- Qt 5.15
- Qt Core
- C++ Classes
- QHash
- Qt 5.15.15 Reference Documentation

# QHash Class

template <typename Key, typename T> class QHash

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

Header: | #include <QHash>
qmake: | QT += core
Inherited By: | QMultiHash

- List of all members, including inherited members
- Obsolete members

Note: All functions in this class are reentrant.

## Public Types

class | const_iterator
class | iterator
class | key_iterator
typedef | ConstIterator
typedef | Iterator
typedef | const_key_value_iterator
typedef | difference_type
typedef | key_type
typedef | key_value_iterator
typedef | mapped_type
typedef | size_type

## Public Functions

| QHash(InputIterator begin, InputIterator end)
| QHash(QHash<K, V> &&other)
| QHash(const QHash<K, V> &other)
| QHash(std::initializer_list<std::pair<Key, T>> list)
| QHash()
QHash<K, V> & | operator=(QHash<K, V> &&other)
QHash<K, V> & | operator=(const QHash<K, V> &other)
| ~QHash()
iterator | begin()
const_iterator | begin() const
int | capacity() const
const_iterator | cbegin() const
const_iterator | cend() const
void | clear()
const_iterator | constBegin() const
const_iterator | constEnd() const
const_iterator | constFind(const Key &key) const
const_key_value_iterator | constKeyValueBegin() const
const_key_value_iterator | constKeyValueEnd() const
bool | contains(const Key &key) const
int | count(const Key &key) const
int | count() const
bool | empty() const
iterator | end()
const_iterator | end() const
QPair<iterator, iterator> | equal_range(const Key &key)
QPair<const_iterator, const_iterator> | equal_range(const Key &key) const
iterator | erase(const_iterator pos)
iterator | erase(iterator pos)
iterator | find(const Key &key)
const_iterator | find(const Key &key) const
iterator | insert(const Key &key, const T &value)
void | insert(const QHash<K, V> &other)
bool | isEmpty() const
const Key | key(const T &value) const
const Key | key(const T &value, const Key &defaultKey) const
key_iterator | keyBegin() const
key_iterator | keyEnd() const
key_value_iterator | keyValueBegin()
const_key_value_iterator | keyValueBegin() const
key_value_iterator | keyValueEnd()
const_key_value_iterator | keyValueEnd() const
QList<Key> | keys() const
QList<Key> | keys(const T &value) const
int | remove(const Key &key)
void | reserve(int size)
int | size() const
void | squeeze()
void | swap(QHash<K, V> &other)
T | take(const Key &key)
const T | value(const Key &key) const
const T | value(const Key &key, const T &defaultValue) const
QList<T> | values() const
bool | operator!=(const QHash<K, V> &other) const
bool | operator==(const QHash<K, V> &other) const
T & | operator[](const Key &key)
const T | operator[](const Key &key) const

## Related Non-Members

int | qGlobalQHashSeed()
uint | qHash(const QUrl &url, uint seed = 0)
uint | qHash(const QHash<Key, T> &key, uint seed = 0)
uint | qHash(const QBitArray &key, uint seed = 0)
uint | qHash(char key, uint seed = 0)
uint | qHash(const QDateTime &key, uint seed = 0)
uint | qHash(QLatin1String key, uint seed = 0)
uint | qHash(uchar key, uint seed = 0)
uint | qHash(const QDate &key, uint seed = 0)
uint | qHash(const T *key, uint seed = 0)
uint | qHash(signed char key, uint seed = 0)
uint | qHash(const QTime &key, uint seed = 0)
uint | qHash(const QSet<T> &key, uint seed = 0)
uint | qHash(ushort key, uint seed = 0)
uint | qHash(short key, uint seed = 0)
uint | qHash(const QPair<T1, T2> &key, uint seed = 0)
uint | qHash(uint key, uint seed = 0)
uint | qHash(const std::pair<T1, T2> &key, uint seed = 0)
uint | qHash(int key, uint seed = 0)
uint | qHash(const QVersionNumber &key, uint seed = 0)
uint | qHash(ulong key, uint seed = 0)
uint | qHash(long key, uint seed = 0)
uint | qHash(quint64 key, uint seed = 0)
uint | qHash(qint64 key, uint seed = 0)
uint | qHash(float key, uint seed = 0)
uint | qHash(double key, uint seed = 0)
uint | qHash(long double key, uint seed = 0)
uint | qHash(const QChar key, uint seed = 0)
uint | qHash(const QByteArray &key, uint seed = 0)
uint | qHash(const QString &key, uint seed = 0)
uint | qHash(const QStringRef &key, uint seed = 0)
uint | qHashBits(const void *p, size_t len, uint seed = 0)
uint | qHashRange(InputIterator first, InputIterator last, uint seed = 0)
uint | qHashRangeCommutative(InputIterator first, InputIterator last, uint seed = 0)
void | qSetGlobalQHashSeed(int newSeed)
QDataStream & | operator<<(QDataStream &out, const QHash<Key, T> &hash)
QDataStream & | operator>>(QDataStream &in, QHash<Key, T> &hash)

## Detailed Description

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 hash function called qHash() (see qHash).

