Collection Classes

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 Qt Template Library documentation for details.

The value-based collections are:

• QValueList, a value-based list.
• QValueVector, a value-based vector.
• QValueStack, a value-based stack.
• QMap, a value-based dictionary (associative array).

The pointer-based collections are:

• QCache and QIntCache, LRU (least recently used) caches.
• QDict, QIntDict and QPtrDict dictionaries.
• QPtrList, a doubly linked list.
• QPtrQueue, a FIFO (first in, first out) queue.
• QPtrStack, a LIFO (last in, first out) stack.
• QPtrVector, a vector.

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:

• QCacheIterator and QIntCacheIterator
• QDictIterator, QIntDictIterator, and QPtrDictIterator
• QPtrListIterator
• QValueListIterator, and QValueListConstIterator
• QMapIterator, and QMapConstIterator

The value-based collections plus algorithms operating on them are grouped together in the Qt Template Library; see also the Qt Template Library Classes.

The rest of this page dicusses the pointer-based containers.

Architecture of the pointer-based containers

    #include <qptrlist.h>
    #include <qstring.h>
    #include <stdio.h>

    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<Employee> 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<Employee> 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;
    }

    Ron earns 60000
    Steve earns 80000
    Bill earns 50000

    QPtrList<Employee> list;            // wherever the list is used

    // Does not work: Employee is not defined
    class Employee;
    QPtrList<Employee> list;

    // This works: Employee is defined before it is used
    class Employee {
        ...
    };
    QPtrList<Employee> list;