projects / ceph.git / blame
| Commit | Line | Data |
|---|---|---|
| 7c673cae FG |
1 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0.1 Transitional//EN"> |
| 2 | ||
| 3 | <html> | |
| 4 | <head> | |
| 5 | <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> | |
| 6 | <title>Boost.Flyweight Documentation - Tutorial - Technical issues</title> | |
| 7 | <link rel="stylesheet" href="../style.css" type="text/css"> | |
| 8 | <link rel="start" href="../index.html"> | |
| 9 | <link rel="prev" href="extension.html"> | |
| 10 | <link rel="up" href="index.html"> | |
| 11 | <link rel="next" href="lambda_expressions.html"> | |
| 12 | </head> | |
| 13 | ||
| 14 | <body> | |
| 15 | <h1><img src="../../../../boost.png" alt="Boost logo" align= | |
| 16 | "middle" width="277" height="86">Boost.Flyweight Tutorial: Technical issues</h1> | |
| 17 | ||
| 18 | <div class="prev_link"><a href="extension.html"><img src="../prev.gif" alt="extending Boost.Flyweight" border="0"><br> | |
| 19 | Extending Boost.Flyweight | |
| 20 | </a></div> | |
| 21 | <div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.Flyweight tutorial" border="0"><br> | |
| 22 | Boost.Flyweight tutorial | |
| 23 | </a></div> | |
| 24 | <div class="next_link"><a href="lambda_expressions.html"><img src="../next.gif" alt="annex: MPL lambda expressions" border="0"><br> | |
| 25 | Annex: MPL lambda expressions | |
| 26 | </a></div><br clear="all" style="clear: all;"> | |
| 27 | ||
| 28 | <hr> | |
| 29 | ||
| 30 | <h2>Contents</h2> | |
| 31 | ||
| 32 | <ul> | |
| 33 | <li><a href="#static_init">Static data initialization</a></li> | |
| 34 | </ul> | |
| 35 | ||
| 36 | <h2><a name="static_init">Static data initialization</a></h2> | |
| 37 | ||
| 38 | <p> | |
| 39 | For any given <code>T</code>, the type <code>flyweight<T></code> | |
| 40 | maintains some class-wide or static data that needs to be properly | |
| 41 | initialized before the class can be used. The internal machinery of | |
| 42 | Boost.Flyweight guarantees that static data initialization | |
| 43 | takes place automatically before the first use of the particular | |
| 44 | <code>flyweight<T></code> instantiation in the program, and in | |
| 45 | any case always during the so-called <i>dynamic initialization phase</i> | |
| 46 | of the program startup sequence. Although this is not strictly | |
| 47 | required by the C++ standard, in current practice dynamic initialization | |
| 48 | is completed before <code>main()</code> begins. | |
| 49 | </p> | |
| 50 | ||
| 51 | <p> | |
| 52 | So, for all practical purposes, static data initialization is performed | |
| 53 | before <code>main()</code> or before the first pre-<code>main()</code> | |
| 54 | usage of the class, for instance if we declare a global | |
| 55 | <code>static flyweight<T></code> object. This covers the vast | |
| 56 | majority of usage cases in a transparent manner, but there are | |
| 57 | some scenarios where the automatic static data initialization | |
| 58 | policy of Boost.Flyweight can fail: | |
| 59 | </p> | |
| 60 | ||
| 61 | <blockquote><pre> | |
| 62 | <span class=comment>// global thread pool</span> | |
| 63 | ||
| 64 | <span class=keyword>class</span> <span class=identifier>thread_pool</span> | |
| 65 | <span class=special>{</span> | |
| 66 | <span class=keyword>public</span><span class=special>:</span> | |
| 67 | <span class=identifier>thread_pool</span><span class=special>()</span> | |
| 68 | <span class=special>{</span> | |
| 69 | <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> | |
| 70 | <span class=special>}</span> | |
| 71 | ||
| 72 | <span class=keyword>private</span><span class=special>:</span> | |
| 73 | <span class=keyword>static</span> <span class=keyword>void</span> <span class=identifier>thread_fun</span><span class=special>()</span> | |
| 74 | <span class=special>{</span> | |
| 75 | <span class=comment>// uses flyweight<std::string></span> | |
| 76 | <span class=special>}</span> | |
| 77 | <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> | |
| 78 | <span class=special>};</span> | |
| 79 | ||
| 80 | <span class=keyword>static</span> <span class=identifier>thread_pool</span> <span class=identifier>thpool</span><span class=special>;</span> | |
| 81 | ||
| 82 | <span class=keyword>int</span> <span class=identifier>main</span><span class=special>()</span> | |
| 83 | <span class=special>{</span> | |
| 84 | <span class=special>...</span> | |
| 85 | </pre></blockquote> | |
| 86 | ||
| 87 | <p> | |
| 88 | The global pool of the example launches several threads, each of which | |
| 89 | internally uses <code>flyweight<std::string></code>. | |
| 90 | Static data initialization can potentially be executed twice concurrently | |
| 91 | if two threads happen to collide on the first usage of | |
| 92 | <code>flyweight<std::string></code>: Boost.Flyweight initialization | |
| 93 | does not consider thread safety. So, we need to explicitly take care of | |
| 94 | static data initialization in a thread safe context before launching | |
| 95 | the threads: | |
| 96 | </p> | |
| 97 | ||
| 98 | <blockquote><pre> | |
