QHash Class Reference
^{[QtCore module]}

Detailed Description

The QHash class is a template class that provides a hash-table-based dictionary.

QHash<Key, T> is one of Qt's generic container classes. It stores (key, value) pairs and provides very fast lookup of the value associated with a key.

QHash provides very similar functionality to QMap. The differences are:

• QHash provides faster lookups than QMap. (See Algorithmic Complexity for details.)
• When iterating over a QMap, the items are always sorted by key. With QHash, the items are arbitrarily ordered.
• The key type of a QMap must provide operator<(). The key type of a QHash must provide operator==() and a global qHash(Key) function.

Here's an example QHash with QString keys and int values:

 QHash<QString, int> hash;

To insert a (key, value) pair into the hash, you can use operator[]():

 hash["one"] = 1;
 hash["three"] = 3;
 hash["seven"] = 7;

This inserts the following three (key, value) pairs into the QHash: ("one", 1), ("three", 3), and ("seven", 7). Another way to insert items into the hash is to use insert():

 hash.insert("twelve", 12);

To look up a value, use operator[]() or value():

 int num1 = hash["thirteen"];
 int num2 = hash.value("thirteen");

If there is no item with the specified key in the hash, these functions return a default-constructed value.

If you want to check whether the hash contains a particular key, use contains():

 int timeout = 30;
 if (hash.contains("TIMEOUT"))
     timeout = hash.value("TIMEOUT");

There is also a value() overload that uses its second argument as a default value if there is no item with the specified key:

 int timeout = hash.value("TIMEOUT", 30);

In general, we recommend that you use contains() and value() rather than operator[]() for looking up a key in a hash. The reason is that operator[]() silently inserts an item into the hash if no item exists with the same key (unless the hash is const). For example, the following code snippet will create 1000 items in memory:

 // WRONG
 QHash<int, QWidget *> hash;
 ...
 for (int i = 0; i < 1000; ++i) {
     if (hash[i] == okButton)
         cout << "Found button at index " << i << endl;
 }

To avoid this problem, replace hash[i] with hash.value(i) in the code above.

If you want to navigate through all the (key, value) pairs stored in a QHash, you can use an iterator. QHash provides both Java-style iterators (QHashIterator and QMutableHashIterator) and STL-style iterators (QHash::const_iterator and QHash::iterator). Here's how to iterate over a QHash<QString, int> using a Java-style iterator:

 QHashIterator<QString, int> i(hash);
 while (i.hasNext()) {
     i.next();
     cout << i.key() << ": " << i.value() << endl;
 }

Here's the same code, but using an STL-style iterator:

 QHash<QString, int>::const_iterator i = hash.constBegin();
 while (i != hash.constEnd()) {
     cout << i.key() << ": " << i.value() << endl;
     ++i;
 }

QHash is unordered, so an iterator's sequence cannot be assumed to be predictable. If ordering by key is required, use a QMap.

Normally, a QHash allows only one value per key. If you call insert() with a key that already exists in the QHash, the previous value is erased. For example:

 hash.insert("plenty", 100);
 hash.insert("plenty", 2000);
 // hash.value("plenty") == 2000

However, you can store multiple values per key by using insertMulti() instead of insert() (or using the convenience subclass QMultiHash). If you want to retrieve all the values for a single key, you can use values(const Key &key), which returns a QList<T>:

 QList<int> values = hash.values("plenty");
 for (int i = 0; i < values.size(); ++i)
     cout << values.at(i) << endl;

The items that share the same key are available from most recently to least recently inserted. A more efficient approach is to call find() to get the iterator for the first item with a key and iterate from there:

 QHash<QString, int>::iterator i = hash.find("plenty");
 while (i != hash.end() && i.key() == "plenty") {
     cout << i.value() << endl;
     ++i;
 }

If you only need to extract the values from a hash (not the keys), you can also use foreach:

 QHash<QString, int> hash;
 ...
 foreach (int value, hash)
     cout << value << endl;

Items can be removed from the hash in several ways. One way is to call remove(); this will remove any item with the given key. Another way is to use QMutableHashIterator::remove(). In addition, you can clear the entire hash using clear().

