388 lines
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388 lines
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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="faq">FAQ</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="#calling-assign-is-very-costly-and-i-do-not-really-need-to-store-cloned-objects-i-merely-need-to-overwrite-the-existing-ones-what-do-i-do" id="id5" name="id5">Calling <tt class="docutils literal"><span class="pre">assign()</span></tt> is very costly and I do not really need to store cloned objects; I merely need to overwrite the existing ones; what do I do?</a></li>
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<li><a class="reference" href="#which-mutating-algorithms-are-safe-to-use-with-pointers" id="id6" name="id6">Which mutating algorithms are safe to use with pointers?</a></li>
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<li><a class="reference" href="#why-does-ptr-map-t-insert-replace-take-two-arguments-the-key-and-the-pointer-instead-of-one-std-pair-and-why-is-the-key-passed-by-non-const-reference" id="id7" name="id7">Why does <tt class="docutils literal"><span class="pre">ptr_map<T>::insert()/replace()</span></tt> take two arguments (the key and the pointer) instead of one <tt class="docutils literal"><span class="pre">std::pair</span></tt>? And why is the key passed by non-const reference?</a></li>
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<li><a class="reference" href="#when-instantiating-a-pointer-container-with-a-type-t-is-t-then-allowed-to-be-incomplete-at-that-point" id="id8" name="id8">When instantiating a pointer container with a type <tt class="docutils literal"><span class="pre">T</span></tt>, is <tt class="docutils literal"><span class="pre">T</span></tt> then allowed to be incomplete at that point?</a></li>
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<li><a class="reference" href="#why-do-iterator-range-inserts-give-the-strong-exception-safety-guarantee" id="id9" name="id9">Why do iterator-range inserts give the strong exception-safety guarantee?</a></li>
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<li><a class="reference" href="#what-is-the-polymorphic-class-problem" id="id10" name="id10">What is the polymorphic class problem?</a></li>
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<li><a class="reference" href="#are-the-pointer-containers-faster-and-do-they-have-a-better-memory-footprint-than-a-container-of-smart-pointers" id="id11" name="id11">Are the pointer containers faster and do they have a better memory footprint than a container of smart pointers?</a></li>
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<li><a class="reference" href="#when-the-stored-pointers-cannot-be-0-how-do-i-allow-this-empty-behavior-anyway" id="id12" name="id12">When the stored pointers cannot be <tt class="docutils literal"><span class="pre">0</span></tt>, how do I allow this "empty" behavior anyway?</a></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="#id5" id="calling-assign-is-very-costly-and-i-do-not-really-need-to-store-cloned-objects-i-merely-need-to-overwrite-the-existing-ones-what-do-i-do" name="calling-assign-is-very-costly-and-i-do-not-really-need-to-store-cloned-objects-i-merely-need-to-overwrite-the-existing-ones-what-do-i-do">Calling <tt class="docutils literal docutils literal"><span class="pre">assign()</span></tt> is very costly and I do not really need to store cloned objects; I merely need to overwrite the existing ones; what do I do?</a></h1>
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<p>Call <tt class="docutils literal"><span class="pre">std::copy(</span> <span class="pre">first,</span> <span class="pre">last,</span> <span class="pre">c.begin()</span> <span class="pre">);</span></tt>.</p>
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</div>
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<div class="section">
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<h1><a class="toc-backref" href="#id6" id="which-mutating-algorithms-are-safe-to-use-with-pointers" name="which-mutating-algorithms-are-safe-to-use-with-pointers">Which mutating algorithms are safe to use with pointers?</a></h1>
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<p>Any mutating algorithm that moves elements around by swapping them. An
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important example is <tt class="docutils literal"><span class="pre">std::sort()</span></tt>; examples of unsafe algorithms are
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<tt class="docutils literal"><span class="pre">std::unique()</span></tt> and <tt class="docutils literal"><span class="pre">std::remove()</span></tt>.</p>
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<!-- That is why these algorithms are
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provided as member functions. -->
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</div>
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<div class="section">
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<h1><a class="toc-backref" href="#id7" id="why-does-ptr-map-t-insert-replace-take-two-arguments-the-key-and-the-pointer-instead-of-one-std-pair-and-why-is-the-key-passed-by-non-const-reference" name="why-does-ptr-map-t-insert-replace-take-two-arguments-the-key-and-the-pointer-instead-of-one-std-pair-and-why-is-the-key-passed-by-non-const-reference">Why does <tt class="docutils literal docutils literal"><span class="pre">ptr_map<T>::insert()/replace()</span></tt> take two arguments (the key and the pointer) instead of one <tt class="docutils literal docutils literal"><span class="pre">std::pair</span></tt>? And why is the key passed by non-const reference?</a></h1>
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<p>This is the only way the function can be implemented in an exception-safe
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manner; since the copy-constructor of the key might throw, and since
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function arguments are not guaranteed to be evaluated from left to right,
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we need to ensure that evaluating the first argument does not throw.
