef will specify the pointer type that</span>
<span class="comment">//scoped_ptr will store</span>
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="special">*</span><span class="identifier">pointer</span><span class="special">;</span>
<span class="comment">//Constructor</span>
<span class="identifier">my_deleter</span><span class="special">(</span><span class="identifier">segment_manager</span> <span class="special">*</span><span class="identifier">s_mngr</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">mp_segment_manager</span><span class="special">(</span><span class="identifier">s_mngr</span><span class="special">){}</span>
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">pointer</span> <span class="identifier">object_to_delete</span><span class="special">)</span>
<span class="special">{</span> <span class="identifier">mp_segment_manager</span><span class="special">-></span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">object_to_delete</span><span class="special">);</span> <span class="special">}</span>
<span class="special">};</span>
<span class="keyword">int</span> <span class="identifier">main</span> <span class="special">()</span>
<span class="special">{</span>
<span class="comment">//Create shared memory</span>
<span class="comment">//Remove shared memory on construction and destruction</span>
<span class="keyword">struct</span> <span class="identifier">shm_remove</span>
<span class="special">{</span>
<span class="identifier">shm_remove</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">~</span><span class="identifier">shm_remove</span><span class="special">(){</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">}</span> <span class="identifier">remover</span><span class="special">;</span>
<span class="identifier">managed_shared_memory</span> <span class="identifier">shmem</span><span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span> <span class="string">"MySharedMemory"</span><span class="special">,</span> <span class="number">10000</span><span class="special">);</span>
<span class="comment">//In the first try, there will be no exceptions</span>
<span class="comment">//in the second try we will throw an exception</span>
<span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="number">2</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">){</span>
<span class="comment">//Create an object in shared memory</span>
<span class="identifier">my_class</span> <span class="special">*</span> <span class="identifier">my_object</span> <span class="special">=</span> <span class="identifier">shmem</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">my_class</span><span class="special">>(</span><span class="string">"my_object"</span><span class="special">)();</span>
<span class="identifier">my_class</span> <span class="special">*</span> <span class="identifier">my_object2</span> <span class="special">=</span> <span class="identifier">shmem</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">my_class</span><span class="special">>(</span><span class="identifier">anonymous_instance</span><span class="special">)();</span>
<span class="identifier">shmem</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">my_object2</span><span class="special">);</span>
<span class="comment">//Since the next shared memory allocation can throw</span>
<span class="comment">//assign it to a scoped_ptr so that if an exception occurs</span>
<span class="comment">//we destroy the object automatically</span>
<span class="identifier">my_deleter</span><span class="special"><</span><span class="identifier">my_class</span><span class="special">></span> <span class="identifier">d</span><span class="special">(</span><span class="identifier">shmem</span><span class="special">.</span><span class="identifier">get_segment_manager</span><span class="special">());</span>
<span class="keyword">try</span><span class="special">{</span>
<span class="identifier">scoped_ptr</span><span class="special"><</span><span class="identifier">my_class</span><span class="special">,</span> <span class="identifier">my_deleter</span><span class="special"><</span><span class="identifier">my_class</span><span class="special">></span> <span class="special">></span> <span class="identifier">s_ptr</span><span class="special">(</span><span class="identifier">my_object</span><span class="special">,</span> <span class="identifier">d</span><span class="special">);</span>
<span class="comment">//Let's emulate a exception capable operation</span>
<span class="comment">//In the second try, throw an exception</span>
<span class="keyword">if</span><span class="special">(</span><span class="identifier">i</span> <span class="special">==</span> <span class="number">1</span><span class="special">){</span>
<span class="keyword">throw</span><span class="special">(</span><span class="identifier">my_exception</span><span class="special">());</span>
<span class="special">}</span>
<span class="comment">//If we have passed the dangerous zone</span>
<span class="comment">//we can release the scoped pointer</span>
<span class="comment">//to avoid destruction</span>
<span class="identifier">s_ptr</span><span class="special">.</span><span class="identifier">release</span><span class="special">();</span>
<span class="special">}</span>
<span class="keyword">catch</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">my_exception</span> <span class="special">&){}</span>
<span class="comment">//Here, scoped_ptr is destroyed</span>
<span class="comment">//so it we haven't thrown an exception</span>
<span class="comment">//the object should be there, otherwise, destroyed</span>
<span class="keyword">if</span><span class="special">(</span><span class="identifier">i</span> <span class="special">==</span> <span class="number">0</span><span class="special">){</span>
<span class="comment">//Make sure the object is alive</span>
<span class="keyword">if</span><span class="special">(!</span><span class="identifier">shmem</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">my_class</span><span class="special">>(</span><span class="string">"my_object"</span><span class="special">).</span><span class="identifier">first</span><span class="special">){</span>
<span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
<span class="special">}</span>
<span class="comment">//Now we can use it and delete it manually</span>
<span class="identifier">shmem</span><span class="special">.</span><span class="identifier">destroy</span><span class="special"><</span><span class="identifier">my_class</span><span class="special">>(</span><span class="string">"my_object"</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">else</span><span class="special">{</span>
<span class="comment">//Make sure the object has been deleted</span>
<span class="keyword">if</span><span class="special">(</span><span class="identifier">shmem</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">my_class</span><span class="special">>(</span><span class="string">"my_object"</span><span class="special">).</span><span class="identifier">first</span><span class="special">){</span>
<span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.interprocess_smart_ptr.shared_ptr"></a><a class="link" href="interprocess_smart_ptr.html#interprocess.interprocess_smart_ptr.shared_ptr" title="Shared pointer and weak pointer">Shared
pointer and weak pointer</a>
</h3></div></div></div>
<p>
<span class="bold"><strong>Boost.Interprocess</strong></span> also offers the possibility
of creating non-intrusive reference-counted objects in managed shared memory
or mapped files.
