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| 4 | <head> |
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| 6 | <meta name="generator" content="Docutils 0.3.9: http://docutils.sourceforge.net/" /> |
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| 7 | <title>Boost Pointer Container Library</title> |
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| 8 | <link rel="stylesheet" href="default.css" type="text/css" /> |
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| 9 | </head> |
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| 10 | <body> |
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| 11 | <div class="document" id="boost-pointer-container-library"> |
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| 12 | <h1 class="title"><img alt="Boost" src="boost.png" /> Pointer Container Library</h1> |
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| 13 | <h2 class="subtitle" id="tutorial">Tutorial</h2> |
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| 14 | <p>The tutorial shows you the most simple usage of the |
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| 15 | library. It is assumed that the reader is familiar |
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| 16 | with the use of standard containers. Although |
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| 17 | the tutorial is devided into sections, it is recommended |
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| 18 | that you read it all from top to bottom.</p> |
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| 19 | <ul class="simple"> |
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| 20 | <li><a class="reference" href="#basic-usage">Basic usage</a></li> |
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| 21 | <li><a class="reference" href="#indirected-interface">Indirected interface</a></li> |
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| 22 | <li><a class="reference" href="#sequence-containers">Sequence containers</a></li> |
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| 23 | <li><a class="reference" href="#associative-containers">Associative containers</a></li> |
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| 24 | <li><a class="reference" href="#null-values">Null values</a></li> |
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| 25 | <li><a class="reference" href="#clonability">Clonability</a></li> |
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| 26 | <li><a class="reference" href="#new-functions">New functions</a></li> |
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| 27 | <li><a class="reference" href="#algorithms">Algorithms</a></li> |
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| 28 | </ul> |
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| 29 | <div class="section" id="basic-usage"> |
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| 30 | <h1><a name="basic-usage">Basic usage</a></h1> |
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| 31 | <p>The most important aspect of a pointer container is that it manages |
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| 32 | memory for you. This means that you in most cases do not need to worry |
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| 33 | about deleting memory.</p> |
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| 34 | <p>Let us assume that we have an OO-hierarchy of animals</p> |
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| 35 | <pre class="literal-block"> |
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| 36 | class animal : <a class="reference" href="http://www.boost.org/libs/utility/utility.htm#Class_noncopyable">boost::noncopyable</a> |
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| 37 | { |
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| 38 | public: |
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| 39 | virtual ~animal() {} |
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| 40 | virtual void eat() = 0; |
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| 41 | // ... |
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| 42 | }; |
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| 43 | |
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| 44 | class mammal : public animal |
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| 45 | { |
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| 46 | // ... |
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| 47 | }; |
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| 48 | |
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| 49 | class bird : public animal |
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| 50 | { |
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| 51 | // ... |
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| 52 | }; |
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| 53 | </pre> |
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| 54 | <p>Then the managing of the animals is straight-forward. Imagine a |
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| 55 | Zoo:</p> |
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| 56 | <pre class="literal-block"> |
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| 57 | class zoo |
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| 58 | { |
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| 59 | boost::ptr_vector<animal> the_animals; |
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| 60 | public: |
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| 61 | |
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| 62 | void add_animal( animal* a ) |
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| 63 | { |
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| 64 | the_animals.push_back( a ); |
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| 65 | } |
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| 66 | }; |
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| 67 | </pre> |
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| 68 | <p>Notice how just pass the class name to the container; there |
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| 69 | is no <tt class="docutils literal"><span class="pre">*</span></tt> to indicate it is a pointer. |
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| 70 | With this declaration we can now say:</p> |
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| 71 | <pre class="literal-block"> |
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| 72 | zoo the_zoo; |
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| 73 | the_zoo.add_animal( new mammal("joe") ); |
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| 74 | the_zoo.add_animal( new bird("dodo") ); |
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| 75 | </pre> |
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| 76 | <p>Thus we heap-allocate all elements of the container |
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| 77 | and never rely on copy-semantics.</p> |
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| 78 | </div> |
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| 79 | <div class="section" id="indirected-interface"> |
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| 80 | <h1><a name="indirected-interface">Indirected interface</a></h1> |
