Qt Toolkit - QMap Class

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# QMap Class Reference

The QMap class is a value based template class that provides a dictionary More...

`#include <qmap.h>`

List of all member functions.

## Public Members

- QMap ()
- QMap ( const QMap<Key,T> & m )
- ~QMap ()
- QMap<Key, T>& operator= ( const QMap<Key, T> & m )
- Iterator begin ()
- Iterator end ()
- ConstIterator begin () const
- ConstIterator end () const
- Iterator find ( const Key & k )
- ConstIterator find ( const Key & k ) const
- T& operator[] ( const Key & k )
- const T& operator[] ( const Key & k ) const
- bool contains ( const Key & k ) const
- uint count () const
- bool isEmpty () const
- Iterator insert ( const Key & key, const T & value )
- void remove ( Iterator it )
- void remove ( const Key & k )
- Iterator replace ( const Key & k, const T & v )
- void clear ()

## Protected Members

- void detach ()

## Related Functions

(Note that these are not member functions.)

- QDataStream& operator>> (QDataStream & s, QMap<Key,T> & m)
- QDataStream& operator<< (QDataStream & s, const QMap<Key,T> & m)

## Detailed Description

The QMap class is a value based template class that provides a dictionary

Define a template instance QMap<Key,Data> to create a dictionary with keys of type Key and values of type Data. QMap does not store pointers to the members of the map. Instead, it holds a copy of every member. For that reason this kind of classes is called "value based" while QList and QDict are "reference based".

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

- a copy constructor,
- an assignment operator and
- 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.

The class used for the key requires that the `operator<` is implemented and defines a total order on the keys.

Example:

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

        class Employee
        {
        public:
            Employee(): s(0) {}
            Employee( const QString& name, int salary )
                : n(name), s(salary)
            {}

            QString     name()   const              { return n; }
            int         salary() const              { return s; }
            void        setSalary( int salary )     { s = salary; }
        private:
            QString     n;
            int         s;
        };

        void main()
        {
            typedef QMap<QString,Employee> EmployeeMap;
            EmployeeMap map;                // map of Employee

            map.insert( "Gates", Employee("Bill", 50000) );
            map.insert( "Ballmer", Employee("Steve",80000) );
            map.insert( "Sommer,", Employee("Ron",  60000) );

            Employee joe( "Joe", 50000 );
            map.insert( "Doe", joe );
            joe.setSalary( 4000 );

            EmployeeMap::Iterator it;
            for( it = map.begin(); it != map.end(); ++it )
                printf( "%s, %s earns %d\n", it.key().latin1(), it.data().name().latin1(), it.data().salary() );
        }

Program output:

            Ballmer, Steve earns 80000
            Doe, Joe earns 50000
            Gates, Bill earns 50000
            Sommer, Ron earns 60000

As you can see, the latest changes to Joe's salary did not affect the value in the list because the map created a copy of Joe's entry. In addition you should notice that the items are alphabetically sorted when iterating over the map.

There are two ways to find values in the list. The first one is to use the find() function. It returns an iterator pointing to the desired item or the end() iterator it no such element exists.

The second approach uses the operator[]. But be warned: If you don't know that the element you are searching for is really in the list, then you should not use operator[]. The following example illustrates that.

            QMap<QString,QString> map;
            map.insert( "Weis", "Torben" );
            str << map["Weis"] << map["Ettrich"] << endl;

            const QMap<QString,QString>& map2 = map;
            str << map2["Weis"] << map2["Reggie"] << endl;

The code fragment will print out "Torben", "" and the second part will print "Torben", "". In addition the first fragment inserted an empty entry with key "Ettrich". The second one did not insert an empty entry with key "Reggie" because the const operator[] was used which can not do insertion. So if you are not sure whether a certain element is in the map you should use find() and iterators.

If you just want to know whether a certain key is contained in the map, the the contains() function is what you are looking for. In addition count() tells you how many keys there are currently in the map.

Another method for traversing a map is to use the functions begin() and end(). With a simple for loop as shown in the example you can iterate over the complete map. It is safe to have multiple iterators at the same time. If some member of the map is removed then only iterators pointing to the removed member become invalid. Inserting in the map does not invalidate any iterator.

Since QMap is value based there is no need to care about deleting elements 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 item with clear().