</p>
<p>
Unlike <a href="http://www.boost.org/libs/smart_ptr/shared_ptr.htm" target="_top">boost::shared_ptr</a>,
due to limitations of mapped segments <code class="computeroutput"><a class="link" href="../boost/interprocess/shared_ptr.html" title="Class template shared_ptr">boost::interprocess::shared_ptr</a></code>
cannot take advantage of virtual functions to maintain the same shared pointer
type while providing user-defined allocators and deleters. The allocator
and the deleter are template parameters of the shared pointer.
</p>
<p>
Since the reference count and other auxiliary data needed by <code class="computeroutput"><a class="link" href="../boost/interprocess/shared_ptr.html" title="Class template shared_ptr">shared_ptr</a></code>
must be created also in the managed segment, and the deleter has to delete
the object from the segment, the user must specify an allocator object and
a deleter object when constructing a non-empty instance of <code class="computeroutput"><a class="link" href="../boost/interprocess/shared_ptr.html" title="Class template shared_ptr">shared_ptr</a></code>,
just like <span class="bold"><strong>Boost.Interprocess</strong></span> containers
need to pass allocators in their constructors.
</p>
<p>
Here is the declaration of <code class="computeroutput"><a class="link" href="../boost/interprocess/shared_ptr.html" title="Class template shared_ptr">shared_ptr</a></code>:
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">VoidAllocator</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Deleter</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">shared_ptr</span><span class="special">;</span>
</pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
T is the type of the pointed type.
</li>
<li class="listitem">
VoidAllocator is the allocator to be used to allocate auxiliary elements
such as the reference count, the deleter... The internal <code class="computeroutput"><span class="identifier">pointer</span></code> typedef of the allocator will
determine the type of pointer that shared_ptr will internally use, so
allocators defining <code class="computeroutput"><span class="identifier">pointer</span></code>
as <code class="computeroutput"><span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span></code>
will make all internal pointers used by <code class="computeroutput"><span class="identifier">shared_ptr</span></code>
to be also relative pointers. See <code class="computeroutput"><a class="link" href="../boost/interprocess/allocator.html" title="Class template allocator">boost::interprocess::allocator</a></code>
for a working allocator.
</li>
<li class="listitem">
Deleter is the function object that will be used to destroy the pointed
object when the last reference to the object is destroyed. The deleter
functor will take a pointer to T of the same category as the void pointer
defined by <code class="computeroutput"><span class="identifier">VoidAllocator</span><span class="special">::</span><span class="identifier">pointer</span></code>.
See <code class="computeroutput"><a class="link" href="../boost/interprocess/deleter.html" title="Class template deleter">boost::interprocess::deleter</a></code>
for a generic deleter that erases a object from a managed segment.
