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| <h1><img src="../../../../boost.png" alt="Boost logo" align= |
| "middle" width="277" height="86">Boost.Flyweight Tutorial: Technical issues</h1> |
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| <h2>Contents</h2> |
| |
| <ul> |
| <li><a href="#static_init">Static data initialization</a></li> |
| </ul> |
| |
| <h2><a name="static_init">Static data initialization</a></h2> |
| |
| <p> |
| For any given <code>T</code>, the type <code>flyweight<T></code> |
| maintains some class-wide or static data that needs to be properly |
| initialized before the class can be used. The internal machinery of |
| Boost.Flyweight guarantees that static data initialization |
| takes place automatically before the first use of the particular |
| <code>flyweight<T></code> instantiation in the program, and in |
| any case always during the so-called <i>dynamic initialization phase</i> |
| of the program startup sequence. Although this is not strictly |
| required by the C++ standard, in current practice dynamic initialization |
| is completed before <code>main()</code> begins. |
| </p> |
| |
| <p> |
| So, for all practical purposes, static data initialization is performed |
| before <code>main()</code> or before the first pre-<code>main()</code> |
| usage of the class, for instance if we declare a global |
| <code>static flyweight<T></code> object. This covers the vast |
| majority of usage cases in a transparent manner, but there are |
| some scenarios where the automatic static data initialization |
| policy of Boost.Flyweight can fail: |
| </p> |
| |
| <blockquote><pre> |
| <span class=comment>// global thread pool</span> |
| |
| <span class=keyword>class</span> <span class=identifier>thread_pool</span> |
| <span class=special>{</span> |
| <span class=keyword>public</span><span class=special>:</span> |
| <span class=identifier>thread_pool</span><span class=special>()</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=identifier>i</span><span class=special>)</span><span class=identifier>p</span><span class=special>[</span><span class=identifier>i</span><span class=special>]=</span><span class=identifier>shared_ptr</span><span class=special><</span><span class=identifier>thread</span><span class=special>>(</span><span class=keyword>new</span> <span class=identifier>thread</span><span class=special>(</span><span class=identifier>thread_fun</span><span class=special>));</span> |
| <span class=special>}</span> |
| |
| <span class=keyword>private</span><span class=special>:</span> |
| <span class=keyword>static</span> <span class=keyword>void</span> <span class=identifier>thread_fun</span><span class=special>()</span> |
| <span class=special>{</span> |
| <span class=comment>// uses flyweight<std::string></span> |
| <span class=special>}</span> |
| <span class=identifier>array</span><span class=special><</span><span class=identifier>shared_ptr</span><span class=special><</span><span class=identifier>thread</span><span class=special>>,</span><span class=number>100</span><span class=special>></span> <span class=identifier>p</span><span class=special>;</span> |
| <span class=special>};</span> |
| |
| <span class=keyword>static</span> <span class=identifier>thread_pool</span> <span class=identifier>thpool</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=special>...</span> |
| </pre></blockquote> |
| |
| <p> |
| The global pool of the example launches several threads, each of which |
| internally uses <code>flyweight<std::string></code>. |
| Static data initialization can potentially be executed twice concurrently |
| if two threads happen to collide on the first usage of |
| <code>flyweight<std::string></code>: Boost.Flyweight initialization |
| does not consider thread safety. So, we need to explicitly take care of |
| static data initialization in a thread safe context before launching |
| the threads: |
| </p> |
| |
| <blockquote><pre> |
| <span class=keyword>class</span> <span class=identifier>thread_pool</span> |
| <span class=special>{</span> |
| <span class=keyword>public</span><span class=special>:</span> |
| <span class=identifier>thread_pool</span><span class=special>()</span> |
| <span class=special>{</span> |
| <b><span class=identifier>flyweight</span><span class=special><</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>>::</span><span class=identifier>init</span><span class=special>();</span></b> |
| <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=identifier>i</span><span class=special>)</span><span class=identifier>p</span><span class=special>[</span><span class=identifier>i</span><span class=special>]=</span><span class=identifier>shared_ptr</span><span class=special><</span><span class=identifier>thread</span><span class=special>>(</span><span class=keyword>new</span> <span class=identifier>thread</span><span class=special>(</span><span class=identifier>thread_fun</span><span class=special>));</span> |
| <span class=special>}</span> |
| <span class=special>...</span> |
| </pre></blockquote> |
| |
| <p> |
| The static member function <code>init</code> is not thread safe, either: in our particular |
| example it just happens to be called in a single threaded environment. |
| When concurrency can happen, <code>flyweight<T>::init</code> must |
| be properly synchronized by the programmer by using some mutual exclusion |
| mechanisms of her own. |
| </p> |
| |
| <p> |
| The following is another example where the default static initialization |
| provided by Boost.Flyweight can fail: |
| </p> |
| |
| <blockquote><pre> |
| <span class=keyword>static</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>vector</span><span class=special><</span><span class=identifier>flyweight</span><span class=special><</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>></span> <span class=special>></span> <span class=identifier>v</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>// use v</span> |
| <span class=special>}</span> |
| </pre></blockquote> |
| |
| <p> |
| In some environments, the program above fails when exiting. For instance, if run |
| from Microsoft Visual C++ environment in debug mode, a breakpoint is triggered |
| at termination time and the debug output window shows a message along the following: |
| </p> |
| |
| <blockquote><pre> |
| HEAP[test.exe]: HEAP: Free Heap block 3a6488 modified at 3a6650 after it was |
| freed |
| Windows has triggered a breakpoint in test.exe. |
| This may be due to a corruption of the heap, and indicates a bug in test.exe |
| or any of the DLLs it has loaded. |
| The output window may have more diagnostic information |
| </pre></blockquote> |
| |
| <p> |
| What is the problem? Although the type of <code>v</code> involves |
| <code>flyweight<std::string></code>, constructing <code>v</code> as an empty vector |
| need not create any flyweight object proper; so, |
| it is perfectly possible that the static initialization of |
| <code>flyweight<std::string></code> happens <i>after</i> the construction |
| of <code>v</code>; when this is the case, the static destruction of |
| the associated factory will occur <i>before</i> <code>v</code>'s |
| destruction, leaving the vector with dangling flyweights. |
| Again, the solution consists in explicitly forcing the static instantiation |
| of <code>flyweight<std::string></code> before <code>v</code> is |
| created. Here, calling |
| the function <code>flyweight<std::string>::init</code> is a little |
| cumbersome, so we can resort to the utility type |
| <code>flyweight<std::string>::initializer</code> to do that job for us: |
| </p> |
| |
| <blockquote><pre> |
| <span class=comment>// equivalent to calling flyweight<std::string>::init()</span> |
| <b><span class=keyword>static</span> <span class=identifier>flyweight</span><span class=special><</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>>::</span><span class=identifier>initializer</span> <span class=identifier>fwinit</span><span class=special>;</span></b> |
| <span class=keyword>static</span> <span class=identifier>std</span><span class=special>::</span><span class=identifier>vector</span><span class=special><</span><span class=identifier>flyweight</span><span class=special><</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>></span> <span class=special>></span> <span class=identifier>v</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>// use v; no dangling flyweights at termination now</span> |
| <span class=special>}</span> |
| </pre></blockquote> |
| |
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| <br> |
| |
| <p>Revised Octber 16th 2010</p> |
| |
| <p>© Copyright 2006-2010 Joaquín M López Muñoz. |
| Distributed under the Boost Software |
| License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt"> |
| LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt"> |
| http://www.boost.org/LICENSE_1_0.txt</a>) |
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