QDict Class

Qt 3.0.5

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QDict Class Reference

The QDict class is a template class that provides a dictionary based on QString keys. More...

#include <qdict.h>

Inherits QPtrCollection.

List of all member functions.

Public Members

QDict ( int size = 17, bool caseSensitive = TRUE )
QDict ( const QDict<type> & dict )
~QDict ()
QDict<type> & operator= ( const QDict<type> & dict )
virtual uint count () const
uint size () const
bool isEmpty () const
void insert ( const QString & key, const type * item )
void replace ( const QString & key, const type * item )
bool remove ( const QString & key )
type * take ( const QString & key )
type * find ( const QString & key ) const
type * operator[] ( const QString & key ) const
virtual void clear ()
void resize ( uint newsize )
void statistics () const

Important Inherited Members

bool autoDelete () const
void setAutoDelete ( bool enable )

Protected Members

virtual QDataStream & read ( QDataStream & s, QPtrCollection::Item & item )
virtual QDataStream & write ( QDataStream & s, QPtrCollection::Item ) const

Detailed Description

The QDict class is a template class that provides a dictionary based on QString keys.

QMap is an STL-compatible alternative to this class.

QDict is implemented as a template class. Define a template instance QDict<X> to create a dictionary that operates on pointers to X (X*).

A dictionary is a collection of key-value pairs. The key is a QString used for insertion, removal and lookup. The value is a pointer. Dictionaries provide very fast insertion and lookup.

If you want to use non-Unicode, plain 8-bit char* keys, use the QAsciiDict template. A QDict has the same performance as a QAsciiDict. If you want to have a dictionary that maps QStrings to QStrings use QMap.

The size() of the dictionary is very important. In order to get good performance, you should use a suitably large prime number. Suitable means equal to or larger than the maximum expected number of dictionary items. Size is set in the constructor but may be changed with resize().

Items are inserted with insert(), and removed with remove(). All the items in a dictionary can be removed with clear(). The number of items in the dictionary is returned by count(). If the dictionary contains no items isEmpty() returns TRUE. You can change an item's value with replace(). Items are looked up with operator[](), or with find() which return a pointer to the value or 0 if the given key does not exist. You can take an item out of the dictionary with take().

Calling setAutoDelete(TRUE) for a dictionary tells it to delete items that are removed. The default behaviour is not to delete items when they are removed.

QDict is implemented by QGDict as a hash array with a fixed number of entries. Each array entry points to a singly linked list of buckets, in which the dictionary items are stored. When an item is inserted with a key, the key is converted (hashed) to an integer index into the hash array. The item is inserted before the first bucket in the list of buckets.

Looking up an item is normally very fast. The key is again hashed to an array index. Then QDict scans the list of buckets and returns the item found or null if the item was not found. You cannot insert null pointers into a dictionary.

Items with equal keys are allowed. When inserting two items with the same key, only the last inserted item will be visible (last in, first out) until it is removed.

The QDictIterator class can traverse the dictionary, but only in an arbitrary order. Multiple iterators may independently traverse the same dictionary.

When inserting an item into a dictionary, only the pointer is copied, not the item itself, i.e. a shallow copy is made. It is possible to make the dictionary copy all of the item's data (a deep copy) when an item is inserted. insert() calls the virtual function QPtrCollection::newItem() for the item to be inserted. Inherit a dictionary and reimplement it if you want deep copies.

When removing a dictionary item, the virtual function QPtrCollection::deleteItem() is called. QDict's default implementation is to delete the item if auto-deletion is enabled.

Example #1:

    QDict<QLineEdit> fields;
    fields.insert( "forename", new QLineEdit( this ) );
    fields.insert( "surname", new QLineEdit( this ) );

    fields["forename"]->setText( "Homer" );
    fields["surname"]->setText( "Simpson" );

    QDictIterator<char> it( extra ); // See QDictIterator
    for( ; it.current(); ++it )
        cout << it.currentKey() << ": " << it.current()->text() << endl;
    cout << endl;

    if ( fields["forename"] && fields["surname"] )
        cout << fields["forename"]->text() << " " 
            << fields["surname"]->text() << endl;  // Prints "Homer Simpson"

    fields.remove( "forename" ); // Does not delete the line edit
    if ( ! fields["forename"] )
        cout << "forename is not in the dictionary" << endl;

In this example we use a dictionary to keep track of the line edits we're using. We insert each line edit into the dictionary with a unique name and then access the line edits via the dictionary.

Example #2:

    QStringList styleList = QStyleFactory::styles();
    styleList.sort();
    QDict<int> letterDict( 17, FALSE );
    for ( QStringList::Iterator it = styleList.begin(); it != styleList.end(); ++it ) {
        QString styleName = *it;
        QString styleAccel = styleName;
        if ( letterDict[styleAccel.left(1)] ) {
            for ( uint i = 0; i < styleAccel.length(); i++ ) {
                if ( ! letterDict[styleAccel.mid( i, 1 )] ) {
                    styleAccel = styleAccel.insert( i, '&' );
                    letterDict.insert(styleAccel.mid( i, 1 ), (const int *)1);
                    break;
                }
            }
        } else {
            styleAccel = "&" + styleAccel;
            letterDict.insert(styleAccel.left(1), (const int *)1);
        }
        (void) new QAction( styleName, QIconSet(), styleAccel, parent );
    }

In the example we are using the dictionary to provide fast random access to the keys, and we don't care what the values are. The example is used to generate a menu of QStyles, each with a unique accelerator key (or no accelerator if there are no unused letters left).