</li>
</ul></div>
<p>
With correctly specified parameters, <span class="bold"><strong>Boost.Interprocess</strong></span>
users can create objects in shared memory that hold shared pointers pointing
to other objects also in shared memory, obtaining the benefits of reference
counting. Let's see how to create a shared pointer in a managed shared memory:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">managed_shared_memory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">smart_ptr</span><span class="special">/</span><span class="identifier">shared_ptr</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">allocators</span><span class="special">/</span><span class="identifier">allocator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">smart_ptr</span><span class="special">/</span><span class="identifier">deleter</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">;</span>
<span class="comment">//This is type of the object we want to share</span>
<span class="keyword">class</span> <span class="identifier">MyType</span>
<span class="special">{};</span>
<span class="keyword">typedef</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">segment_manager</span> <span class="identifier">segment_manager_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">allocator</span><span class="special"><</span><span class="keyword">void</span><span class="special">,</span> <span class="identifier">segment_manager_type</span><span class="special">></span> <span class="identifier">void_allocator_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">deleter</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">,</span> <span class="identifier">segment_manager_type</span><span class="special">></span> <span class="identifier">deleter_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">,</span> <span class="identifier">void_allocator_type</span><span class="special">,</span> <span class="identifier">deleter_type</span><span class="special">></span> <span class="identifier">my_shared_ptr</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">main</span> <span class="special">()</span>
<span class="special">{</span>
<span class="comment">//Remove shared memory on construction and destruction</span>
<span class="keyword">struct</span> <span class="identifier">shm_remove</span>
<span class="special">{</span>
<span class="identifier">shm_remove</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">~</span><span class="identifier">shm_remove</span><span class="special">(){</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">}</span> <span class="identifier">remover</span><span class="special">;</span>
<span class="identifier">managed_shared_memory</span> <span class="identifier">segment</span><span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span> <span class="string">"MySharedMemory"</span><span class="special">,</span> <span class="number">4096</span><span class="special">);</span>
<span class="comment">//Create a shared pointer in shared memory</span>
<span class="comment">//pointing to a newly created object in the segment</span>
<span class="identifier">my_shared_ptr</span> <span class="special">&</span><span class="identifier">shared_ptr_instance</span> <span class="special">=</span>
<span class="special">*</span><span class="identifier">segment</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">my_shared_ptr</span><span class="special">>(</span><span class="string">"shared ptr"</span><span class="special">)</span>
<span class="comment">//Arguments to construct the shared pointer</span>
<span class="special">(</span> <span class="identifier">segment</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"object to share"</span><span class="special">)()</span> <span class="comment">//object to own</span>
<span class="special">,</span> <span class="identifier">void_allocator_type</span><span class="special">(</span><span class="identifier">segment</span><span class="special">.</span><span class="identifier">get_segment_manager</span><span class="special">())</span> <span class="comment">//allocator</span>
<span class="special">,</span> <span class="identifier">deleter_type</span><span class="special">(</span><span class="identifier">segment</span><span class="special">.</span><span class="identifier">get_segment_manager</span><span class="special">())</span> <span class="comment">//deleter</span>
<span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">shared_ptr_instance</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="number">1</span><span class="special">);</span>
<span class="comment">//Destroy "shared ptr". "object to share" will be automatically destroyed</span>
<span class="identifier">segment</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(&</span><span class="identifier">shared_ptr_instance</span><span class="special">);</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
<code class="computeroutput"><a class="link" href="../boost/interprocess/shared_ptr.html" title="Class template shared_ptr">boost::interprocess::shared_ptr</a></code>
is very flexible and configurable (we can specify the allocator and the deleter,
for example), but as shown the creation of a shared pointer in managed segments
need too much typing.
</p>
<p>
To simplify this usage, <code class="computeroutput"><a class="link" href="../boost/interprocess/shared_ptr.html" title="Class template shared_ptr">boost::interprocess::shared_ptr</a></code>
header offers a shared pointer definition helper class (<code class="computeroutput"><a class="link" href="../boost/interprocess/managed_shared_ptr.html" title="Struct template managed_shared_ptr">managed_shared_ptr</a></code>)
and a function (<code class="computeroutput"><a class="link" href="../boost/interprocess/make_man_idm46037322335680.html" title="Function template make_managed_shared_ptr">make_managed_shared_ptr</a></code>)
to easily construct a shared pointer from a type allocated in a managed segment
with an allocator that will allocate the reference count also in the managed
segment and a deleter that will erase the object from the segment.
</p>
<p>
These utilities will use a <span class="bold"><strong>Boost.Interprocess</strong></span>
allocator (<code class="computeroutput"><a class="link" href="../boost/interprocess/allocator.html" title="Class template allocator">boost::interprocess::allocator</a></code>)
and deleter (<code class="computeroutput"><a class="link" href="../boost/interprocess/deleter.html" title="Class template deleter">boost::interprocess::deleter</a></code>)
to do their job. The definition of the previous shared pointer could be simplified
to the following:
</p>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">managed_shared_ptr</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">,</span> <span class="identifier">managed_shared_memory</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">my_shared_ptr</span><span class="special">;</span>
</pre>
<p>
And the creation of a shared pointer can be simplified to this:
</p>
<pre class="programlisting"><span class="identifier">my_shared_ptr</span> <span class="identifier">sh_ptr</span> <span class="special">=</span> <span class="identifier">make_managed_shared_ptr</span>
<span class="special">(</span><span class="identifier">segment</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"object to share"</span><span class="special">)(),</span> <span class="identifier">segment</span><span class="special">);</span>
</pre>
<p>
<span class="bold"><strong>Boost.Interprocess</strong></span> also offers a weak pointer
named <code class="computeroutput"><a class="link" href="../boost/interprocess/weak_ptr.html" title="Class template weak_ptr">weak_ptr</a></code>
(with its corresponding <code class="computeroutput"><a class="link" href="../boost/interprocess/managed_weak_ptr.html" title="Struct template managed_weak_ptr">managed_weak_ptr</a></code>
and <code class="computeroutput"><a class="link" href="../boost/interprocess/make_managed_weak_ptr.html" title="Function template make_managed_weak_ptr">make_managed_weak_ptr</a></code>
utilities) to implement non-owning observers of an object owned by <code class="computeroutput"><a class="link" href="../boost/interprocess/shared_ptr.html" title="Class template shared_ptr">shared_ptr</a></code>.