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 << Qt::endl;
     }

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

Internally, QHash uses a hash table to perform lookups. This 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.

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() << Qt::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() << Qt::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

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 << Qt::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 *.

#### The qHash() hashing function

A QHash's key type has additional requirements other than being an assignable data type: it must provide operator==(), and there must also be a qHash() function in the type's namespace that returns a hash value for an argument of the key's type.

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.

For a key type `K`, the qHash function must have one of these signatures:

     uint qHash(K key);
     uint qHash(const K &key);

     uint qHash(K key, uint seed);
     uint qHash(const K &key, uint seed);

The two-arguments overloads take an unsigned integer that should be used to seed the calculation of the hash function. This seed is provided by QHash in order to prevent a family of algorithmic complexity attacks. If both a one-argument and a two-arguments overload are defined for a key type, the latter is used by QHash (note that you can simply define a two-arguments version, and use a default value for the seed parameter).

Here's a partial 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. Many other Qt classes also declare a qHash overload for their type; please refer to the documentation of each class.

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, 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, uint seed)
     {
         return qHash(key.name(), seed) ^ key.dateOfBirth().day();
     }

     #endif // EMPLOYEE_H

In the example above, we've relied on Qt's global qHash(const QString &, uint) 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.

Note that the implementation of the qHash() overloads offered by Qt may change at any time. You must not rely on the fact that qHash() will give the same results (for the same inputs) across different Qt versions.

#### Algorithmic complexity attacks

All hash tables are vulnerable to a particular class of denial of service attacks, in which the attacker carefully pre-computes a set of different keys that are going to be hashed in the same bucket of a hash table (or even have the very same hash value). The attack aims at getting the worst-case algorithmic behavior (O(n) instead of amortized O(1), see Algorithmic Complexity for the details) when the data is fed into the table.

In order to avoid this worst-case behavior, the calculation of the hash value done by qHash() can be salted by a random seed, that nullifies the attack's extent. This seed is automatically generated by QHash once per process, and then passed by QHash as the second argument of the two-arguments overload of the qHash() function.

This randomization of QHash is enabled by default. Even though programs should never depend on a particular QHash ordering, there may be situations where you temporarily need deterministic behavior, for example for debugging or regression testing. To disable the randomization, define the environment variable `QT_HASH_SEED` to have the value 0. Alternatively, you can call the qSetGlobalQHashSeed() function with the value 0.

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::const_key_value_iterator

The QMap::const_key_value_iterator typedef provides an STL-style const iterator for QHash and QMultiHash.

QHash::const_key_value_iterator is essentially the same as QHash::const_iterator with the difference that operator*() returns a key/value pair instead of a value.

This typedef was introduced in Qt 5.10.

See also QKeyValueIterator.

### 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::key_value_iterator

The QMap::key_value_iterator typedef provides an STL-style iterator for QHash and QMultiHash.

QHash::key_value_iterator is essentially the same as QHash::iterator with the difference that operator*() returns a key/value pair instead of a value.

This typedef was introduced in Qt 5.10.

See also QKeyValueIterator.

### 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

### template <typename InputIterator> QHash::QHash(InputIterator begin, InputIterator end)

Constructs a hash with a copy of each of the elements in the iterator range [begin, end). Either the elements iterated by the range must be objects with `first` and `second` data members (like `QPair`, `std::pair`, etc.) convertible to `Key` and to `T` respectively; or the iterators must have `key()` and `value()` member functions, returning a key convertible to `Key` and a value convertible to `T` respectively.

This function was introduced in Qt 5.14.

### QHash::QHash(QHash<K, V> &&other)

Move-constructs a QHash instance, making it point at the same object that other was pointing to.

This function was introduced in Qt 5.2.

### QHash::QHash(const QHash<K, V> &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(std::initializer_list<std::pair<Key, T>> list)

Constructs a hash with a copy of each of the elements in the initializer list list.

This function is only available if the program is being compiled in C++11 mode.

This function was introduced in Qt 5.1.

### QHash::QHash()

Constructs an empty hash.

See also clear().

### QHash<K, V> &QHash::operator=(QHash<K, V> &&other)

Move-assigns other to this QHash instance.

This function was introduced in Qt 5.2.

### QHash<K, V> &QHash::operator=(const QHash<K, V> &other)

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

### 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 function.

### 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().

### const_iterator QHash::cbegin() const

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

This function was introduced in Qt 5.0.

