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Licensees holding valid Qt ** Commercial licenses may use this file in accordance with the Qt ** Commercial License Agreement provided with the Software. ** ** This file is provided "AS IS" with NO WARRANTY OF ANY KIND, ** INCLUDING THE WARRANTIES OF DESIGN, MERCHANTABILITY AND FITNESS FOR ** A PARTICULAR PURPOSE. Trolltech reserves all rights not granted ** herein. ** **********************************************************************/ /*! \defgroup collection \title Collection Classes \keyword collection classes \keyword persistent data A collection class is a container which holds a number of items in a data structure and provides various operations to manipulate the contents of the collection, such as insert item, remove item, find item, etc. Qt has several value-based and several pointer-based collection classes. The pointer-based collection classes work with pointers to items, while the value-based classes store copies of their items. The value-based collections are very similar to STL container classes, and can be used with STL algorithms and containers. See the \link qt-template-lib.html Qt Template Library\endlink documentation for details. The value-based collections are: \list \i \l QValueList, a value-based list. \i \l QValueVector, a value-based vector. \i \l QValueStack, a value-based stack. \i \l QMap, a value-based dictionary (associative array). \endlist The pointer-based collections are: \list \i \l QCache and \l QIntCache, LRU (least recently used) caches. \i \l QDict, \l QIntDict and \l QPtrDict dictionaries. \i \l QPtrList, a doubly linked list. \i \l QPtrQueue, a FIFO (first in, first out) queue. \i \l QPtrStack, a LIFO (last in, first out) stack. \i \l QPtrVector, a vector. \endlist \l QMemArray is exceptional; it is neither pointer nor value based, but memory based. For maximum efficiency with the simple data types usually used in arrays, it uses bitwise operations to copy and compare array elements. Some of these classes have corresponding iterators. An iterator is a class for traversing the items in a collection: \list \i \link QCacheIterator QCacheIterator\endlink and \link QIntCacheIterator QIntCacheIterator\endlink \i \link QDictIterator QDictIterator\endlink, \link QIntDictIterator QIntDictIterator\endlink, and \link QPtrDictIterator QPtrDictIterator\endlink \i \link QPtrListIterator QPtrListIterator\endlink \i \link QValueListIterator QValueListIterator\endlink, and \link QValueListConstIterator QValueListConstIterator\endlink \i \link QMapIterator QMapIterator\endlink, and \link QMapConstIterator QMapConstIterator\endlink \endlist The value-based collections plus algorithms operating on them are grouped together in the \link qt-template-lib.html Qt Template Library\endlink; see also the \link qtl.html Qt Template Library Classes\endlink. The rest of this page dicusses the pointer-based containers. \section1 Architecture of the pointer-based containers There are four internal base classes for the pointer-based containers (QGCache, QGDict, QGList and QGVector) that operate on void pointers. A thin template layer implements the actual collections by casting item pointers to and from void pointers. This strategy allows Qt's templates to be very economical on space (instantiating one of these templates adds only inlinable calls to the base classes), without hurting performance. \section1 A QPtrList Example This example shows how to store Employee items in a list and prints them out in reverse order: \code #include #include #include class Employee { public: Employee( const char *name, int salary ) { n=name; s=salary; } const char *name() const { return n; } int salary() const { return s; } private: QString n; int s; }; int main() { QPtrList list; // list of pointers to Employee list.setAutoDelete( TRUE ); // delete items when they are removed list.append( new Employee("Bill", 50000) ); list.append( new Employee("Steve",80000) ); list.append( new Employee("Ron", 60000) ); QPtrListIterator it(list); // iterator for employee list for ( it.toLast(); it.current(); --it) ) { Employee *emp = it.current(); printf( "%s earns %d\n", emp->name(), emp->salary() ); } return 0; } \endcode Program output: \code Ron earns 60000 Steve earns 80000 Bill earns 50000 \endcode \section1 Managing Collection Items All pointer-based collections inherit the \l QPtrCollection base class. This class only knows about the number of items in the collection and the deletion strategy. By default, items in a collection are not deleted when they are removed from the collection. The \l QPtrCollection::setAutoDelete() function specifies the deletion strategy. In the list example, we enable auto-deletion to make the list delete the items when they are removed from the list. When inserting an item into a collection, only the pointer is copied, not the item itself. This is called a shallow copy. It is possible to make the collection copy all of the item's data (known as a deep copy) when an item is inserted. All collection functions that insert an item call the virtual function \l QPtrCollection::newItem() for the item to be inserted. Inherit a collection and reimplement it if you want to have deep copies in your collection. When removing an item from a list, the virtual function \l{QPtrCollection::deleteItem()} is called. The default implementation in all collection classes deletes the item if auto-deletion is enabled. \section1 Usage A pointer-based collection class, such as QPtrList\, defines a collection of \e pointers to \e type objects. The pointer (*) is implicit. We discuss \l QPtrList here, but the same techniques apply to all pointer-based collection classes and all collection class iterators. Template instantiation: \code QPtrList list; // wherever the list is used \endcode The item's class or type, Employee in our example, must be defined prior to the list definition. \code // Does not work: Employee is not defined class Employee; QPtrList list; // This works: Employee is defined before it is used class Employee { ... }; QPtrList list; \endcode \section1 Iterators Although \l QPtrList has member functions to traverse the list, it can often be better to make use of an iterator. \l QPtrListIterator is very safe and can traverse lists that are being modified at the same time. Multiple iterators can work independently on the same collection. A QPtrList has an internal list of all the iterators that are currently operating on it. When a list entry is removed, the list updates all iterators accordingly. The \l QDict and \l QCache collections have no traversal functions. To traverse these collections, you must use \l QDictIterator or \l QCacheIterator. \section1 Predefined Collections Qt has the following predefined collection classes: \list \i String lists: \l QStrList, \l QStrIList (\l qstrlist.h) and \l QStringList (\l qstringlist.h) \i String vectors: QStrVec and QStrIVec (qstrvec.h); these are obsolete \endlist In almost all cases you would choose \l QStringList, a value list of implicitly shared QString Unicode strings. QPtrStrList and QPtrStrIList store only char pointers, not the strings themselves. \section1 List of Pointer-based Collection Classes and Related Iterator Classes */