</p>
<p>
Now let's see a detailed example of the use of <code class="computeroutput"><a class="link" href="../boost/interprocess/shared_ptr.html" title="Class template shared_ptr">shared_ptr</a></code>:
and <code class="computeroutput"><a class="link" href="../boost/interprocess/weak_ptr.html" title="Class template weak_ptr">weak_ptr</a></code>
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">managed_mapped_file</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">smart_ptr</span><span class="special">/</span><span class="identifier">shared_ptr</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">smart_ptr</span><span class="special">/</span><span class="identifier">weak_ptr</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">;</span>
<span class="comment">//This is type of the object we want to share</span>
<span class="keyword">struct</span> <span class="identifier">type_to_share</span>
<span class="special">{};</span>
<span class="comment">//This is the type of a shared pointer to the previous type</span>
<span class="comment">//that will be built in the mapped file</span>
<span class="keyword">typedef</span> <span class="identifier">managed_shared_ptr</span><span class="special"><</span><span class="identifier">type_to_share</span><span class="special">,</span> <span class="identifier">managed_mapped_file</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">shared_ptr_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">managed_weak_ptr</span><span class="special"><</span><span class="identifier">type_to_share</span><span class="special">,</span> <span class="identifier">managed_mapped_file</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">weak_ptr_type</span><span class="special">;</span>
<span class="comment">//This is a type holding a shared pointer</span>
<span class="keyword">struct</span> <span class="identifier">shared_ptr_owner</span>
<span class="special">{</span>
<span class="identifier">shared_ptr_owner</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">shared_ptr_type</span> <span class="special">&</span><span class="identifier">other_shared_ptr</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">shared_ptr_</span><span class="special">(</span><span class="identifier">other_shared_ptr</span><span class="special">)</span>
<span class="special">{}</span>
<span class="identifier">shared_ptr_owner</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">shared_ptr_owner</span> <span class="special">&</span><span class="identifier">other_owner</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">shared_ptr_</span><span class="special">(</span><span class="identifier">other_owner</span><span class="special">.</span><span class="identifier">shared_ptr_</span><span class="special">)</span>
<span class="special">{}</span>
<span class="identifier">shared_ptr_type</span> <span class="identifier">shared_ptr_</span><span class="special">;</span>
<span class="comment">//...</span>
<span class="special">};</span>
<span class="keyword">int</span> <span class="identifier">main</span> <span class="special">()</span>
<span class="special">{</span>
<span class="comment">//Define file names</span>
<span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">MappedFile</span> <span class="special">=</span> <span class="string">"MyMappedFile"</span><span class="special">;</span>
<span class="comment">//Destroy any previous file with the name to be used.</span>
<span class="keyword">struct</span> <span class="identifier">file_remove</span>
<span class="special">{</span>
<span class="identifier">file_remove</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">MappedFile</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">MappedFile_</span><span class="special">(</span><span class="identifier">MappedFile</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">file_mapping</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="identifier">MappedFile_</span><span class="special">);</span> <span class="special">}</span>
<span class="special">~</span><span class="identifier">file_remove</span><span class="special">(){</span> <span class="identifier">file_mapping</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="identifier">MappedFile_</span><span class="special">);</span> <span class="special">}</span>
<span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">MappedFile_</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">remover</span><span class="special">(</span><span class="identifier">MappedFile</span><span class="special">);</span>
<span class="special">{</span>
<span class="identifier">managed_mapped_file</span> <span class="identifier">file</span><span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span> <span class="identifier">MappedFile</span><span class="special">,</span> <span class="number">65536</span><span class="special">);</span>
<span class="comment">//Construct the shared type in the file and</span>
<span class="comment">//pass ownership to this local shared pointer</span>
<span class="identifier">shared_ptr_type</span> <span class="identifier">local_shared_ptr</span> <span class="special">=</span> <span class="identifier">make_managed_shared_ptr</span>
<span class="special">(</span><span class="identifier">file</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">type_to_share</span><span class="special">>(</span><span class="string">"object to share"</span><span class="special">)(),</span> <span class="identifier">file</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">local_shared_ptr</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="number">1</span><span class="special">);</span>
<span class="comment">//Share ownership of the object between local_shared_ptr and a new "owner1"</span>
<span class="identifier">shared_ptr_owner</span> <span class="special">*</span><span class="identifier">owner1</span> <span class="special">=</span>
<span class="identifier">file</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">shared_ptr_owner</span><span class="special">>(</span><span class="string">"owner1"</span><span class="special">)(</span><span class="identifier">local_shared_ptr</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">local_shared_ptr</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="number">2</span><span class="special">);</span>
<span class="comment">//local_shared_ptr releases object ownership</span>
<span class="identifier">local_shared_ptr</span><span class="special">.</span><span class="identifier">reset</span><span class="special">();</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">local_shared_ptr</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">owner1</span><span class="special">-></span><span class="identifier">shared_ptr_</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="number">1</span><span class="special">);</span>
<span class="comment">//Share ownership of the object between "owner1" and a new "owner2"</span>
<span class="identifier">shared_ptr_owner</span> <span class="special">*</span><span class="identifier">owner2</span> <span class="special">=</span>
