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| <body> |
| <div class="document" id="boost-pointer-container-library"> |
| <h1 class="title"><img alt="Boost" src="boost.png" /> Pointer Container Library</h1> |
| <h2 class="subtitle" id="usage-guidelines">Usage Guidelines</h2> |
| <div class="contents local topic"> |
| <ul class="simple"> |
| <li><a class="reference" href="#choosing-the-right-container" id="id8" name="id8">Choosing the right container</a></li> |
| <li><a class="reference" href="#recommended-practice-for-object-oriented-programming" id="id9" name="id9">Recommended practice for Object-Oriented Programming</a><ul> |
| <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> |
| <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> |
| <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> |
| <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> |
| </ul> |
| </li> |
| </ul> |
| </div> |
| <div class="section"> |
| <h1><a class="toc-backref" href="#id8" id="choosing-the-right-container" name="choosing-the-right-container">Choosing the right container</a></h1> |
| <p>The recommended usage pattern of the container classes is the same as |
| for normal standard containers.</p> |
| <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 |
| complexity tradeoffs and should be used accordingly. <tt class="docutils literal"><span class="pre">ptr_vector</span></tt> is the |
| type of sequence that should be used by default. <tt class="docutils literal"><span class="pre">ptr_list</span></tt> should be used |
| when there are frequent insertions and deletions from the middle of the |
| sequence and if the container is fairly large (eg. more than 100 |
| elements). <tt class="docutils literal"><span class="pre">ptr_deque</span></tt> is the data structure of choice when most insertions |
| and deletions take place at the beginning or at the end of the sequence. |
| 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 |
| and known at compile time.</p> |
| <p>An associative container supports unique keys if it may contain at most |
| one element for each key. Otherwise, it supports equivalent keys. |
| <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. |
| <tt class="docutils literal"><span class="pre">ptr_multiset</span></tt> and <tt class="docutils literal"><span class="pre">ptr_multimap</span></tt> |
| support equivalent keys.</p> |
| </div> |
| <div class="section"> |
| <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> |
| <p>Idiomatic Object-Oriented Programming in C++ looks a bit different from |
| the way it is done in other languages. This is partly because C++ |
| has both value and reference semantics, and partly because C++ is more flexible |
| than other languages. Below is a list of recommendations that you are |
| encouraged to follow:</p> |
| <div class="section"> |
| <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> |
| <p>This has the following advantages:</p> |
| <blockquote> |
| <ol class="loweralpha simple"> |
| <li>It reduces <em>coupling</em> because you do not have to maintain or update state</li> |
| </ol> |
| <!-- --> |
| <ol class="loweralpha simple" start="2"> |
| <li>It helps you to avoid <em>slicing</em></li> |
| </ol> |
| <!-- --> |
| <ol class="loweralpha simple" start="3"> |
| <li>It ensures you <em>override</em> the right function</li> |
| </ol> |
| </blockquote> |
| <p>You might also want to read the following articles:</p> |
| <ul class="simple"> |
| <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> |
| </ul> |
| <ul class="simple"> |
| <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> |
| </ul> |
| </div> |
| <div class="section"> |
| <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> |
| <p>In code:</p> |
| <pre class="literal-block"> |
| class Polymorphic |
| { |
| private: |
| virtual int do_foo() = 0; |
| |
| public: |
| int foo() |
| { |
| return do_foo(); |
| } |
| ... |
| }; |
| </pre> |
| <p>This has the following advantages:</p> |
| <blockquote> |
| <ol class="loweralpha simple"> |
| <li>It makes sure all calls to the virtual function always goes through one place in your code</li> |
| </ol> |
| <!-- --> |
| <ol class="loweralpha simple" start="2"> |
| <li>It enables you to check preconditions and postconditions inside the forwarding function</li> |
| </ol> |
| </blockquote> |
| <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> |
| </div> |
| <div class="section"> |
| <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> |
| <p>Having an abstact base class prevents slicing when the base class is involved, but |
| it does not prevent it for classes further down the hierarchy. This is where |
| <a class="reference" href="http://www.boost.org/libs/utility/utility.htm#Class_noncopyable">boost::noncopyable</a> is handy to use:</p> |
| <pre class="literal-block"> |
| class Polymorphic : boost::noncopyable |
| { |
| ... |
| }; |
| </pre> |
| </div> |
| <div class="section"> |
| <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> |
| <p>By default the pointer containers do not allow you to store null-pointer in them. |
| As you might know, this behavior can be changed explicitly with the use |
| of <a class="reference" href="reference.html#class-nullable">boost::nullable</a>.</p> |
| <p>The primary reason to avoid null-pointers |
| is that you have to check for null-pointers every time the container is |
| used. This extra checking is easy to forget, and it is somewhat contradictory to |
| the spirit of OO where you replace special cases with dynamic dispatch.</p> |
| <p>Often, however, you need to place some special object in the container because you |
| do not have enough information to construct a full object. In that case |
| you might be able to use the Null Object pattern which simply dictates that |
| you implement virtual functions from the abstract base-class |
| as empty functions or with dummy return values. This means that |
| your OO-code still does not need to worry about null-pointers.</p> |
| <p>You might want to read</p> |
| <ul class="simple"> |
| <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> |
| </ul> |
| <p>Finally you might end up in a situation where not even the Null Object can help |
| 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> |
| <hr><p><strong>Navigate:</strong></p> |
| <ul class="simple"> |
| <li><a class="reference" href="ptr_container.html">home</a></li> |
| <li><a class="reference" href="reference.html">reference</a></li> |
| </ul> |
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| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <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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