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| 81 | <p>As particular feature of the pointer containers is that |
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| 82 | the query interface is indirected. For example,</p> |
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| 83 | <pre class="literal-block"> |
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| 84 | boost::ptr_vector<animal> vec; |
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| 85 | vec.push_back( new animal ); // you add it as pointer ... |
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| 86 | vec[0].eat(); // but get a reference back |
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| 87 | </pre> |
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| 88 | <p>This indirection also happens to iterators, so</p> |
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| 89 | <pre class="literal-block"> |
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| 90 | typedef std::vector<animal*> std_vec; |
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| 91 | std_vec vec; |
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| 92 | ... |
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| 93 | std_vec::iterator i = vec.begin(); |
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| 94 | (*i)->eat(); // '*' needed |
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| 95 | </pre> |
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| 96 | <p>now becomes</p> |
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| 97 | <pre class="literal-block"> |
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| 98 | typedef boost::ptr_vector<animal> ptr_vec; |
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| 99 | ptr_vec vec; |
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| 100 | ptr_vec::iterator i = vec.begin(); |
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| 101 | i->eat(); // no indirection needed |
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| 102 | </pre> |
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| 103 | </div> |
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| 104 | <div class="section" id="sequence-containers"> |
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| 105 | <h1><a name="sequence-containers">Sequence containers</a></h1> |
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| 106 | <p>The sequence containers used when you do not need to |
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| 107 | keep an ordering on your elements. You can basically |
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| 108 | expect all operations of the normal standard containers |
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| 109 | to be available. So, for example, with a <tt class="docutils literal"><span class="pre">ptr_deque</span></tt> |
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| 110 | and <tt class="docutils literal"><span class="pre">ptr_list</span></tt> object you can say:</p> |
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| 111 | <pre class="literal-block"> |
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| 112 | boost::ptr_deque<animal> deq; |
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| 113 | deq.push_front( new animal ); |
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| 114 | deq.pop_front(); |
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| 115 | </pre> |
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| 116 | <p>because <tt class="docutils literal"><span class="pre">std::deque</span></tt> and <tt class="docutils literal"><span class="pre">std::list</span></tt> has <tt class="docutils literal"><span class="pre">push_front()</span></tt> |
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| 117 | and <tt class="docutils literal"><span class="pre">pop_front</span></tt> members.</p> |
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| 118 | <p>If the standard sequence support |
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| 119 | random access, so does the pointer container; for example:</p> |
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| 120 | <pre class="literal-block"> |
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| 121 | for( boost::ptr_deque<animal>::size_type i = 0u; |
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| 122 | i != deq.size(); ++i ) |
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| 123 | deq[i].eat(); |
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| 124 | </pre> |
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| 125 | <p>The <tt class="docutils literal"><span class="pre">ptr_vector</span></tt> also allows you to specify the size of |
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| 126 | the buffer to allocate; for example</p> |
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| 127 | <pre class="literal-block"> |
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| 128 | boost::ptr_vector<animal> animals( 10u ); |
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| 129 | </pre> |
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| 130 | <p>will reserve room for 10 animals.</p> |
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| 131 | </div> |
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| 132 | <div class="section" id="associative-containers"> |
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| 133 | <h1><a name="associative-containers">Associative containers</a></h1> |
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| 134 | <p>To keep an ordering on our animals, we could use a <tt class="docutils literal"><span class="pre">ptr_set</span></tt>:</p> |
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| 135 | <pre class="literal-block"> |
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| 136 | boost::ptr_set<animal> set; |
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| 137 | set.insert( new monkey("bobo") ); |
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| 138 | set.insert( new whale("anna") ); |
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| 139 | ... |
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| 140 | </pre> |
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| 141 | <p>This requires that <tt class="docutils literal"><span class="pre">operator<()</span></tt> is defined for animals. One |
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| 142 | way to do this could be</p> |
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| 143 | <pre class="literal-block"> |
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| 144 | inline bool operator<( const animal& l, const animal& r ) |
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| 145 | { |
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| 146 | return l.name() < r.name(); |
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| 147 | } |
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| 148 | </pre> |
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| 149 | <p>if we wanted to keep the animals sorted by name.</p> |
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| 150 | <p>Maybe you want to keep all the animals in zoo ordered wrt. |
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| 151 | their name, but it so happens that many animals have the |
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| 152 | same name. We can then use a <tt class="docutils literal"><span class="pre">ptr_multimap</span></tt>:</p> |
