435 lines
14 KiB
HTML
435 lines
14 KiB
HTML
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</head>
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<body>
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<div class="document" id="boost-pointer-container-library">
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<h1 class="title"><img alt="Boost" src="boost.png" /> Pointer Container Library</h1>
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<h2 class="subtitle" id="usage-guidelines">Usage Guidelines</h2>
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<div class="contents local topic">
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<ul class="simple">
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<li><a class="reference" href="#choosing-the-right-container" id="id8" name="id8">Choosing the right container</a></li>
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<li><a class="reference" href="#recommended-practice-for-object-oriented-programming" id="id9" name="id9">Recommended practice for Object-Oriented Programming</a><ul>
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<li><a class="reference" href="#make-base-classes-abstract-and-without-data" id="id10" name="id10">1. Make base classes abstract and without data</a></li>
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<li><a class="reference" href="#make-virtual-functions-private-and-provide-a-non-virtual-public-forwarding-function" id="id11" name="id11">2. Make virtual functions private and provide a non-virtual public forwarding function</a></li>
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<li><a class="reference" href="#derive-your-base-class-from-boost-noncopyable" id="id12" name="id12">3. Derive your base class from <tt class="docutils literal"><span class="pre">boost::noncopyable</span></tt></a></li>
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<li><a class="reference" href="#avoid-null-pointers-in-containers-if-possible" id="id13" name="id13">4. Avoid null-pointers in containers (if possible)</a></li>
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</ul>
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</li>
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</ul>
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</div>
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<div class="section">
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<h1><a class="toc-backref" href="#id8" id="choosing-the-right-container" name="choosing-the-right-container">Choosing the right container</a></h1>
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<p>The recommended usage pattern of the container classes is the same as
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for normal standard containers.</p>
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<p><tt class="docutils literal"><span class="pre">ptr_vector</span></tt>, <tt class="docutils literal"><span class="pre">ptr_list</span></tt> and <tt class="docutils literal"><span class="pre">ptr_deque</span></tt> offer the programmer different
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complexity tradeoffs and should be used accordingly. <tt class="docutils literal"><span class="pre">ptr_vector</span></tt> is the
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type of sequence that should be used by default. <tt class="docutils literal"><span class="pre">ptr_list</span></tt> should be used
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when there are frequent insertions and deletions from the middle of the
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sequence and if the container is fairly large (eg. more than 100
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elements). <tt class="docutils literal"><span class="pre">ptr_deque</span></tt> is the data structure of choice when most insertions
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and deletions take place at the beginning or at the end of the sequence.
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The special container <tt class="docutils literal"><span class="pre">ptr_array</span></tt> may be used when the size of the container is invariant
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and known at compile time.</p>
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<p>An associative container supports unique keys if it may contain at most
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one element for each key. Otherwise, it supports equivalent keys.
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<tt class="docutils literal"><span class="pre">ptr_set</span></tt> and <tt class="docutils literal"><span class="pre">ptr_map</span></tt> support unique keys.
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<tt class="docutils literal"><span class="pre">ptr_multiset</span></tt> and <tt class="docutils literal"><span class="pre">ptr_multimap</span></tt>
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support equivalent keys.</p>
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</div>
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<div class="section">
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<h1><a class="toc-backref" href="#id9" id="recommended-practice-for-object-oriented-programming" name="recommended-practice-for-object-oriented-programming">Recommended practice for Object-Oriented Programming</a></h1>
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<p>Idiomatic Object-Oriented Programming in C++ looks a bit different from
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the way it is done in other languages. This is partly because C++
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has both value and reference semantics, and partly because C++ is more flexible
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than other languages. Below is a list of recommendations that you are
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encouraged to follow:</p>
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<div class="section">
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<h2><a class="toc-backref" href="#id10" id="make-base-classes-abstract-and-without-data" name="make-base-classes-abstract-and-without-data">1. Make base classes abstract and without data</a></h2>
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<p>This has the following advantages:</p>
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<blockquote>
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<ol class="loweralpha simple">
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<li>It reduces <em>coupling</em> because you do not have to maintain or update state</li>
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</ol>
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<!-- -->
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<ol class="loweralpha simple" start="2">
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<li>It helps you to avoid <em>slicing</em></li>
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</ol>
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<!-- -->
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<ol class="loweralpha simple" start="3">
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<li>It ensures you <em>override</em> the right function</li>
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</ol>
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</blockquote>
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<p>You might also want to read the following articles:</p>
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<ul class="simple">
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<li>Kevlin Henney's <a class="reference" href="http://www.two-sdg.demon.co.uk/curbralan/papers/SixOfTheBest.pdf">Six of the best</a></li>
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</ul>
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<ul class="simple">
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<li>Jack Reeves' <a class="reference" href="http://www.ddj.com/documents/s=10011/q=1/cuj0602reeves/0602reeves.html">Multiple Inheritance Considered Useful</a></li>
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</ul>
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</div>
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<div class="section">
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<h2><a class="toc-backref" href="#id11" id="make-virtual-functions-private-and-provide-a-non-virtual-public-forwarding-function" name="make-virtual-functions-private-and-provide-a-non-virtual-public-forwarding-function">2. Make virtual functions private and provide a non-virtual public forwarding function</a></h2>
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<p>In code:</p>
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<pre class="literal-block">
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class Polymorphic
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{
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private:
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virtual int do_foo() = 0;
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public:
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int foo()
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{
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return do_foo();
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}
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...
