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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="tutorial">Tutorial</h2>
- <p>The tutorial shows you the most simple usage of the
- library. It is assumed that the reader is familiar
- with the use of standard containers. Although
- the tutorial is devided into sections, it is recommended
- that you read it all from top to bottom.</p>
- <ul class="simple">
- <li><a class="reference internal" href="#basic-usage">Basic usage</a></li>
- <li><a class="reference internal" href="#indirected-interface">Indirected interface</a></li>
- <li><a class="reference internal" href="#sequence-containers">Sequence containers</a></li>
- <li><a class="reference internal" href="#associative-containers">Associative containers</a></li>
- <li><a class="reference internal" href="#null-values">Null values</a></li>
- <li><a class="reference internal" href="#cloneability">Cloneability</a></li>
- <li><a class="reference internal" href="#new-functions">New functions</a></li>
- <li><a class="reference internal" href="#compatible-smart-pointer-overloads">Compatible smart pointer overloads</a></li>
- <li><a class="reference internal" href="#algorithms">Algorithms</a></li>
- </ul>
- <div class="section" id="basic-usage">
- <h1>Basic usage</h1>
- <p>The most important aspect of a pointer container is that it manages
- memory for you. This means that you in most cases do not need to worry
- about deleting memory.</p>
- <p>Let us assume that we have an OO-hierarchy of animals</p>
- <pre class="literal-block">
- class animal : <a class="reference external" href="http://www.boost.org/libs/utility/utility.htm#Class_noncopyable">boost::noncopyable</a>
- {
- public:
- virtual ~animal() {}
- virtual void eat() = 0;
- virtual int age() const = 0;
- // ...
- };
- class mammal : public animal
- {
- // ...
- };
- class bird : public animal
- {
- // ...
- };
- </pre>
- <p>Then the managing of the animals is straight-forward. Imagine a
- Zoo:</p>
- <pre class="literal-block">
- class zoo
- {
- boost::ptr_vector<animal> the_animals;
- public:
- void add_animal( animal* a )
- {
- the_animals.push_back( a );
- }
- };
- </pre>
- <p>Notice how we just pass the class name to the container; there
- is no <tt class="docutils literal">*</tt> to indicate it is a pointer.
- With this declaration we can now say:</p>
- <pre class="literal-block">
- zoo the_zoo;
- the_zoo.add_animal( new mammal("joe") );
- the_zoo.add_animal( new bird("dodo") );
- </pre>
- <p>Thus we heap-allocate all elements of the container
- and never rely on copy-semantics.</p>
- </div>
- <div class="section" id="indirected-interface">
- <h1>Indirected interface</h1>
- <p>A particular feature of the pointer containers is that
- the query interface is indirected. For example,</p>
- <pre class="literal-block">
- boost::ptr_vector<animal> vec;
- vec.push_back( new animal ); // you add it as pointer ...
- vec[0].eat(); // but get a reference back
- </pre>
- <p>This indirection also happens to iterators, so</p>
- <pre class="literal-block">
- typedef std::vector<animal*> std_vec;
- std_vec vec;
- ...
- std_vec::iterator i = vec.begin();
- (*i)->eat(); // '*' needed
- </pre>
- <p>now becomes</p>
- <pre class="literal-block">
- typedef boost::ptr_vector<animal> ptr_vec;
- ptr_vec vec;
- ptr_vec::iterator i = vec.begin();
- i->eat(); // no indirection needed
- </pre>
- </div>
- <div class="section" id="sequence-containers">
- <h1>Sequence containers</h1>
- <p>The sequence containers are used when you do not need to
- keep an ordering on your elements. You can basically
- expect all operations of the normal standard containers
- to be available. So, for example, with a <tt class="docutils literal">ptr_deque</tt>
- and <tt class="docutils literal">ptr_list</tt> object you can say:</p>
- <pre class="literal-block">
- boost::ptr_deque<animal> deq;
- deq.push_front( new animal );
- deq.pop_front();
- </pre>
- <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> have <tt class="docutils literal">push_front()</tt>
- and <tt class="docutils literal">pop_front()</tt> members.</p>
- <p>If the standard sequence supports
- random access, so does the pointer container; for example:</p>
- <pre class="literal-block">
- for( boost::ptr_deque<animal>::size_type i = 0u;
- i != deq.size(); ++i )
- deq[i].eat();
- </pre>
- <p>The <tt class="docutils literal">ptr_vector</tt> also allows you to specify the size of
- the buffer to allocate; for example</p>
- <pre class="literal-block">
- boost::ptr_vector<animal> animals( 10u );
- </pre>
- <p>will reserve room for 10 animals.</p>
- </div>
- <div class="section" id="associative-containers">
- <h1>Associative containers</h1>
- <p>To keep an ordering on our animals, we could use a <tt class="docutils literal">ptr_set</tt>:</p>
- <pre class="literal-block">
- boost::ptr_set<animal> set;
- set.insert( new monkey("bobo") );
- set.insert( new whale("anna") );
- ...