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| 153 | <pre class="literal-block"> |
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| 154 | typedef boost::ptr_multimap<std::string,animal> zoo_type; |
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| 155 | zoo_type zoo; |
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| 156 | std::string bobo = "bobo", |
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| 157 | anna = "anna"; |
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| 158 | zoo.insert( bobo, new monkey(bobo) ); |
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| 159 | zoo.insert( bobo, new elephant(bobo) ); |
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| 160 | zoo.insert( anna, new whale(anna) ); |
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| 161 | zoo.insert( anna, new emu(anna) ); |
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| 162 | </pre> |
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| 163 | <p>Note that must create the key as an lvalue |
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| 164 | (due to exception-safety issues); the following would not |
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| 165 | have compiled</p> |
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| 166 | <pre class="literal-block"> |
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| 167 | zoo.insert( "bobo", // this is bad, but you get compile error |
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| 168 | new monkey("bobo") ); |
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| 169 | </pre> |
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| 170 | <p>If a multimap is not needed, we can use <tt class="docutils literal"><span class="pre">operator[]()</span></tt> |
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| 171 | to avoid the clumsiness:</p> |
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| 172 | <pre class="literal-block"> |
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| 173 | boost::ptr_map<std::string,animal> animals; |
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| 174 | animals["bobo"].set_name("bobo"); |
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| 175 | </pre> |
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| 176 | <p>This requires a default constructor for animals and |
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| 177 | a function to do the initialization, in this case <tt class="docutils literal"><span class="pre">set_name()</span></tt>;</p> |
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| 178 | </div> |
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| 179 | <div class="section" id="null-values"> |
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| 180 | <h1><a name="null-values">Null values</a></h1> |
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| 181 | <p>By default, if you try to insert null into a container, an exception |
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| 182 | is thrown. If you want to allow nulls, then you must |
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| 183 | say so explicitly when declaring the container variable</p> |
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| 184 | <pre class="literal-block"> |
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| 185 | boost::ptr_vector< boost::nullable<animal> > animals_type; |
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| 186 | animals_type animals; |
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| 187 | ... |
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| 188 | animals.insert( animals.end(), new dodo("fido") ); |
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| 189 | animals.insert( animals.begin(), 0 ) // ok |
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| 190 | </pre> |
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| 191 | <p>Once you have inserted a null into the container, you must |
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| 192 | always check if the value is null before accessing the object</p> |
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| 193 | <pre class="literal-block"> |
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| 194 | for( animals_type::iterator i = animals.begin(); |
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| 195 | i != animals.end(); ++i ) |
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| 196 | { |
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| 197 | if( !boost::is_null(i) ) // always check for validity |
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| 198 | i->eat(); |
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| 199 | } |
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| 200 | </pre> |
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| 201 | <p>If the container support random access, you may also check this as</p> |
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| 202 | <pre class="literal-block"> |
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| 203 | for( animals_type::size_type i = 0u; |
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| 204 | i != animals.size(); ++i ) |
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| 205 | { |
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| 206 | if( !animals.is_null(i) ) |
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| 207 | animals[i].eat(); |
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| 208 | } |
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| 209 | </pre> |
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| 210 | <p>Note that it is meaningless to insert |
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| 211 | null into <tt class="docutils literal"><span class="pre">ptr_set</span></tt> and <tt class="docutils literal"><span class="pre">ptr_multiset</span></tt>.</p> |
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| 212 | </div> |
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| 213 | <div class="section" id="clonability"> |
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| 214 | <h1><a name="clonability">Clonability</a></h1> |
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| 215 | <p>In OO programming it is typical to prohibit copying of objects; the |
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| 216 | objects may sometimes be allowed to be clonable; for example,:</p> |
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| 217 | <pre class="literal-block"> |
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| 218 | animal* animal::clone() const |
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| 219 | { |
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| 220 | return do_clone(); // implemented by private virtual function |
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| 221 | } |
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| 222 | </pre> |
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| 223 | <p>If the OO hierarchy thus allows cloning, we need to tell the |
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| 224 | pointer containers how cloning is to be done. This is simply |
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| 225 | done by defining a free-standing function, <tt class="docutils literal"><span class="pre">new_clone()</span></tt>, |