QHash's key and value data types must be assignable data types. You cannot, for example, store a QWidget as a value; instead, store a QWidget *. In addition, QHash's key type must provide operator==(), and there must also be a global qHash() function that returns a hash value for an argument of the key's type.

Here's a list of the C++ and Qt types that can serve as keys in a QHash: any integer type (char, unsigned long, etc.), any pointer type, QChar, QString, and QByteArray. For all of these, the <QHash> header defines a qHash() function that computes an adequate hash value. If you want to use other types as the key, make sure that you provide operator==() and a qHash() implementation.

Example:

 #ifndef EMPLOYEE_H
 #define EMPLOYEE_H

 class Employee
 {
 public:
     Employee() {}
     Employee(const QString &name, const QDate &dateOfBirth);
     ...

 private:
     QString myName;
     QDate myDateOfBirth;
 };

 inline bool operator==(const Employee &e1, const Employee &e2)
 {
     return e1.name() == e2.name()
            && e1.dateOfBirth() == e2.dateOfBirth();
 }

 inline uint qHash(const Employee &key)
 {
     return qHash(key.name()) ^ key.dateOfBirth().day();
 }

 #endif // EMPLOYEE_H

The qHash() function computes a numeric value based on a key. It can use any algorithm imaginable, as long as it always returns the same value if given the same argument. In other words, if e1 == e2, then qHash(e1) == qHash(e2) must hold as well. However, to obtain good performance, the qHash() function should attempt to return different hash values for different keys to the largest extent possible.

In the example above, we've relied on Qt's global qHash(const QString &) to give us a hash value for the employee's name, and XOR'ed this with the day they were born to help produce unique hashes for people with the same name.

Internally, QHash uses a hash table to perform lookups. Unlike Qt 3's QDict class, which needed to be initialized with a prime number, QHash's hash table automatically grows and shrinks to provide fast lookups without wasting too much memory. You can still control the size of the hash table by calling reserve() if you already know approximately how many items the QHash will contain, but this isn't necessary to obtain good performance. You can also call capacity() to retrieve the hash table's size.

See also QHashIterator, QMutableHashIterator, QMap, and QSet.

Member Type Documentation

typedef QHash::ConstIterator

Qt-style synonym for QHash::const_iterator.

typedef QHash::Iterator

Qt-style synonym for QHash::iterator.

typedef QHash::difference_type

Typedef for ptrdiff_t. Provided for STL compatibility.

typedef QHash::key_type

Typedef for Key. Provided for STL compatibility.

typedef QHash::mapped_type

Typedef for T. Provided for STL compatibility.

typedef QHash::size_type

Typedef for int. Provided for STL compatibility.

Member Function Documentation

QHash::QHash ()

Constructs an empty hash.

See also clear().

QHash::QHash ( const QHash<Key, T> & other )

Constructs a copy of other.

This operation occurs in constant time, because QHash is implicitly shared. This makes returning a QHash from a function very fast. If a shared instance is modified, it will be copied (copy-on-write), and this takes linear time.

See also operator=().

QHash::~QHash ()

Destroys the hash. References to the values in the hash and all iterators of this hash become invalid.

iterator QHash::begin ()

Returns an STL-style iterator pointing to the first item in the hash.

See also constBegin() and end().

const_iterator QHash::begin () const

This is an overloaded member function, provided for convenience.

int QHash::capacity () const

Returns the number of buckets in the QHash's internal hash table.

The sole purpose of this function is to provide a means of fine tuning QHash's memory usage. In general, you will rarely ever need to call this function. If you want to know how many items are in the hash, call size().

See also reserve() and squeeze().

void QHash::clear ()

Removes all items from the hash.

See also remove().

const_iterator QHash::constBegin () const

Returns a const STL-style iterator pointing to the first item in the hash.

See also begin() and constEnd().

const_iterator QHash::constEnd () const

Returns a const STL-style iterator pointing to the imaginary item after the last item in the hash.

See also constBegin() and end().

const_iterator QHash::constFind ( const Key & key ) const

Returns an iterator pointing to the item with key key in the hash.

If the hash contains no item with key key, the function returns constEnd().

This function was introduced in Qt 4.1.

See also find().

bool QHash::contains ( const Key & key ) const

Returns true if the hash contains an item with key key; otherwise returns false.

See also count().

int QHash::count ( const Key & key ) const

Returns the number of items associated with key key.