- </pre>
- <p>This requires that <tt class="docutils literal"><span class="pre">operator<()</span></tt> is defined for animals. One
- way to do this could be</p>
- <pre class="literal-block">
- inline bool operator<( const animal& l, const animal& r )
- {
- return l.name() < r.name();
- }
- </pre>
- <p>if we wanted to keep the animals sorted by name.</p>
- <p>Maybe you want to keep all the animals in zoo ordered wrt.
- their name, but it so happens that many animals have the
- same name. We can then use a <tt class="docutils literal">ptr_multimap</tt>:</p>
- <pre class="literal-block">
- typedef boost::ptr_multimap<std::string,animal> zoo_type;
- zoo_type zoo;
- std::string bobo = "bobo",
- anna = "anna";
- zoo.insert( bobo, new monkey(bobo) );
- zoo.insert( bobo, new elephant(bobo) );
- zoo.insert( anna, new whale(anna) );
- zoo.insert( anna, new emu(anna) );
- </pre>
- <p>Note that must create the key as an lvalue
- (due to exception-safety issues); the following would not
- have compiled</p>
- <pre class="literal-block">
- zoo.insert( "bobo", // this is bad, but you get compile error
- new monkey("bobo") );
- </pre>
- <p>If a multimap is not needed, we can use <tt class="docutils literal"><span class="pre">operator[]()</span></tt>
- to avoid the clumsiness:</p>
- <pre class="literal-block">
- boost::ptr_map<std::string,animal> animals;
- animals["bobo"].set_name("bobo");
- </pre>
- <p>This requires a default constructor for animals and
- a function to do the initialization, in this case <tt class="docutils literal">set_name()</tt>.</p>
- <p>A better alternative is to use <a class="reference external" href="../../assign/index.html">Boost.Assign</a>
- to help you out. In particular, consider</p>
- <ul class="simple">
- <li><a class="reference external" href="../../assign/doc/index.html#ptr_push_back">ptr_push_back(), ptr_push_front(), ptr_insert() and ptr_map_insert()</a></li>
- <li><a class="reference external" href="../../assign/doc/index.html#ptr_list_of">ptr_list_of()</a></li>
- </ul>
- <p>For example, the above insertion may now be written</p>
- <pre class="literal-block">
- boost::ptr_multimap<std::string,animal> animals;
- using namespace boost::assign;
- ptr_map_insert<monkey>( animals )( "bobo", "bobo" );
- ptr_map_insert<elephant>( animals )( "bobo", "bobo" );
- ptr_map_insert<whale>( animals )( "anna", "anna" );
- ptr_map_insert<emu>( animals )( "anna", "anna" );
- </pre>
- </div>
- <div class="section" id="null-values">
- <h1>Null values</h1>
- <p>By default, if you try to insert null into a container, an exception
- is thrown. If you want to allow nulls, then you must
- say so explicitly when declaring the container variable</p>
- <pre class="literal-block">
- boost::ptr_vector< boost::nullable<animal> > animals_type;
- animals_type animals;
- ...
- animals.insert( animals.end(), new dodo("fido") );
- animals.insert( animals.begin(), 0 ) // ok
- </pre>
- <p>Once you have inserted a null into the container, you must
- always check if the value is null before accessing the object</p>
- <pre class="literal-block">
- for( animals_type::iterator i = animals.begin();
- i != animals.end(); ++i )
- {
- if( !boost::is_null(i) ) // always check for validity
- i->eat();
- }
- </pre>
- <p>If the container support random access, you may also check this as</p>
- <pre class="literal-block">
- for( animals_type::size_type i = 0u;
- i != animals.size(); ++i )
- {
- if( !animals.is_null(i) )
- animals[i].eat();
- }
- </pre>
- <p>Note that it is meaningless to insert
- null into <tt class="docutils literal">ptr_set</tt> and <tt class="docutils literal">ptr_multiset</tt>.</p>
- </div>
- <div class="section" id="cloneability">
- <h1>Cloneability</h1>
- <p>In OO programming it is typical to prohibit copying of objects; the
- objects may sometimes be allowed to be Cloneable; for example,:</p>
- <pre class="literal-block">
- animal* animal::clone() const
- {
- return do_clone(); // implemented by private virtual function
- }
- </pre>
- <p>If the OO hierarchy thus allows cloning, we need to tell the
- pointer containers how cloning is to be done. This is simply
- done by defining a free-standing function, <tt class="docutils literal">new_clone()</tt>,
- in the same namespace as
- the object hierarchy:</p>
- <pre class="literal-block">
- inline animal* new_clone( const animal& a )
- {
- return a.clone();
- }
- </pre>
- <p>That is all, now a lot of functions in a pointer container
- can exploit the cloneability of the animal objects. For example</p>
- <pre class="literal-block">
- typedef boost::ptr_list<animal> zoo_type;
- zoo_type zoo, another_zoo;
- ...