<span class="identifier">file</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">shared_ptr_owner</span><span class="special">>(</span><span class="string">"owner2"</span><span class="special">)(*</span><span class="identifier">owner1</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">owner1</span><span class="special">-></span><span class="identifier">shared_ptr_</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="number">2</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">owner2</span><span class="special">-></span><span class="identifier">shared_ptr_</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="number">2</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">owner1</span><span class="special">-></span><span class="identifier">shared_ptr_</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">owner2</span><span class="special">-></span><span class="identifier">shared_ptr_</span><span class="special">.</span><span class="identifier">get</span><span class="special">());</span>
<span class="comment">//The mapped file is unmapped here. Objects have been flushed to disk</span>
<span class="special">}</span>
<span class="special">{</span>
<span class="comment">//Reopen the mapped file and find again all owners</span>
<span class="identifier">managed_mapped_file</span> <span class="identifier">file</span><span class="special">(</span><span class="identifier">open_only</span><span class="special">,</span> <span class="identifier">MappedFile</span><span class="special">);</span>
<span class="identifier">shared_ptr_owner</span> <span class="special">*</span><span class="identifier">owner1</span> <span class="special">=</span> <span class="identifier">file</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">shared_ptr_owner</span><span class="special">>(</span><span class="string">"owner1"</span><span class="special">).</span><span class="identifier">first</span><span class="special">;</span>
<span class="identifier">shared_ptr_owner</span> <span class="special">*</span><span class="identifier">owner2</span> <span class="special">=</span> <span class="identifier">file</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">shared_ptr_owner</span><span class="special">>(</span><span class="string">"owner2"</span><span class="special">).</span><span class="identifier">first</span><span class="special">;</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">owner1</span> <span class="special">&&</span> <span class="identifier">owner2</span><span class="special">);</span>
<span class="comment">//Check everything is as expected</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">file</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">type_to_share</span><span class="special">>(</span><span class="string">"object to share"</span><span class="special">).</span><span class="identifier">first</span> <span class="special">!=</span> <span class="number">0</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">owner1</span><span class="special">-></span><span class="identifier">shared_ptr_</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="number">2</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">owner2</span><span class="special">-></span><span class="identifier">shared_ptr_</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="number">2</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">owner1</span><span class="special">-></span><span class="identifier">shared_ptr_</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">owner2</span><span class="special">-></span><span class="identifier">shared_ptr_</span><span class="special">.</span><span class="identifier">get</span><span class="special">());</span>
<span class="comment">//Now destroy one of the owners, the reference count drops.</span>
<span class="identifier">file</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">owner1</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">owner2</span><span class="special">-></span><span class="identifier">shared_ptr_</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="number">1</span><span class="special">);</span>
<span class="comment">//Create a weak pointer</span>
<span class="identifier">weak_ptr_type</span> <span class="identifier">local_observer1</span><span class="special">(</span><span class="identifier">owner2</span><span class="special">-></span><span class="identifier">shared_ptr_</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">local_observer1</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">owner2</span><span class="special">-></span><span class="identifier">shared_ptr_</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">());</span>
<span class="special">{</span> <span class="comment">//Create a local shared pointer from the weak pointer</span>
<span class="identifier">shared_ptr_type</span> <span class="identifier">local_shared_ptr</span> <span class="special">=</span> <span class="identifier">local_observer1</span><span class="special">.</span><span class="identifier">lock</span><span class="special">();</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">local_observer1</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">owner2</span><span class="special">-></span><span class="identifier">shared_ptr_</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">());</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">local_observer1</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="number">2</span><span class="special">);</span>
<span class="special">}</span>
<span class="comment">//Now destroy the remaining owner. "object to share" will be destroyed</span>
<span class="identifier">file</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">owner2</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">file</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">type_to_share</span><span class="special">>(</span><span class="string">"object to share"</span><span class="special">).</span><span class="identifier">first</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
<span class="comment">//Test observer</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">local_observer1</span><span class="special">.</span><span class="identifier">expired</span><span class="special">());</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">local_observer1</span><span class="special">.</span><span class="identifier">use_count</span><span class="special">()</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
<span class="comment">//The reference count will be deallocated when all weak pointers</span>
<span class="comment">//disappear. After that, the file is unmapped.</span>
<span class="special">}</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
In general, using <span class="bold"><strong>Boost.Interprocess</strong></span>' <code class="computeroutput"><a class="link" href="../boost/interprocess/shared_ptr.html" title="Class template shared_ptr">shared_ptr</a></code> and <code class="computeroutput"><a class="link" href="../boost/interprocess/weak_ptr.html" title="Class template weak_ptr">weak_ptr</a></code> is very similar
to their counterparts <a href="http://www.boost.org/libs/smart_ptr/shared_ptr.htm" target="_top">boost::shared_ptr</a>
and <a href="http://www.boost.org/libs/smart_ptr/weak_ptr.htm" target="_top">boost::weak_ptr</a>,
but they need more template parameters and more run-time parameters in their
constructors.
