++++++++++++++++++++++++++++++++++ |Boost| Pointer Container Library ++++++++++++++++++++++++++++++++++ .. |Boost| image:: boost.png ======== Tutorial ======== 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. * `Basic usage`_ * `Indirected interface`_ * `Sequence containers`_ * `Associative containers`_ * `Null values`_ * `Clonability`_ * `New functions`_ * `std::auto_ptr overloads`_ * `Algorithms`_ Basic usage ----------- 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. Let us assume that we have an OO-hierarchy of animals .. parsed-literal:: class animal : `boost::noncopyable `_ { public: virtual ~animal() {} virtual void eat() = 0; virtual int age() const = 0; // ... }; class mammal : public animal { // ... }; class bird : public animal { // ... }; Then the managing of the animals is straight-forward. Imagine a Zoo:: class zoo { boost::ptr_vector the_animals; public: void add_animal( animal* a ) { the_animals.push_back( a ); } }; Notice how just pass the class name to the container; there is no ``*`` to indicate it is a pointer. With this declaration we can now say:: zoo the_zoo; the_zoo.add_animal( new mammal("joe") ); the_zoo.add_animal( new bird("dodo") ); Thus we heap-allocate all elements of the container and never rely on copy-semantics. Indirected interface -------------------- As particular feature of the pointer containers is that the query interface is indirected. For example, :: boost::ptr_vector vec; vec.push_back( new animal ); // you add it as pointer ... vec[0].eat(); // but get a reference back This indirection also happens to iterators, so :: typedef std::vector std_vec; std_vec vec; ... std_vec::iterator i = vec.begin(); (*i)->eat(); // '*' needed now becomes :: typedef boost::ptr_vector ptr_vec; ptr_vec vec; ptr_vec::iterator i = vec.begin(); i->eat(); // no indirection needed Sequence containers ------------------- The sequence containers 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 ``ptr_deque`` and ``ptr_list`` object you can say:: boost::ptr_deque deq; deq.push_front( new animal ); deq.pop_front(); because ``std::deque`` and ``std::list`` has ``push_front()`` and ``pop_front`` members. If the standard sequence support random access, so does the pointer container; for example:: for( boost::ptr_deque::size_type i = 0u; i != deq.size(); ++i ) deq[i].eat(); The ``ptr_vector`` also allows you to specify the size of the buffer to allocate; for example :: boost::ptr_vector animals( 10u ); will reserve room for 10 animals. Associative containers ---------------------- To keep an ordering on our animals, we could use a ``ptr_set``:: boost::ptr_set set; set.insert( new monkey("bobo") ); set.insert( new whale("anna") ); ... This requires that ``operator<()`` is defined for animals. One way to do this could be :: inline bool operator<( const animal& l, const animal& r ) { return l.name() < r.name(); } if we wanted to keep the animals sorted by name. 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 ``ptr_multimap``:: typedef boost::ptr_multimap 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) ); Note that must create the key as an lvalue (due to exception-safety issues); the following would not have compiled :: zoo.insert( "bobo", // this is bad, but you get compile error new monkey("bobo") ); If a multimap is not needed, we can use ``operator[]()`` to avoid the clumsiness:: boost::ptr_map animals; animals["bobo"].set_name("bobo"); This requires a default constructor for animals and a function to do the initialization, in this case ``set_name()``. A better alternative is to use `Boost.Assign <../../assign/index.html>`_ to help you out. In particular, consider - `ptr_push_back(), ptr_push_front(), ptr_insert() and ptr_map_insert() <../../assign/doc/index.html#ptr_push_back>`_ - `ptr_list_of() <../../assign/doc/index.html#ptr_list_of>`_ For example, the above insertion may now be written :: boost::ptr_multimap animals; using namespace boost::assign; ptr_map_insert( animals )( "bobo", "bobo" ); ptr_map_insert( animals )( "bobo", "bobo" ); ptr_map_insert( animals )( "anna", "anna" ); ptr_map_insert( animals )( "anna", "anna" ); Null values ----------- 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 :: boost::ptr_vector< boost::nullable > animals_type; animals_type animals; ... animals.insert( animals.end(), new dodo("fido") ); animals.insert( animals.begin(), 0 ) // ok Once you have inserted a null into the container, you must always check if the value is null before accessing the object :: for( animals_type::iterator i = animals.begin(); i != animals.end(); ++i ) { if( !boost::is_null(i) ) // always check for validity i->eat(); } If the container support random access, you may also check this as :: for( animals_type::size_type