See also contains() and insertMulti().

int QHash::count () const

This is an overloaded member function, provided for convenience.

Same as size().

bool QHash::empty () const

This function is provided for STL compatibility. It is equivalent to isEmpty(), returning true if the hash is empty; otherwise returns false.

iterator QHash::end ()

Returns an STL-style iterator pointing to the imaginary item after the last item in the hash.

See also begin() and constEnd().

const_iterator QHash::end () const

This is an overloaded member function, provided for convenience.

iterator QHash::erase ( iterator pos )

Removes the (key, value) pair associated with the iterator pos from the hash, and returns an iterator to the next item in the hash.

Unlike remove() and take(), this function never causes QHash to rehash its internal data structure. This means that it can safely be called while iterating, and won't affect the order of items in the hash. For example:

 QHash<QObject *, int> objectHash;
 ...
 QHash<QObject *, int>::iterator i = objectHash.find(obj);
 while (i != objectHash.end() && i.key() == obj) {
     if (i.value() == 0) {
         i = objectHash.erase(i);
     } else {
         ++i;
     }
 }

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

iterator QHash::find ( const Key & key )

Returns an iterator pointing to the item with key key in the hash.

If the hash contains no item with key key, the function returns end().

If the hash contains multiple items with key key, this function returns an iterator that points to the most recently inserted value. The other values are accessible by incrementing the iterator. For example, here's some code that iterates over all the items with the same key:

 QHash<QString, int> hash;
 ...
 QHash<QString, int>::const_iterator i = hash.find("HDR");
 while (i != hash.end() && i.key() == "HDR") {
     cout << i.value() << endl;
     ++i;
 }

See also value() and values().

const_iterator QHash::find ( const Key & key ) const

This is an overloaded member function, provided for convenience.

iterator QHash::insert ( const Key & key, const T & value )

Inserts a new item with the key key and a value of value.

If there is already an item with the key key, that item's value is replaced with value.

If there are multiple items with the key key, the most recently inserted item's value is replaced with value.

See also insertMulti().

iterator QHash::insertMulti ( const Key & key, const T & value )

Inserts a new item with the key key and a value of value.

If there is already an item with the same key in the hash, this function will simply create a new one. (This behavior is different from insert(), which overwrites the value of an existing item.)

See also insert() and values().

bool QHash::isEmpty () const

Returns true if the hash contains no items; otherwise returns false.

See also size().

const Key QHash::key ( const T & value ) const

Returns the first key with value value.

If the hash contains no item with value value, the function returns a default-constructed key.

This function can be slow (linear time), because QHash's internal data structure is optimized for fast lookup by key, not by value.

See also value() and values().

QList<Key> QHash::keys () const

Returns a list containing all the keys in the hash, in an arbitrary order. Keys that occur multiple times in the hash (because items were inserted with insertMulti(), or unite() was used) also occur multiple times in the list.

To obtain a list of unique keys, where each key from the map only occurs once, use uniqueKeys().

The order is guaranteed to be the same as that used by values().

See also uniqueKeys(), values(), and key().

QList<Key> QHash::keys ( const T & value ) const

This is an overloaded member function, provided for convenience.

Returns a list containing all the keys associated with value value, in an arbitrary order.

This function can be slow (linear time), because QHash's internal data structure is optimized for fast lookup by key, not by value.

int QHash::remove ( const Key & key )

Removes all the items that have the key key from the hash. Returns the number of items removed which is usually 1 but will be 0 if the key isn't in the hash, or greater than 1 if insertMulti() has been used with the key.

See also clear() and take().

void QHash::reserve ( int size )

Ensures that the QHash's internal hash table consists of at least size buckets.

This function is useful for code that needs to build a huge hash and wants to avoid repeated reallocation. For example:

 QHash<QString, int> hash;
 hash.reserve(20000);
 for (int i = 0; i < 20000; ++i)
     hash.insert(keys[i], values[i]);

Ideally, size should be slightly more than the maximum number of items expected in the hash. size doesn't have to be prime, because QHash will use a prime number internally anyway. If size is an underestimate, the worst that will happen is that the QHash will be a bit slower.

In general, you will rarely ever need to call this function. QHash's internal hash table automatically shrinks or grows to provide good performance without wasting too much memory.