- another_zoo.assign( zoo.begin(), zoo.end() );
- </pre>
- <p>will fill another zoo with clones of the first zoo. Similarly,
- <tt class="docutils literal">insert()</tt> can now insert clones into your pointer container</p>
- <pre class="literal-block">
- another_zoo.insert( another_zoo.begin(), zoo.begin(), zoo.end() );
- </pre>
- <p>The whole container can now also be cloned</p>
- <pre class="literal-block">
- zoo_type yet_another_zoo = zoo.clone();
- </pre>
- <p>Copying or assigning the container has the same effect as cloning (though it is slightly cheaper):</p>
- <pre class="literal-block">
- zoo_type yet_another_zoo = zoo;
- </pre>
- <p>Copying also support derived-to-base class conversions:</p>
- <pre class="literal-block">
- boost::ptr_vector<monkey> monkeys = boost::assign::ptr_list_of<monkey>( "bobo" )( "bebe")( "uhuh" );
- boost::ptr_vector<animal> animals = monkeys;
- </pre>
- <p>This also works for maps:</p>
- <pre class="literal-block">
- boost::ptr_map<std::string,monkey> monkeys = ...;
- boost::ptr_map<std::string,animal> animals = monkeys;
- </pre>
- </div>
- <div class="section" id="new-functions">
- <h1>New functions</h1>
- <p>Given that we know we are working with pointers, a few new functions
- make sense. For example, say you want to remove an
- animal from the zoo</p>
- <pre class="literal-block">
- zoo_type::auto_type the_animal = zoo.release( zoo.begin() );
- the_animal->eat();
- animal* the_animal_ptr = the_animal.release(); // now this is not deleted
- zoo.release(2); // for random access containers
- </pre>
- <p>You can think of <tt class="docutils literal">auto_type</tt> as a non-copyable form of
- <tt class="docutils literal"><span class="pre">std::auto_ptr</span></tt>. Notice that when you release an object, the
- pointer is removed from the container and the containers size
- shrinks. For containers that store nulls, we can exploit that
- <tt class="docutils literal">auto_type</tt> is convertible to <tt class="docutils literal">bool</tt>:</p>
- <pre class="literal-block">
- if( ptr_vector< nullable<T> >::auto_type r = vec.pop_back() )
- {
- ...
- }
- </pre>
- <p>You can also release the entire container if you
- want to return it from a function</p>
- <pre class="literal-block">
- <a class="reference external" href="compatible_smart_ptr.html"><em>compatible-smart-ptr</em></a>< boost::ptr_deque<animal> > get_zoo()
- {
- boost::ptr_deque<animal> result;
- ...
- return result.release(); // give up ownership
- }
- ...
- boost::ptr_deque<animal> animals = get_zoo();
- </pre>
- <p>Let us assume we want to move an animal object from
- one zoo to another. In other words, we want to move the
- animal and the responsibility of it to another zoo</p>
- <pre class="literal-block">
- another_zoo.transfer( another_zoo.end(), // insert before end
- zoo.begin(), // insert this animal ...
- zoo ); // from this container
- </pre>
- <p>This kind of "move-semantics" is different from
- normal value-based containers. You can think of <tt class="docutils literal">transfer()</tt>
- as the same as <tt class="docutils literal">splice()</tt> on <tt class="docutils literal"><span class="pre">std::list</span></tt>.</p>
- <p>If you want to replace an element, you can easily do so</p>
- <pre class="literal-block">
- zoo_type::auto_type old_animal = zoo.replace( zoo.begin(), new monkey("bibi") );
- zoo.replace( 2, old_animal.release() ); // for random access containers
- </pre>
- <p>A map is slightly different to iterate over than standard maps.
- Now we say</p>
- <pre class="literal-block">
- typedef boost::ptr_map<std::string, boost::nullable<animal> > animal_map;
- animal_map map;
- ...