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Passing the key as a reference achieves just that.</p>
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</div>
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<div class="section">
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<h1><a class="toc-backref" href="#id8" id="when-instantiating-a-pointer-container-with-a-type-t-is-t-then-allowed-to-be-incomplete-at-that-point" name="when-instantiating-a-pointer-container-with-a-type-t-is-t-then-allowed-to-be-incomplete-at-that-point">When instantiating a pointer container with a type <tt class="docutils literal docutils literal"><span class="pre">T</span></tt>, is <tt class="docutils literal docutils literal"><span class="pre">T</span></tt> then allowed to be incomplete at that point?</a></h1>
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<p>No. This is a distinct property of <tt class="docutils literal"><span class="pre">shared_ptr</span></tt> which implies some overhead.</p>
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<p>However, one can leave <tt class="docutils literal"><span class="pre">T</span></tt> incomplete in the header file:</p>
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<pre class="literal-block">
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// foo.hpp
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class Foo { ... };
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new_clone( const Foo& ) { ... }
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delete_clone( const Foo* ) { ... }
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// x.hpp
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class Foo; // Foo is incomplete here
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class X { ptr_deque<Foo> container; ... }
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// x.cpp
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#include <x.hpp>
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#include <foo.hpp> // now Foo is not incomplete anymore
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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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<h1><a class="toc-backref" href="#id9" id="why-do-iterator-range-inserts-give-the-strong-exception-safety-guarantee" name="why-do-iterator-range-inserts-give-the-strong-exception-safety-guarantee">Why do iterator-range inserts give the strong exception-safety guarantee?</a></h1>
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<p>Is this not very inefficient? It is because it is actually affordable to
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do so; the overhead is one heap-allocation which is relatively small
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compared to cloning N objects.</p>
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</div>
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<div class="section">
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<h1><a class="toc-backref" href="#id10" id="what-is-the-polymorphic-class-problem" name="what-is-the-polymorphic-class-problem">What is the <span class="target" id="polymorphic-class-problem">polymorphic class problem</span>?</a></h1>
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<p>The problem refers to the relatively troublesome way C++ supports Object
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Oriented programming in connection with containers of pointers to
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polymorphic objects. In a language without garbage collection, you end up
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using either a container of smart pointers or a container that takes
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ownership of the pointers. The hard part is to find a safe, fast and
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elegant solution.</p>
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</div>
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<div class="section">
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<h1><a class="toc-backref" href="#id11" id="are-the-pointer-containers-faster-and-do-they-have-a-better-memory-footprint-than-a-container-of-smart-pointers" name="are-the-pointer-containers-faster-and-do-they-have-a-better-memory-footprint-than-a-container-of-smart-pointers">Are the pointer containers faster and do they have a better memory footprint than a container of smart pointers?</a></h1>
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<p>The short answer is yes: they are faster and they do use less memory; in
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fact, they are the only way to obtain the zero-overhead hallmark of C++.
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Smart pointers usually have one word or more of memory overhead per
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pointer because a reference count must be maintained. And since the
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reference count must be maintained, there is also a runtime-overhead. If
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your objects are big, then the memory overhead is often negligible, but if
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you have many small objects, it is not. Further reading can be found in
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these references: <a class="reference" href="ptr_container.html#references">[11]</a> and <a class="reference" href="ptr_container.html#references">[12]</a>.</p>
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</div>
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<div class="section">
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<h1><a class="toc-backref" href="#id12" id="when-the-stored-pointers-cannot-be-0-how-do-i-allow-this-empty-behavior-anyway" name="when-the-stored-pointers-cannot-be-0-how-do-i-allow-this-empty-behavior-anyway">When the stored pointers cannot be <tt class="docutils literal docutils literal"><span class="pre">0</span></tt>, how do I allow this "empty" behavior anyway?</a></h1>
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<p>Storing a null-pointer among a list of pointers does not fit well into the Object Oriented paradigm.
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The most elegant design is to use the Null-Object Pattern where one basically makes a concrete
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class with dummy implementations of the virtual functions. See <a class="reference" href="ptr_container.html#references">[13]</a> for details.</p>
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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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