| 99 | <span class=keyword>class</span> <span class=identifier>thread_pool</span> | |
| 100 | <span class=special>{</span> | |
| 101 | <span class=keyword>public</span><span class=special>:</span> | |
| 102 | <span class=identifier>thread_pool</span><span class=special>()</span> | |
| 103 | <span class=special>{</span> | |
| 104 | <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> | |
| 105 | <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> | |
| 106 | <span class=special>}</span> | |
| 107 | <span class=special>...</span> | |
| 108 | </pre></blockquote> | |
| 109 | ||
| 110 | <p> | |
| 111 | The static member function <code>init</code> is not thread safe, either: in our particular | |
| 112 | example it just happens to be called in a single threaded environment. | |
| 113 | When concurrency can happen, <code>flyweight<T>::init</code> must | |
| 114 | be properly synchronized by the programmer by using some mutual exclusion | |
| 115 | mechanisms of her own. | |
| 116 | </p> | |
| 117 | ||
| 118 | <p> | |
| 119 | The following is another example where the default static initialization | |
| 120 | provided by Boost.Flyweight can fail: | |
| 121 | </p> | |
| 122 | ||
| 123 | <blockquote><pre> | |
| 124 | <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> | |
| 125 | ||
| 126 | <span class=keyword>int</span> <span class=identifier>main</span><span class=special>()</span> | |
| 127 | <span class=special>{</span> | |
| 128 | <span class=comment>// use v</span> | |
| 129 | <span class=special>}</span> | |
| 130 | </pre></blockquote> | |
| 131 | ||
| 132 | <p> | |
| 133 | In some environments, the program above fails when exiting. For instance, if run | |
| 134 | from Microsoft Visual C++ environment in debug mode, a breakpoint is triggered | |
| 135 | at termination time and the debug output window shows a message along the following: | |
| 136 | </p> | |
| 137 | ||
| 138 | <blockquote><pre> | |
| 139 | HEAP[test.exe]: HEAP: Free Heap block 3a6488 modified at 3a6650 after it was | |
| 140 | freed | |
| 141 | Windows has triggered a breakpoint in test.exe. | |
| 142 | This may be due to a corruption of the heap, and indicates a bug in test.exe | |
| 143 | or any of the DLLs it has loaded. | |
| 144 | The output window may have more diagnostic information | |
| 145 | </pre></blockquote> | |
| 146 | ||
| 147 | <p> | |
| 148 | What is the problem? Although the type of <code>v</code> involves | |
| 149 | <code>flyweight<std::string></code>, constructing <code>v</code> as an empty vector | |
| 150 | need not create any flyweight object proper; so, | |
| 151 | it is perfectly possible that the static initialization of | |
| 152 | <code>flyweight<std::string></code> happens <i>after</i> the construction | |
| 153 | of <code>v</code>; when this is the case, the static destruction of | |
| 154 | the associated factory will occur <i>before</i> <code>v</code>'s | |
| 155 | destruction, leaving the vector with dangling flyweights. | |
| 156 | Again, the solution consists in explicitly forcing the static instantiation | |
| 157 | of <code>flyweight<std::string></code> before <code>v</code> is | |
| 158 | created. Here, calling | |
| 159 | the function <code>flyweight<std::string>::init</code> is a little | |
| 160 | cumbersome, so we can resort to the utility type | |
| 161 | <code>flyweight<std::string>::initializer</code> to do that job for us: | |
| 162 | </p> | |
| 163 | ||
| 164 | <blockquote><pre> | |
| 165 | <span class=comment>// equivalent to calling flyweight<std::string>::init()</span> | |
| 166 | <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> | |
| 167 | <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> | |
| 168 | ||
| 169 | <span class=keyword>int</span> <span class=identifier>main</span><span class=special>()</span> | |
| 170 | <span class=special>{</span> | |
| 171 | <span class=comment>// use v; no dangling flyweights at termination now</span> | |
| 172 | <span class=special>}</span> | |
| 173 | </pre></blockquote> | |
| 174 | ||
| 175 | <hr> | |
| 176 | ||
| 177 | <div class="prev_link"><a href="extension.html"><img src="../prev.gif" alt="extending Boost.Flyweight" border="0"><br> | |
| 178 | Extending Boost.Flyweight | |
| 179 | </a></div> | |
| 180 | <div class="up_link"><a href="index.html"><img src="../up.gif" alt="Boost.Flyweight tutorial" border="0"><br> | |
| 181 | Boost.Flyweight tutorial | |
| 182 | </a></div> | |
| 183 | <div class="next_link"><a href="lambda_expressions.html"><img src="../next.gif" alt="annex: MPL lambda expressions" border="0"><br> | |
| 184 | Annex: MPL lambda expressions | |
| 185 | </a></div><br clear="all" style="clear: all;"> | |
| 186 | ||
| 187 | <br> | |
| 188 | ||
| 189 | <p>Revised Octber 16th 2010</p> | |
| 190 | ||
| 191 | <p>© Copyright 2006-2010 Joaquín M López Muñoz. | |
| 192 | Distributed under the Boost Software | |
| 193 | License, Version 1.0. (See accompanying file <a href="../../../../LICENSE_1_0.txt"> | |
| 194 | LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt"> | |
| 195 | http://www.boost.org/LICENSE_1_0.txt</a>) | |
| 196 | </p> | |
| 197 | ||
| 198 | </body> | |
| 199 | </html> |