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| 226 | in the same namespace as |
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| 227 | the object hierarchy:</p> |
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| 228 | <pre class="literal-block"> |
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| 229 | inline animal* new_clone( const animal& a ) |
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| 230 | { |
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| 231 | return a.clone(); |
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| 232 | } |
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| 233 | </pre> |
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| 234 | <p>That is all, now a lot of functions in a pointer container |
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| 235 | can exploit the clonability of the animal objects. For example</p> |
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| 236 | <pre class="literal-block"> |
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| 237 | typedef boost::ptr_list<animal> zoo_type; |
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| 238 | zoo_type zoo, another_zoo; |
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| 239 | ... |
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| 240 | another_zoo.assign( zoo.begin(), zoo.end() ); |
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| 241 | </pre> |
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| 242 | <p>will fill another zoo with clones of the first zoo. Similarly, |
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| 243 | insert() can now insert clones into your pointer container</p> |
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| 244 | <pre class="literal-block"> |
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| 245 | another_zoo.insert( another_zoo.begin(), zoo.begin(), zoo.end() ); |
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| 246 | </pre> |
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| 247 | <p>The whole container can now also be cloned</p> |
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| 248 | <pre class="literal-block"> |
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| 249 | zoo_type yet_another_zoo = zoo.clone(); |
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| 250 | </pre> |
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| 251 | </div> |
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| 252 | <div class="section" id="new-functions"> |
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| 253 | <h1><a name="new-functions">New functions</a></h1> |
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| 254 | <p>Given that we know we are working with pointers, a few new functions |
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| 255 | make sense. For example, say you want to remove an |
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| 256 | animal from the zoo</p> |
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| 257 | <pre class="literal-block"> |
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| 258 | zoo_type::auto_type the_animal = zoo.release( zoo.begin() ); |
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| 259 | the_animal->eat(); |
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| 260 | animal* the_animal_ptr = the_animal.release(); // now this is not deleted |
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| 261 | zoo.release(2); // for random access containers |
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| 262 | </pre> |
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| 263 | <p>You can think of <tt class="docutils literal"><span class="pre">auto_type</span></tt> as a non-copyable form of |
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| 264 | <tt class="docutils literal"><span class="pre">std::auto_ptr</span></tt>. Notice that when you release an object, the |
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| 265 | pointer is removed from the container and the containers size |
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| 266 | shrinks. You can also release the entire container if you |
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| 267 | want to return it from a function</p> |
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| 268 | <pre class="literal-block"> |
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| 269 | std::auto_ptr< boost::ptr_deque<animal> > get_zoo() |
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| 270 | { |
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| 271 | boost::ptr_deque<animal> result; |
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| 272 | ... |
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| 273 | return result.release(); // give up ownership |
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| 274 | } |
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| 275 | ... |
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| 276 | boost::ptr_deque<animal> animals = get_zoo(); |
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| 277 | </pre> |
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| 278 | <p>Let us assume we want to move an animal object from |
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| 279 | one zoo to another. In other words, we want to move the |
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| 280 | animal and the responsibility of it to another zoo</p> |
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| 281 | <pre class="literal-block"> |
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| 282 | another_zoo.transfer( another_zoo.end(), // insert before end |
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| 283 | zoo.begin(), // insert this animal ... |
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| 284 | zoo ); // from this container |
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| 285 | </pre> |
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| 286 | <p>This kind of "move-semantics" is different from |
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| 287 | normal value-based containers. You can think of <tt class="docutils literal"><span class="pre">transfer()</span></tt> |
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| 288 | as the same as <tt class="docutils literal"><span class="pre">splice()</span></tt> on <tt class="docutils literal"><span class="pre">std::list</span></tt>.</p> |
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| 289 | <p>If you want to replace an element, you can easily do so</p> |
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| 290 | <pre class="literal-block"> |
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| 291 | zoo_type::auto_type old_animal = zoo.replace( zoo.begin(), new monkey("bibi") ); |
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| 292 | zoo.replace( 2, old_animal.release() ); // for random access containers |
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| 293 | </pre> |
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| 294 | <p>A map is a little different to iterator over than standard maps. |
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| 295 | Now we say</p> |
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| 296 | <pre class="literal-block"> |