QMap is implicitly shared. This means you can just make copies of the map in time O(1). If multiple QMap instances share the same data and one is modifying the map's data then this modifying instance makes a copy and modifies its private copy - thus it does not affect other instances. From a developer's point of view you can think that a QMap and a copy of this map have nothing to do with each other.

There are two ways of inserting new elements in a list. One uses the insert() method while the other one uses operator[] like this:

            QMap<QString,QString> map;
            map["Weis"] = "Torben";

;

Items can be removed from the map in two ways. The first is to pass an iterator to the remove(). The other possibility is to pass a key value to remove() which will delete the entry with the requested key. In addition you can clear the entire map using the clear() method.

See also QMapIterator.

## Member Function Documentation

### QMap::QMap ()

Constructs an empty map.

### QMap::QMap ( const QMap<Key,T> & m )

Constructs a copy of m.

This operation costs O(1) time since QMap is implicit shared. The first instance applying modifications to a shared list will create a copy which takes in turn O(n) time. However returning a QMap from a function is very fast.

### QMap::~QMap ()

Destroys the map. References to the values in the map and all iterators of this map become invalidated. Since QMap is highly tuned for performance you won't see warnings if you use invalid iterators, because it is impossible for an iterator to check whether it is valid or not.

### ConstIterator QMap::begin () const

Returns an iterator pointing to the first element in the map. This iterator equals end() if the map is empty;

See also end() and QMapConstIterator.

### Iterator QMap::begin ()

Returns an iterator pointing to the first element in the map. This iterator equals end() if the map is empty;

See also end() and QMapIterator.

### void QMap::clear ()

Removes all items from the map.

See also remove().

### bool QMap::contains ( const Key & k ) const

Returns TRUE if the key k is contained in the map.

### uint QMap::count () const

Returns the number of items in the ap.

See also isEmpty().

### void QMap::detach () `[protected]`

If the map does not share its data with another QMap instance, then nothing happens, otherwise the function creates a new copy of this data and detaches from the shared one. This function is called whenever the map is modified. The implicit sharing mechanism is implemented this way.

### ConstIterator QMap::end () const

Returns an iterator pointing behind the last element in the map. This iterator equals begin() if the map is empty.

See also begin() and QMapConstIterator.

### Iterator QMap::end ()

Returns an iterator pointing behind the last element in the map. This iterator equals begin() if the map is empty.

See also begin() and QMapIterator.

### ConstIterator QMap::find ( const Key & k ) const

Finds the key k in the map.

Returns end() if no key did match.

See also QMapConstIterator.

### Iterator QMap::find ( const Key & k )

Finds the key k in the map.

Returns end() if no key did match.

See also QMapIterator.

### Iterator QMap::insert ( const Key & key, const T & value )

Inserts the value with key k.

Returns an iterator pointing at the inserted value.

See also QMapIterator.

### bool QMap::isEmpty () const

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

See also count().

### QMap<Key, T>& QMap::operator= ( const QMap<Key, T> & m )

Assigns m to this map and returns a reference to this map.

All iterators of the current map become invalidated by this operation. The cost of such an assignment is O(1) since QMap is implicitly shared.

### T& QMap::operator[] ( const Key & k )

Returns the value associated with the key k. If no such key is present then an empty item is inserted with this key and a reference to the item is returned.

You can use this operator in two directions: For reading and for writing:

            QMap<QString,QString> map;
            map[ "Weis" ] = "Torben";
            stream << map[ "Weis" ];

### const T& QMap::operator[] ( const Key & k ) const

Returns the value associated with the key k. If no such key is present then a reference to an empty item is returned.

### void QMap::remove ( Iterator it )

Removes the item at position it in the map.

See also clear() and QMapIterator.

### void QMap::remove ( const Key & k )

Removes the item with the key k.

See also clear().

### Iterator QMap::replace ( const Key & k, const T & v )

Replaces the value with key k from the map if possible and inserts the new value v with key k in the map.

See also insert(), remove() and QMapIterator.

## Related Functions

### QDataStream& operator>> (QDataStream & s, QMap<Key,T> & m)

Reads a map from the stream. The types Key and T must implement the streaming operator, too.

### QDataStream& operator<< (QDataStream & s, const QMap<Key,T> & m)

Writes a map to the stream. The types Key and T must implement the streaming operator, too.

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