We first obtain the list of available styles, then sort them so that the menu items will be ordered alphabetically. Next we create a dictionary of int pointers. The keys in the dictionary are each one character long, representing letters that have been used for accelerators. We iterate through our list of style names. If the first letter of the style name is in the dictionary, i.e. has been used, we iterate over all the characters in the style name to see if we can find a letter that hasn't been used. If we find an unused letter we put the accelerator ampersand (&) in front of it and add that letter to the dictionary. If we can't find an unused letter the style will simply have no accelerator. If the first letter of the style name is not in the dictionary we use it for the accelerator and add it to the dictionary. Finally we create a QAction for each style.

Member Function Documentation

QDict::QDict ( int size = 17, bool caseSensitive = TRUE )

Constructs a dictionary optimized for less than size entries.

We recommend setting size to a suitably large prime number (e.g. a prime that's slightly larger than the expected number of entries). This makes the hash distribution better which will lead to faster lookup.

If caseSensitive is TRUE (the default), keys which differ only in case are considered different.

QDict::QDict ( const QDict<type> & dict )

Constructs a copy of dict.

Each item in dict is inserted into this dictionary. Only the pointers are copied (shallow copy).

QDict::~QDict ()

Removes all items from the dictionary and destroys it. If setAutoDelete() is TRUE each value is deleted. All iterators that access this dictionary will be reset.

bool QPtrCollection::autoDelete () const

Returns the setting of the auto-delete option. The default is FALSE.

void QDict::clear () `[virtual]`

Removes all items from the dictionary.

The removed items are deleted if auto-deletion is enabled.

All dictionary iterators that operate on the dictionary are reset.

See also remove(), take() and setAutoDelete().

Reimplemented from QPtrCollection.

uint QDict::count () const `[virtual]`

Returns the number of items in the dictionary.

type * QDict::find ( const QString & key ) const

Returns the item with key key, or null if the key does not exist in the dictionary.

If there are two or more items with equal keys, then the last item that was inserted will be found.

Equivalent to the [] operator.

Warning: Your application will crash if you call find() on an empty dictionary; you can check with isEmpty() or count(). We don't perform this check for efficiency reasons.

void QDict::insert ( const QString & key, const type * item )

Inserts the key key with value item into the dictionary.

The key does not have to be unique. If multiple items are inserted with the same key, only the last item will be visible.

Null items are not allowed.

bool QDict::isEmpty () const

Returns TRUE if the dictionary is empty, i.e. count() == 0; otherwise returns FALSE.

QDict<type> & QDict::operator= ( const QDict<type> & dict )

Assigns dict to this dictionary and returns a reference to this dictionary.

This dictionary is first cleared, then each item in dict is inserted into this dictionary. Only the pointers are copied (shallow copy), unless newItem() has been reimplemented().

type * QDict::operator[] ( const QString & key ) const

Returns the item with key key, or null if the key does not exist in the dictionary.

If there are two or more items with equal keys, then the last item that was inserted will be found.

Equivalent to the find() function.

Warning: Your application will crash if you call find() on an empty dictionary; you can check with isEmpty() or count(). We don't perform this check for efficiency reasons.

QDataStream & QDict::read ( QDataStream & s, QPtrCollection::Item & item ) `[virtual protected]`

Reads a dictionary item from the stream s and returns a reference to the stream.

The default implementation sets item to 0.

bool QDict::remove ( const QString & key )

Removes the item with key from the dictionary. Returns TRUE if successful, or FALSE if the key does not exist in the dictionary.

If there are two or more items with equal keys, then the last item that was inserted will be removed.

The removed item is deleted if auto-deletion is enabled.

All dictionary iterators that refer to the removed item will be set to point to the next item in the dictionary traversing order.

See also take(), clear() and setAutoDelete().

void QDict::replace ( const QString & key, const type * item )

Replaces the value of the key, key with item.

If the item does not already exist, it will be inserted.

Null items are not allowed.

Equivalent to:

    QDict<char> dict;
        ...
    if ( dict.find( key ) )
        dict.remove( key );
    dict.insert( key, item );

If there are two or more items with equal keys, then the last item that was inserted will be replaced.

void QDict::resize ( uint newsize )

Changes the size of the hashtable the newsize. The contents of the dictionary are preserved, but all iterators on the dictionary become invalid.

void QPtrCollection::setAutoDelete ( bool enable )

Sets the collection to auto-delete its contents if enable is TRUE and to never delete them if enable is FALSE.

If auto-deleting is turned on, all the items in a collection are deleted when the collection itself is deleted. This is convenient if the collection has the only pointer to the items.

The default setting is FALSE, for safety. If you turn it on, be careful about copying the collection - you might find yourself with two collections deleting the same items.

Note that the auto-delete setting may also affect other functions in subclasses. For example, a subclass that has a remove() function will remove the item from its data structure, and if auto-delete is enabled, will also delete the item.

uint QDict::size () const

Returns the size of the internal hash array (as specified in the constructor).

void QDict::statistics () const

Debugging-only function that prints out the dictionary distribution using qDebug().

type * QDict::take ( const QString & key )

Takes the item with key out of the dictionary without deleting it (even if auto-deletion is enabled).

If there are two or more items with equal keys, then the last item that was inserted will be taken.

Returns a pointer to the item taken out, or null if the key does not exist in the dictionary.

All dictionary iterators that refer to the taken item will be set to point to the next item in the dictionary traversal order.

See also remove(), clear() and setAutoDelete().

QDataStream & QDict::write ( QDataStream & s, QPtrCollection::Item ) const `[virtual protected]`

Writes a dictionary item to the stream s and returns a reference to the stream.