</p>
<p>
Just like <a href="http://www.boost.org/libs/smart_ptr/shared_ptr.htm" target="_top">boost::shared_ptr</a>
can be stored in a STL container, <code class="computeroutput"><a class="link" href="../boost/interprocess/shared_ptr.html" title="Class template shared_ptr">shared_ptr</a></code>
can also be stored in <span class="bold"><strong>Boost.Interprocess</strong></span>
containers.
</p>
<p>
If a programmer just uses <code class="computeroutput"><a class="link" href="../boost/interprocess/shared_ptr.html" title="Class template shared_ptr">shared_ptr</a></code>
to be able to insert dynamically constructed objects into a container constructed
in the managed segment, but he does not need to share the ownership of that
object with other objects <code class="computeroutput"><a class="link" href="../boost/interprocess/managed_unique_ptr.html" title="Struct template managed_unique_ptr">managed_unique_ptr</a></code>
is a much faster and easier to use alternative.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.interprocess_smart_ptr.unique_ptr"></a><a class="link" href="interprocess_smart_ptr.html#interprocess.interprocess_smart_ptr.unique_ptr" title="Unique pointer">Unique
pointer</a>
</h3></div></div></div>
<p>
Unique ownership smart pointers are really useful to free programmers from
manual resource liberation of non-shared objects. <span class="bold"><strong>Boost.Interprocess</strong></span>'
<code class="computeroutput"><span class="identifier">unique_ptr</span></code> is much like
<code class="computeroutput"><a class="link" href="../boost/interprocess/scoped_ptr.html" title="Class template scoped_ptr">scoped_ptr</a></code> but
it's <span class="bold"><strong>moveable</strong></span> and can be easily inserted
in <span class="bold"><strong>Boost.Interprocess</strong></span> containers. Interprocess
had its own <code class="computeroutput"><span class="identifier">unique_ptr</span></code> implementation
but from Boost 1.57, <span class="bold"><strong>Boost.Interprocess</strong></span>
uses the improved and generic <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique_ptr</span></code>
implementation. Here is the declaration of the unique pointer class:
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">D</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">unique_ptr</span><span class="special">;</span>
</pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
T is the type of the object pointed by <code class="computeroutput"><span class="identifier">unique_ptr</span></code>.
</li>
<li class="listitem">
D is the deleter that will erase the object type of the object pointed
by the unique_ptr when the unique pointer is destroyed (and if still
has the ownership of the object). If the deleter defines an internal
<code class="computeroutput"><span class="identifier">pointer</span></code> typedef, <code class="computeroutput"><span class="identifier">unique_ptr</span></code>] will use an internal pointer
of the same type. So if <code class="computeroutput"><span class="identifier">D</span><span class="special">::</span><span class="identifier">pointer</span></code>
is <code class="computeroutput"><span class="identifier">offset_ptr</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
the unique pointer will store a relative pointer instead of a raw one.
This allows placing <code class="computeroutput"><span class="identifier">unique_ptr</span></code>
in shared memory and memory-mapped files.
</li>
</ul></div>
<p>
<code class="computeroutput"><span class="identifier">unique_ptr</span></code> can release the
ownership of the stored pointer so it's useful also to be used as a rollback
function. One of the main properties of the class is that <span class="bold"><strong>is
not copyable, but only moveable</strong></span>. When a unique pointer is moved
to another one, the ownership of the pointer is transferred from the source
unique pointer to the target unique pointer. If the target unique pointer
owned an object, that object is first deleted before taking ownership of
the new object.
</p>
<p>
<span class="bold"><strong>Boost.Interprocess</strong></span> also offers auxiliary
types to easily define and construct unique pointers that can be placed in
managed segments and will correctly delete owned object from the segment:
<code class="computeroutput"><a class="link" href="../boost/interprocess/managed_unique_ptr.html" title="Struct template managed_unique_ptr">managed_unique_ptr</a></code>
and <code class="computeroutput"><a class="link" href="../boost/interprocess/make_managed_unique_ptr.html" title="Function template make_managed_unique_ptr">make_managed_unique_ptr</a></code>
utilities.