See also squeeze() and capacity().

int QHash::size () const

Returns the number of items in the hash.

See also isEmpty() and count().

void QHash::squeeze ()

Reduces the size of the QHash's internal hash table to save memory.

The sole purpose of this function is to provide a means of fine tuning QHash's memory usage. In general, you will rarely ever need to call this function.

See also reserve() and capacity().

T QHash::take ( const Key & key )

Removes the item with the key key from the hash and returns the value associated with it.

If the item does not exist in the hash, the function simply returns a default-constructed value. If there are multiple items for key in the hash, only the most recently inserted one is removed.

If you don't use the return value, remove() is more efficient.

See also remove().

QList<Key> QHash::uniqueKeys () const

Returns a list containing all the keys in the map in ascending order. Keys that occur multiple times in the map (because items were inserted with insertMulti(), or unite() was used) occur only once in the returned list.

This function was introduced in Qt 4.2.

See also keys() and values().

QHash<Key, T> & QHash::unite ( const QHash<Key, T> & other )

Inserts all the items in the other hash into this hash. If a key is common to both hashes, the resulting hash will contain the key multiple times.

See also insertMulti().

const T QHash::value ( const Key & key ) const

Returns the value associated with the key key.

If the hash contains no item with key key, the function returns a default-constructed value. If there are multiple items for key in the hash, the value of the most recently inserted one is returned.

See also key(), values(), contains(), and operator[]().

const T QHash::value ( const Key & key, const T & defaultValue ) const

This is an overloaded member function, provided for convenience.

If the hash contains no item with the given key, the function returns defaultValue.

QList<T> QHash::values () const

Returns a list containing all the values in the hash, in an arbitrary order. If a key is associated multiple values, all of its values will be in the list, and not just the most recently inserted one.

The order is guaranteed to be the same as that used by keys().

See also keys().

QList<T> QHash::values ( const Key & key ) const

This is an overloaded member function, provided for convenience.

Returns a list of all the values associated with key key, from the most recently inserted to the least recently inserted.

See also count() and insertMulti().

bool QHash::operator!= ( const QHash<Key, T> & other ) const

Returns true if other is not equal to this hash; otherwise returns false.

Two hashes are considered equal if they contain the same (key, value) pairs.

This function requires the value type to implement operator==().

See also operator==().

QHash<Key, T> & QHash::operator= ( const QHash<Key, T> & other )

Assigns other to this hash and returns a reference to this hash.

bool QHash::operator== ( const QHash<Key, T> & other ) const

Returns true if other is equal to this hash; otherwise returns false.

Two hashes are considered equal if they contain the same (key, value) pairs.

This function requires the value type to implement operator==().

See also operator!=().

T & QHash::operator[] ( const Key & key )

Returns the value associated with the key key as a modifiable reference.

If the hash contains no item with key key, the function inserts a default-constructed value into the hash with key key, and returns a reference to it. If the hash contains multiple items with key key, this function returns a reference to the most recently inserted value.

See also insert() and value().

const T QHash::operator[] ( const Key & key ) const

This is an overloaded member function, provided for convenience.

Same as value().

Related Non-Members

uint qHash ( char key )

Returns the hash value for key.

uint qHash ( uchar key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

uint qHash ( signed char key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

uint qHash ( ushort key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

uint qHash ( short key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

uint qHash ( uint key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

uint qHash ( int key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

uint qHash ( ulong key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

uint qHash ( long key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

uint qHash ( quint64 key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

uint qHash ( qint64 key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

uint qHash ( QChar key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

uint qHash ( const QByteArray & key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

uint qHash ( const QString & key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

uint qHash ( const T * key )

This is an overloaded member function, provided for convenience.

Returns the hash value for key.

QDataStream & operator<< ( QDataStream & out, const QHash<Key, T> & hash )

This is an overloaded member function, provided for convenience.

Writes the hash hash to stream out.

This function requires the key and value types to implement operator<<().

See also Format of the QDataStream operators.

QDataStream & operator>> ( QDataStream & in, QHash<Key, T> & hash )

This is an overloaded member function, provided for convenience.

Reads a hash from stream in into hash.

This function requires the key and value types to implement operator>>().

See also Format of the QDataStream operators.