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| 297 | typedef boost::ptr_map<std::string, boost::nullable<animal> > animal_map; |
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| 298 | animal_map map; |
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| 299 | ... |
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| 300 | for( animal_map::iterator i = map.begin(); |
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| 301 | i != map.end(); ++i ) |
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| 302 | { |
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| 303 | std::cout << "\n key: " << i.key(); |
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| 304 | std::cout << "\n age: "; |
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| 305 | |
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| 306 | if( boost::is_null(i) ) |
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| 307 | std::cout << "unknown"; |
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| 308 | else |
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| 309 | std::cout << i->age(); |
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| 310 | } |
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| 311 | </pre> |
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| 312 | <p>Maps can also be indexed with bounds-checking</p> |
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| 313 | <pre class="literal-block"> |
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| 314 | try |
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| 315 | { |
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| 316 | animal& bobo = map.at("bobo"); |
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| 317 | } |
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| 318 | catch( boost::bad_ptr_container_operation& e ) |
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| 319 | { |
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| 320 | // "bobo" not found |
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| 321 | } |
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| 322 | </pre> |
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| 323 | </div> |
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| 324 | <div class="section" id="algorithms"> |
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| 325 | <h1><a name="algorithms">Algorithms</a></h1> |
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| 326 | <p>Unfortunately it is not possible to use pointer containers with |
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| 327 | mutating algorithms from the standard library. However, |
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| 328 | the most useful ones |
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| 329 | are instead provided as member functions:</p> |
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| 330 | <pre class="literal-block"> |
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| 331 | boost::ptr_vector<animal> zoo; |
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| 332 | ... |
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| 333 | zoo.sort(); // assume 'bool operator<( const animal&, const animal& )' |
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| 334 | zoo.sort( std::less<animal>() ); // the same, notice no '*' is present |
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| 335 | zoo.sort( zoo.begin(), zoo.begin() + 5 ); // sort selected range |
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| 336 | </pre> |
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| 337 | <p>Notice that predicates are automatically wrapped in an <a class="reference" href="indirect_fun.html">indirect_fun</a> object.</p> |
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| 338 | <p>You can remove equal and adjacent elements using <tt class="docutils literal"><span class="pre">unique()</span></tt>:</p> |
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| 339 | <pre class="literal-block"> |
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| 340 | zoo.unique(); // assume 'bool operator==( const animal&, const animal& )' |
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| 341 | zoo.unique( zoo.begin(), zoo.begin() + 5, my_comparison_predicate() ); |
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| 342 | </pre> |
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| 343 | <p>If you just want to remove certain elements, use <tt class="docutils literal"><span class="pre">erase_if</span></tt>:</p> |
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| 344 | <pre class="literal-block"> |
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| 345 | zoo.erase_if( my_predicate() ); |
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| 346 | </pre> |
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| 347 | <p>Finally you may want to merge together two sorted containers:</p> |
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| 348 | <pre class="literal-block"> |
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| 349 | boost::ptr_vector<animal> another_zoo = ...; |
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| 350 | another_zoo.sort(); // sorted wrt. to same order as 'zoo' |
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| 351 | zoo.merge( another_zoo ); |
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| 352 | BOOST_ASSERT( another_zoo.empty() ); |
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| 353 | </pre> |
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| 354 | <p>That is all; now you have learned all the basics!</p> |
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| 355 | <p><strong>Navigate</strong></p> |
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| 356 | <blockquote> |
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| 357 | <ul class="simple"> |
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| 358 | <li><a class="reference" href="ptr_container.html">home</a></li> |
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| 359 | <li><a class="reference" href="examples.html">examples</a></li> |
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| 360 | </ul> |
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| 361 | </blockquote> |
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| 362 | <table class="docutils field-list" frame="void" rules="none"> |
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| 363 | <col class="field-name" /> |
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| 364 | <col class="field-body" /> |
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| 365 | <tbody valign="top"> |
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| 366 | <tr class="field"><th class="field-name">copyright:</th><td class="field-body">Thorsten Ottosen 2004-2005.</td> |
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| 367 | </tr> |
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| 368 | </tbody> |
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| 369 | </table> |
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| 370 | </div> |
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| 371 | </div> |
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| 372 | </body> |
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| 373 | </html> |
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