</p>
<p>
Here we see an example of the use <code class="computeroutput"><span class="identifier">unique_ptr</span></code>
including creating containers of such objects:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">managed_mapped_file</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">smart_ptr</span><span class="special">/</span><span class="identifier">unique_ptr</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">containers</span><span class="special">/</span><span class="identifier">vector</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">containers</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">allocators</span><span class="special">/</span><span class="identifier">allocator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">;</span>
<span class="comment">//This is type of the object we'll allocate dynamically</span>
<span class="keyword">struct</span> <span class="identifier">MyType</span>
<span class="special">{</span>
<span class="identifier">MyType</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">number</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">number_</span><span class="special">(</span><span class="identifier">number</span><span class="special">)</span>
<span class="special">{}</span>
<span class="keyword">int</span> <span class="identifier">number_</span><span class="special">;</span>
<span class="special">};</span>
<span class="comment">//This is the type of a unique pointer to the previous type</span>
<span class="comment">//that will be built in the mapped file</span>
<span class="keyword">typedef</span> <span class="identifier">managed_unique_ptr</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">,</span> <span class="identifier">managed_mapped_file</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">unique_ptr_type</span><span class="special">;</span>
<span class="comment">//Define containers of unique pointer. Unique pointer simplifies object management</span>
<span class="keyword">typedef</span> <span class="identifier">vector</span>
<span class="special"><</span> <span class="identifier">unique_ptr_type</span>
<span class="special">,</span> <span class="identifier">allocator</span><span class="special"><</span><span class="identifier">unique_ptr_type</span><span class="special">,</span> <span class="identifier">managed_mapped_file</span><span class="special">::</span><span class="identifier">segment_manager</span><span class="special">></span>
<span class="special">></span> <span class="identifier">unique_ptr_vector_t</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">list</span>
<span class="special"><</span> <span class="identifier">unique_ptr_type</span>
<span class="special">,</span> <span class="identifier">allocator</span><span class="special"><</span><span class="identifier">unique_ptr_type</span><span class="special">,</span> <span class="identifier">managed_mapped_file</span><span class="special">::</span><span class="identifier">segment_manager</span><span class="special">></span>
<span class="special">></span> <span class="identifier">unique_ptr_list_t</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">main</span> <span class="special">()</span>
<span class="special">{</span>
<span class="comment">//Define file names</span>
<span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">MappedFile</span> <span class="special">=</span> <span class="string">"MyMappedFile"</span><span class="special">;</span>
<span class="comment">//Destroy any previous file with the name to be used.</span>
<span class="keyword">struct</span> <span class="identifier">file_remove</span>
<span class="special">{</span>
<span class="identifier">file_remove</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">MappedFile</span><span class="special">)</span>
<span class="special">:</span> <span class="identifier">MappedFile_</span><span class="special">(</span><span class="identifier">MappedFile</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">file_mapping</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="identifier">MappedFile_</span><span class="special">);</span> <span class="special">}</span>
<span class="special">~</span><span class="identifier">file_remove</span><span class="special">(){</span> <span class="identifier">file_mapping</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="identifier">MappedFile_</span><span class="special">);</span> <span class="special">}</span>
<span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">MappedFile_</span><span class="special">;</span>
<span class="special">}</span> <span class="identifier">remover</span><span class="special">(</span><span class="identifier">MappedFile</span><span class="special">);</span>
<span class="special">{</span>
<span class="identifier">managed_mapped_file</span> <span class="identifier">file</span><span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span> <span class="identifier">MappedFile</span><span class="special">,</span> <span class="number">65536</span><span class="special">);</span>
<span class="comment">//Construct an object in the file and</span>
<span class="comment">//pass ownership to this local unique pointer</span>
<span class="identifier">unique_ptr_type</span> <span class="identifier">local_unique_ptr</span> <span class="special">(</span><span class="identifier">make_managed_unique_ptr</span>
<span class="special">(</span><span class="identifier">file</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"unique object"</span><span class="special">)(),</span> <span class="identifier">file</span><span class="special">));</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">local_unique_ptr</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">!=</span> <span class="number">0</span><span class="special">);</span>
<span class="comment">//Reset the unique pointer. The object is automatically destroyed</span>
<span class="identifier">local_unique_ptr</span><span class="special">.</span><span class="identifier">reset</span><span class="special">();</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">file</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"unique object"</span><span class="special">).</span><span class="identifier">first</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
<span class="comment">//Now create a vector of unique pointers</span>
<span class="identifier">unique_ptr_vector_t</span> <span class="special">*</span><span class="identifier">unique_vector</span> <span class="special">=</span>
<span class="identifier">file</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">unique_ptr_vector_t</span><span class="special">>(</span><span class="string">"unique vector"</span><span class="special">)(</span><span class="identifier">file</span><span class="special">.</span><span class="identifier">get_segment_manager</span><span class="special">());</span>
