[ceph.git] / ceph / src / boost / libs / ptr_container / doc / guidelines.html

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<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&lt;</span> <span class="pre">nullable&lt;T&gt;</span> <span class="pre">&gt;</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>
<hr><table class="docutils field-list" frame="void" rules="none">
<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>
</tr>
</tbody>
</table>
</div>
</div>
</div>
</body>
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289	<body>
290	<div class="document" id="boost-pointer-container-library">
291	<h1 class="title"><img alt="Boost" src="boost.png" /> Pointer Container Library</h1>
292	<h2 class="subtitle" id="usage-guidelines">Usage Guidelines</h2>
293	<div class="contents local topic">
294	<ul class="simple">
295	<li><a class="reference" href="#choosing-the-right-container" id="id8" name="id8">Choosing the right container</a></li>
296	<li><a class="reference" href="#recommended-practice-for-object-oriented-programming" id="id9" name="id9">Recommended practice for Object-Oriented Programming</a><ul>
297	<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>
298	<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>
299	<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>
300	<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>
301	</ul>
302	</li>
303	</ul>
304	</div>
305	<div class="section">
306	<h1><a class="toc-backref" href="#id8" id="choosing-the-right-container" name="choosing-the-right-container">Choosing the right container</a></h1>
307	<p>The recommended usage pattern of the container classes is the same as
308	for normal standard containers.</p>
309	<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
310	complexity tradeoffs and should be used accordingly. <tt class="docutils literal"><span class="pre">ptr_vector</span></tt> is the
311	type of sequence that should be used by default. <tt class="docutils literal"><span class="pre">ptr_list</span></tt> should be used
312	when there are frequent insertions and deletions from the middle of the
313	sequence and if the container is fairly large (eg. more than 100
314	elements). <tt class="docutils literal"><span class="pre">ptr_deque</span></tt> is the data structure of choice when most insertions
315	and deletions take place at the beginning or at the end of the sequence.
316	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
317	and known at compile time.</p>
318	<p>An associative container supports unique keys if it may contain at most
319	one element for each key. Otherwise, it supports equivalent keys.
320	<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.
321	<tt class="docutils literal"><span class="pre">ptr_multiset</span></tt> and <tt class="docutils literal"><span class="pre">ptr_multimap</span></tt>
322	support equivalent keys.</p>
323	</div>
324	<div class="section">
325	<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>
326	<p>Idiomatic Object-Oriented Programming in C++ looks a bit different from
327	the way it is done in other languages. This is partly because C++
328	has both value and reference semantics, and partly because C++ is more flexible
329	than other languages. Below is a list of recommendations that you are
330	encouraged to follow:</p>
331	<div class="section">
332	<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>
333	<p>This has the following advantages:</p>
334	<blockquote>
335	<ol class="loweralpha simple">
336	<li>It reduces <em>coupling</em> because you do not have to maintain or update state</li>
337	</ol>
338	<!-- -->
339	<ol class="loweralpha simple" start="2">
340	<li>It helps you to avoid <em>slicing</em></li>
341	</ol>
342	<!-- -->
343	<ol class="loweralpha simple" start="3">
344	<li>It ensures you <em>override</em> the right function</li>
345	</ol>
346	</blockquote>
347	<p>You might also want to read the following articles:</p>
348	<ul class="simple">
349	<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>
350	</ul>
351	<ul class="simple">
352	<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>
353	</ul>
354	</div>
355	<div class="section">
356	<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>
357	<p>In code:</p>
358	<pre class="literal-block">
359	class Polymorphic
360	{
361	private:
362	virtual int do_foo() = 0;
363
364	public:
365	int foo()
366	{
367	return do_foo();
368	}
369	...
370	};
371	</pre>
372	<p>This has the following advantages:</p>
373	<blockquote>
374	<ol class="loweralpha simple">
375	<li>It makes sure all calls to the virtual function always goes through one place in your code</li>
376	</ol>
377	<!-- -->
378	<ol class="loweralpha simple" start="2">
379	<li>It enables you to check preconditions and postconditions inside the forwarding function</li>
380	</ol>
381	</blockquote>
382	<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>
383	</div>
384	<div class="section">
385	<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>
386	<p>Having an abstact base class prevents slicing when the base class is involved, but
387	it does not prevent it for classes further down the hierarchy. This is where
388	<a class="reference" href="http://www.boost.org/libs/utility/utility.htm#Class_noncopyable">boost::noncopyable</a> is handy to use:</p>
389	<pre class="literal-block">
390	class Polymorphic : boost::noncopyable
391	{
392	...
393	};
394	</pre>
395	</div>
396	<div class="section">
397	<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>
398	<p>By default the pointer containers do not allow you to store null-pointer in them.
399	As you might know, this behavior can be changed explicitly with the use
400	of <a class="reference" href="reference.html#class-nullable">boost::nullable</a>.</p>
401	<p>The primary reason to avoid null-pointers
402	is that you have to check for null-pointers every time the container is
403	used. This extra checking is easy to forget, and it is somewhat contradictory to
404	the spirit of OO where you replace special cases with dynamic dispatch.</p>
405	<p>Often, however, you need to place some special object in the container because you
406	do not have enough information to construct a full object. In that case
407	you might be able to use the Null Object pattern which simply dictates that
408	you implement virtual functions from the abstract base-class
409	as empty functions or with dummy return values. This means that
410	your OO-code still does not need to worry about null-pointers.</p>
411	<p>You might want to read</p>
412	<ul class="simple">
413	<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>
414	</ul>
415	<p>Finally you might end up in a situation where not even the Null Object can help
416	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>
417	<hr><p><strong>Navigate:</strong></p>
418	<ul class="simple">
419	<li><a class="reference" href="ptr_container.html">home</a></li>
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421	</ul>
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425	<tbody valign="top">
426	<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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