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};
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</pre>
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<p>This has the following advantages:</p>
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<blockquote>
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<ol class="loweralpha simple">
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<li>It makes sure all calls to the virtual function always goes through one place in your code</li>
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</ol>
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<!-- -->
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<ol class="loweralpha simple" start="2">
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<li>It enables you to check preconditions and postconditions inside the forwarding function</li>
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</ol>
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</blockquote>
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<p>You might also want to read Herb Sutter's article <a class="reference" href="http://www.gotw.ca/publications/mill18.htm">Virtuality</a>.</p>
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</div>
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<div class="section">
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<h2><a class="toc-backref" href="#id12" id="derive-your-base-class-from-boost-noncopyable" name="derive-your-base-class-from-boost-noncopyable">3. Derive your base class from <tt class="docutils literal docutils literal"><span class="pre">boost::noncopyable</span></tt></a></h2>
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<p>Having an abstact base class prevents slicing when the base class is involved, but
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it does not prevent it for classes further down the hierarchy. This is where
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<a class="reference" href="http://www.boost.org/libs/utility/utility.htm#Class_noncopyable">boost::noncopyable</a> is handy to use:</p>
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<pre class="literal-block">
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class Polymorphic : boost::noncopyable
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{
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...
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};
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</pre>
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</div>
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<div class="section">
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<h2><a class="toc-backref" href="#id13" id="avoid-null-pointers-in-containers-if-possible" name="avoid-null-pointers-in-containers-if-possible">4. Avoid null-pointers in containers (if possible)</a></h2>
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<p>By default the pointer containers do not allow you to store null-pointer in them.
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As you might know, this behavior can be changed explicitly with the use
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of <a class="reference" href="reference.html#class-nullable">boost::nullable</a>.</p>
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<p>The primary reason to avoid null-pointers
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is that you have to check for null-pointers every time the container is
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used. This extra checking is easy to forget, and it is somewhat contradictory to
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the spirit of OO where you replace special cases with dynamic dispatch.</p>
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<p>Often, however, you need to place some special object in the container because you
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do not have enough information to construct a full object. In that case
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you might be able to use the Null Object pattern which simply dictates that
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you implement virtual functions from the abstract base-class
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as empty functions or with dummy return values. This means that
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your OO-code still does not need to worry about null-pointers.</p>
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<p>You might want to read</p>
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<ul class="simple">
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<li>Kevlin Henney's <a class="reference" href="http://www.two-sdg.demon.co.uk/curbralan/papers/europlop/NullObject.pdf">Null Object - Something for Nothing</a></li>
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</ul>
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<p>Finally you might end up in a situation where not even the Null Object can help
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you. That is when you truly need <tt class="docutils literal"><span class="pre">container<</span> <span class="pre">nullable<T></span> <span class="pre">></span></tt>.</p>
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<hr><p><strong>Navigate:</strong></p>
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<ul class="simple">
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<li><a class="reference" href="ptr_container.html">home</a></li>
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<li><a class="reference" href="reference.html">reference</a></li>
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</ul>
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<hr><table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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<tr class="field"><th class="field-name">Copyright:</th><td class="field-body">Thorsten Ottosen 2004-2006. Use, modification and distribution is subject to the Boost Software License, Version 1.0 (see <a class="reference" href="http://www.boost.org/LICENSE_1_0.txt">LICENSE_1_0.txt</a>).</td>
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</tr>
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</tbody>
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</table>
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</div>
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</div>
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</div>
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</body>
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