- for( animal_map::const_iterator i = map.begin(), e = map.end(); i != e; ++i )
- {
- std::cout << "\n key: " << i->first;
- std::cout << "\n age: ";
- if( boost::is_null(i) )
- std::cout << "unknown";
- else
- std::cout << i->second->age();
- }
- </pre>
- <p>Except for the check for null, this looks like it would with a normal map. But if <tt class="docutils literal">age()</tt> had
- not been a <tt class="docutils literal">const</tt> member function,
- it would not have compiled.</p>
- <p>Maps can also be indexed with bounds-checking</p>
- <pre class="literal-block">
- try
- {
- animal& bobo = map.at("bobo");
- }
- catch( boost::bad_ptr_container_operation& e )
- {
- // "bobo" not found
- }
- </pre>
- </div>
- <div class="section" id="compatible-smart-pointer-overloads">
- <h1>Compatible smart pointer overloads</h1>
- <p>Every time there is a function that takes a <tt class="docutils literal">T*</tt> parameter, there is
- also a function overload (or two) taking a <tt class="docutils literal"><span class="pre"><a class="reference external" href="compatible_smart_ptr.html"><em>compatible-smart-ptr</em></a><U></span></tt>
- parameter. This is of course done to make the library intregrate
- seamlessly with <tt class="docutils literal"><span class="pre">std::auto_ptr</span></tt> or <tt class="docutils literal"><span class="pre">std::unique_ptr</span></tt>. For example,
- consider a statement like</p>
- <pre class="literal-block">
- std::ptr_vector<Base> vec;
- vec.push_back( new Base );
- </pre>
- <p>If the compiler supports <tt class="docutils literal"><span class="pre">std::auto_ptr</span></tt>, this is complemented
- by</p>
- <pre class="literal-block">
- std::auto_ptr<Derived> p( new Derived );
- vec.push_back( p );
- </pre>
- <p>Similarly if <tt class="docutils literal"><span class="pre">std::unique_ptr</span></tt> is available, we can write</p>
- <pre class="literal-block">
- std::unique_ptr<Derived> p( new Derived );
- vec.push_back( std::move( p ) );
- </pre>
- <p>Notice that the template argument for <tt class="docutils literal"><span class="pre"><a class="reference external" href="compatible_smart_ptr.html"><em>compatible-smart-ptr</em></a></span></tt> does not need to
- follow the template argument for <tt class="docutils literal">ptr_vector</tt> as long as <tt class="docutils literal">Derived*</tt>
- can be implicitly converted to <tt class="docutils literal">Base*</tt>.</p>
- </div>
- <div class="section" id="algorithms">
- <h1>Algorithms</h1>
- <p>Unfortunately it is not possible to use pointer containers with
- mutating algorithms from the standard library. However,
- the most useful ones
- are instead provided as member functions:</p>
- <pre class="literal-block">
- boost::ptr_vector<animal> zoo;
- ...
- zoo.sort(); // assume 'bool operator<( const animal&, const animal& )'
- zoo.sort( std::less<animal>() ); // the same, notice no '*' is present
- zoo.sort( zoo.begin(), zoo.begin() + 5 ); // sort selected range
- </pre>
- <p>Notice that predicates are automatically wrapped in an <a class="reference external" href="indirect_fun.html">indirect_fun</a> object.</p>
- <p>You can remove equal and adjacent elements using <tt class="docutils literal">unique()</tt>:</p>
- <pre class="literal-block">
- zoo.unique(); // assume 'bool operator==( const animal&, const animal& )'
- zoo.unique( zoo.begin(), zoo.begin() + 5, my_comparison_predicate() );
- </pre>
- <p>If you just want to remove certain elements, use <tt class="docutils literal">erase_if</tt>:</p>
- <pre class="literal-block">
- zoo.erase_if( my_predicate() );
- </pre>
- <p>Finally you may want to merge two sorted containers:</p>
- <pre class="literal-block">
- boost::ptr_vector<animal> another_zoo = ...;
- another_zoo.sort(); // sorted wrt. to same order as 'zoo'
- zoo.merge( another_zoo );
- BOOST_ASSERT( another_zoo.empty() );
- </pre>
- <p>That is all; now you have learned all the basics!</p>
- <hr><p><strong>See also</strong></p>
- <ul class="simple">
- <li><a class="reference external" href="guidelines.html">Usage guidelines</a></li>
- <li><a class="reference external" href="../../conversion/cast.htm#Polymorphic_castl">Cast utilities</a></li>
- </ul>
- <p><strong>Navigate</strong></p>
- <ul class="simple">
- <li><a class="reference external" href="ptr_container.html">home</a></li>
- <li><a class="reference external" href="examples.html">examples</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 external" href="http://www.boost.org/LICENSE_1_0.txt">LICENSE_1_0.txt</a>).</td>
- </tr>
- </tbody>
- </table>
- </div>
- </div>
- </body>
- </html>
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