Q3ValueList Class Reference
[Qt3Support module]

Detailed Description

The Q3ValueList class is a value-based template class that provides lists.

Q3ValueList is a Qt implementation of an STL-like list container. It can be used in your application if the standard list is not available for your target platforms.

Q3ValueList<T> defines a template instance to create a list of values that all have the class T. Note that Q3ValueList does not store pointers to the members of the list; it holds a copy of every member. This is why these kinds of classes are called "value based"; Q3PtrList and Q3Dict are "pointer based".

Q3ValueList contains and manages a collection of objects of type T and provides iterators that allow the contained objects to be addressed. Q3ValueList owns the contained items. For more relaxed ownership semantics, see Q3PtrCollection and friends which are pointer-based containers.

Some classes cannot be used within a Q3ValueList, for example, all classes derived from QObject and thus all classes that implement widgets. Only values can be used in a Q3ValueList. To qualify as a value the class must provide:

• a copy constructor;
• an assignment operator;
• a default constructor, i.e. a constructor that does not take any arguments.

Note that C++ defaults to field-by-field assignment operators and copy constructors if no explicit version is supplied. In many cases this is sufficient.

In addition, some compilers (e.g. Sun CC) might require that the class provides an equality operator (operator==()).

Q3ValueList's function naming is consistent with the other Qt classes (e.g. count(), isEmpty()). Q3ValueList also provides extra functions for compatibility with STL algorithms, such as size() and empty(). Programmers already familiar with the STL list may prefer to use the STL-compatible functions.

Example:

 class Employee
 {
 public:
     Employee(): sn(0) {}
     Employee( const QString& forename, const QString& surname, int salary )
         : fn(forename), sn(surname), sal(salary)
     {}

     QString forename() const { return fn; }
     QString surname() const { return sn; }
     int salary() const { return sal; }
     void setSalary( int salary ) { sal = salary; }

 private:
     QString fn;
     QString sn;
     int sal;
 };

 typedef Q3ValueList<Employee> EmployeeList;
 EmployeeList list;

 list.append( Employee("John", "Doe", 50000) );
 list.append( Employee("Jane", "Williams", 80000) );
 list.append( Employee("Tom", "Jones", 60000) );

 Employee mary( "Mary", "Hawthorne", 90000 );
 list.append( mary );
 mary.setSalary( 100000 );

 EmployeeList::iterator it;
 for ( it = list.begin(); it != list.end(); ++it )
     cout << (*it).surname().latin1() << ", " <<
             (*it).forename().latin1() << " earns " <<
             (*it).salary() << endl;

 // Output:
 // Doe, John earns 50000
 // Williams, Jane earns 80000
 // Hawthorne, Mary earns 90000
 // Jones, Tom earns 60000

Notice that the latest changes to Mary's salary did not affect the value in the list because the list created a copy of Mary's entry.

There are several ways to find items in the list. The begin() and end() functions return iterators to the beginning and end of the list. The advantage of getting an iterator is that you can move forward or backward from this position by incrementing/decrementing the iterator. The iterator returned by end() points to the item which is one past the last item in the container. The past-the-end iterator is still associated with the list it belongs to, however it is not dereferenceable; operator*() will not return a well-defined value. If the list is empty(), the iterator returned by begin() will equal the iterator returned by end().

It is safe to have multiple iterators a the list at the same time. If some member of the list is removed, only iterators pointing to the removed member become invalid. Inserting into the list does not invalidate any iterator. For convenience, the function last() returns a reference to the last item in the list, and first() returns a reference to the first item. If the list is empty(), both last() and first() have undefined behavior (your application will crash or do unpredictable things). Use last() and first() with caution, for example:

 Q3ValueList<int> list;
 list.append( 1 );
 list.append( 2 );
 list.append( 3 );
 ...
 if ( !list.empty() ) {
     // OK, modify the first item
     int& i = list.first();
     i = 18;
 }
 ...
 Q3ValueList<double> dlist;
 double d = dlist.last(); // undefined

Because Q3ValueList is value-based there is no need to be careful about deleting items in the list. The list holds its own copies and will free them if the corresponding member or the list itself is deleted. You can force the list to free all of its items with clear().

Q3ValueList is shared implicitly, which means it can be copied in constant time, i.e. O(1). If multiple Q3ValueList instances share the same data and one needs to modify its contents, this modifying instance makes a copy and modifies its private copy; therefore it does not affect the other instances; this takes O(n) time. This is often called "copy on write". If a Q3ValueList is being used in a multi-threaded program, you must protect all access to the list. See QMutex.

There are several ways to insert items into the list. The prepend() and append() functions insert items at the beginning and the end of the list respectively. The insert() function comes in several flavors and can be used to add one or more items at specific positions within the list.

Items can also be removed from the list in several ways. There are several variants of the remove() function, which removes a specific item from the list. The remove() function will find and remove items according to a specific item value.

See also Q3ValueListIterator.

Member Type Documentation

 Q3ValueList<int> l;
 ...
 Q3ValueList<int>::iterator it = l.end();
 --it;
 if ( it != end() )
     // ...

 Q3ValueList<int> l;
 ...
 Q3ValueList<int>::iterator it = l.end();
 --it;
 if ( it != end() )
     // ...