<span class="comment">//Speed optimization</span>
<span class="identifier">unique_vector</span><span class="special">-></span><span class="identifier">reserve</span><span class="special">(</span><span class="number">100</span><span class="special">);</span>
<span class="comment">//Now insert all values</span>
<span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="number">100</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">){</span>
<span class="identifier">unique_ptr_type</span> <span class="identifier">p</span><span class="special">(</span><span class="identifier">make_managed_unique_ptr</span><span class="special">(</span><span class="identifier">file</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="identifier">anonymous_instance</span><span class="special">)(</span><span class="identifier">i</span><span class="special">),</span> <span class="identifier">file</span><span class="special">));</span>
<span class="identifier">unique_vector</span><span class="special">-></span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">p</span><span class="special">));</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">unique_vector</span><span class="special">-></span><span class="identifier">back</span><span class="special">()-></span><span class="identifier">number_</span> <span class="special">==</span> <span class="identifier">i</span><span class="special">);</span>
<span class="special">}</span>
<span class="comment">//Now create a list of unique pointers</span>
<span class="identifier">unique_ptr_list_t</span> <span class="special">*</span><span class="identifier">unique_list</span> <span class="special">=</span>
<span class="identifier">file</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">unique_ptr_list_t</span><span class="special">>(</span><span class="string">"unique list"</span><span class="special">)(</span><span class="identifier">file</span><span class="special">.</span><span class="identifier">get_segment_manager</span><span class="special">());</span>
<span class="comment">//Pass ownership of all values to the list</span>
<span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">99</span><span class="special">;</span> <span class="special">!</span><span class="identifier">unique_vector</span><span class="special">-></span><span class="identifier">empty</span><span class="special">();</span> <span class="special">--</span><span class="identifier">i</span><span class="special">){</span>
<span class="identifier">unique_list</span><span class="special">-></span><span class="identifier">push_front</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">unique_vector</span><span class="special">-></span><span class="identifier">back</span><span class="special">()));</span>
<span class="comment">//The unique ptr of the vector is now empty...</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">unique_vector</span><span class="special">-></span><span class="identifier">back</span><span class="special">()</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
<span class="identifier">unique_vector</span><span class="special">-></span><span class="identifier">pop_back</span><span class="special">();</span>
<span class="comment">//...and the list has taken ownership of the value</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">unique_list</span><span class="special">-></span><span class="identifier">front</span><span class="special">()</span> <span class="special">!=</span> <span class="number">0</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">unique_list</span><span class="special">-></span><span class="identifier">front</span><span class="special">()-></span><span class="identifier">number_</span> <span class="special">==</span> <span class="identifier">i</span><span class="special">);</span>
<span class="special">}</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">unique_list</span><span class="special">-></span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="number">100</span><span class="special">);</span>
<span class="comment">//Now destroy the empty vector.</span>
<span class="identifier">file</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">unique_vector</span><span class="special">);</span>
<span class="comment">//The mapped file is unmapped here. Objects have been flushed to disk</span>
<span class="special">}</span>
<span class="special">{</span>
<span class="comment">//Reopen the mapped file and find again the list</span>
<span class="identifier">managed_mapped_file</span> <span class="identifier">file</span><span class="special">(</span><span class="identifier">open_only</span><span class="special">,</span> <span class="identifier">MappedFile</span><span class="special">);</span>
<span class="identifier">unique_ptr_list_t</span> <span class="special">*</span><span class="identifier">unique_list</span> <span class="special">=</span>
<span class="identifier">file</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">unique_ptr_list_t</span><span class="special">>(</span><span class="string">"unique list"</span><span class="special">).</span><span class="identifier">first</span><span class="special">;</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">unique_list</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">unique_list</span><span class="special">-></span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="number">100</span><span class="special">);</span>
<span class="identifier">unique_ptr_list_t</span><span class="special">::</span><span class="identifier">const_iterator</span> <span class="identifier">list_it</span> <span class="special">=</span> <span class="identifier">unique_list</span><span class="special">-></span><span class="identifier">begin</span><span class="special">();</span>
<span class="keyword">for</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="number">100</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">,</span> <span class="special">++</span><span class="identifier">list_it</span><span class="special">){</span>
<span class="identifier">assert</span><span class="special">((*</span><span class="identifier">list_it</span><span class="special">)-></span><span class="identifier">number_</span> <span class="special">==</span> <span class="identifier">i</span><span class="special">);</span>
<span class="special">}</span>
<span class="comment">//Now destroy the list. All elements will be automatically deallocated.</span>
<span class="identifier">file</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">unique_list</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
</div>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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