i = 0u; i != animals.size(); ++i ) { if( !animals.is_null(i) ) animals[i].eat(); } Note that it is meaningless to insert null into ``ptr_set`` and ``ptr_multiset``. Clonability ----------- In OO programming it is typical to prohibit copying of objects; the objects may sometimes be allowed to be clonable; for example,:: animal* animal::clone() const { return do_clone(); // implemented by private virtual function } 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, ``new_clone()``, in the same namespace as the object hierarchy:: inline animal* new_clone( const animal& a ) { return a.clone(); } That is all, now a lot of functions in a pointer container can exploit the clonability of the animal objects. For example :: typedef boost::ptr_list zoo_type; zoo_type zoo, another_zoo; ... another_zoo.assign( zoo.begin(), zoo.end() ); will fill another zoo with clones of the first zoo. Similarly, ``insert()`` can now insert clones into your pointer container :: another_zoo.insert( another_zoo.begin(), zoo.begin(), zoo.end() ); The whole container can now also be cloned :: zoo_type yet_another_zoo = zoo.clone(); New functions ------------- 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 :: 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 You can think of ``auto_type`` as a non-copyable form of ``std::auto_ptr``. 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 ``auto_type`` is convertible to ``bool``:: if( ptr_vector< nullable >::auto_type r = vec.pop_back() ) { ... } You can also release the entire container if you want to return it from a function :: std::auto_ptr< boost::ptr_deque > get_zoo() { boost::ptr_deque result; ... return result.release(); // give up ownership } ... boost::ptr_deque animals = get_zoo(); 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 :: another_zoo.transfer( another_zoo.end(), // insert before end zoo.begin(), // insert this animal ... zoo ); // from this container This kind of "move-semantics" is different from normal value-based containers. You can think of ``transfer()`` as the same as ``splice()`` on ``std::list``. If you want to replace an element, you can easily do so :: zoo_type::auto_type old_animal = zoo.replace( zoo.begin(), new monkey("bibi") ); zoo.replace( 2, old_animal.release() ); // for random access containers A map is slightly different to iterator over than standard maps. Now we say :: typedef boost::ptr_map > 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(); } Except for the check for null, this looks like it would with a normal map. But if ``age()`` had not been a ``const`` member function, it would not have compiled. Maps can also be indexed with bounds-checking :: try { animal& bobo = map.at("bobo"); } catch( boost::bad_ptr_container_operation& e ) { // "bobo" not found } ``std::auto_ptr`` overloads ------------------------------ Evetime there is a function that takes a ``T*`` parameter, there is also a function taking an ``std::auto_ptr`` parameter. This is of course done to make the library intregrate seamless with ``std::auto_ptr``. For example :: std::ptr_vector vec; vec.push_back( new Base ); is complemented by :: std::auto_ptr p( new Derived ); vec.push_back( p ); Notice that the template argument for ``std::auto_ptr`` does not need to follow the template argument for ``ptr_vector`` as long as ``Derived*`` can be implicitly converted to ``Base*``. Algorithms ---------- 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:: boost::ptr_vector zoo; ... zoo.sort(); // assume 'bool operator<( const animal&, const animal& )' zoo.sort( std::less() ); // the same, notice no '*' is present zoo.sort( zoo.begin(), zoo.begin() + 5 ); // sort selected range Notice that predicates are automatically wrapped in an `indirect_fun`_ object. .. _`indirect_fun`: indirect_fun.html You can remove equal and adjacent elements using ``unique()``:: zoo.unique(); // assume 'bool operator==( const animal&, const animal& )' zoo.unique( zoo.begin(), zoo.begin() + 5, my_comparison_predicate() ); If you just want to remove certain elements, use ``erase_if``:: zoo.erase_if( my_predicate() ); Finally you may want to merge together two sorted containers:: boost::ptr_vector another_zoo = ...; another_zoo.sort(); // sorted wrt. to same order as 'zoo' zoo.merge( another_zoo ); BOOST_ASSERT( another_zoo.empty() ); That is all; now you have learned all the basics! .. raw:: html
**See also** - `Usage guidelines `_ - `Cast utilities <../../conversion/cast.htm#Polymorphic_castl>`_ **Navigate** - `home `_ - `examples `_ .. raw:: html
:Copyright: Thorsten Ottosen 2004-2006. Use, modification and distribution is subject to the Boost Software License, Version 1.0 (see LICENSE_1_0.txt__). __ http://www.boost.org/LICENSE_1_0.txt