See also begin() and cend().

### const_iterator QHash::cend() const

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

This function was introduced in Qt 5.0.

See also cbegin() and end().

### 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 the key in the hash.

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

This function was introduced in Qt 4.1.

See also find() and QMultiHash::constFind().

### const_key_value_iterator QHash::constKeyValueBegin() const

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

This function was introduced in Qt 5.10.

See also keyValueBegin().

### const_key_value_iterator QHash::constKeyValueEnd() const

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

This function was introduced in Qt 5.10.

See also constKeyValueBegin().

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

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

See also count() and QMultiHash::contains().

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

Returns the number of items associated with the key.

See also contains() and insertMulti().

### int QHash::count() const

This is an overloaded function.

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 function.

### QPair<iterator, iterator> QHash::equal_range(const Key &key)

Returns a pair of iterators delimiting the range of values `[first, second)`, that are stored under key. If the range is empty then both iterators will be equal to end().

This function was introduced in Qt 5.7.

### QPair<const_iterator, const_iterator> QHash::equal_range(const Key &key) const

This is an overloaded function.

This function was introduced in Qt 5.7.

### iterator QHash::erase(const_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;
         }
     }

This function was introduced in Qt 5.7.

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

### iterator QHash::erase(iterator pos)

This is an overloaded function.

### iterator QHash::find(const Key &key)

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

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

If the hash contains multiple items with the 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() << Qt::endl;
         ++i;
     }

See also value(), values(), and QMultiHash::find().

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

This is an overloaded function.

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

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

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

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

### void QHash::insert(const QHash<K, V> &other)

Inserts all the items in the other hash into this hash.

If a key is common to both hashes, its value will be replaced with the value stored in other.

Note: If other contains multiple entries with the same key then the final value of the key is undefined.

This function was introduced in Qt 5.15.

### 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 mapped to value.

If the hash contains no item with the 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 keys().

### const Key QHash::key(const T &value, const Key &defaultKey) const

This is an overloaded function.

Returns the first key mapped to value, or defaultKey if the hash contains no item mapped to value.

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

This function was introduced in Qt 4.3.

### key_iterator QHash::keyBegin() const

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

This function was introduced in Qt 5.6.

See also keyEnd().

### key_iterator QHash::keyEnd() const

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

This function was introduced in Qt 5.6.

See also keyBegin().

### key_value_iterator QHash::keyValueBegin()

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

This function was introduced in Qt 5.10.

See also keyValueEnd().

### const_key_value_iterator QHash::keyValueBegin() const

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

This function was introduced in Qt 5.10.

See also keyValueEnd().

### key_value_iterator QHash::keyValueEnd()

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

This function was introduced in Qt 5.10.

See also keyValueBegin().

### const_key_value_iterator QHash::keyValueEnd() const

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

This function was introduced in Qt 5.10.

See also keyValueBegin().

### 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 the method is operating on a QMultiHash) also occur multiple times in the list.

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

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

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

This is an overloaded function.

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 from the hash. Returns the number of items removed which is 1 if the key exists in the hash, and 0 otherwise.

See also clear(), take(), and QMultiHash::remove().

### 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().

### void QHash::swap(QHash<K, V> &other)

Swaps hash other with this hash. This operation is very fast and never fails.

This function was introduced in Qt 4.8.

### T QHash::take(const Key &key)

Removes the item with the 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().

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

Returns the value associated with the key.

If the hash contains no item with the key, the function returns a default-constructed value. If there are multiple items for the 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 function.

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 with 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() and value().

### bool QHash::operator!=(const QHash<K, V> &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==().

### bool QHash::operator==(const QHash<K, V> &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 as a modifiable reference.

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

Same as value().

## Related Non-Members

### int qGlobalQHashSeed()

Returns the current global QHash seed.

The seed is set in any newly created QHash. See qHash about how this seed is being used by QHash.

This function was introduced in Qt 5.6.

See also qSetGlobalQHashSeed.

### uint qHash(const QUrl &url, uint seed = 0)

Returns the hash value for the url. If specified, seed is used to initialize the hash.

This function was introduced in Qt 5.0.

### template <typename Key, typename T> uint qHash(const QHash<Key, T> &key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

Type `T` must be supported by qHash().

This function was introduced in Qt 5.8.

### uint qHash(const QBitArray &key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(char key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(const QDateTime &key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(QLatin1String key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(uchar key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(const QDate &key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### template <typename T> uint qHash(const T *key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(signed char key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(const QTime &key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### template <typename T> uint qHash(const QSet<T> &key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

The hash value is independent of the order of elements in key, that is, sets that contain the same elements hash to the same value.

This function was introduced in Qt 5.5.

### uint qHash(ushort key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(short key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### template <typename T1, typename T2> uint qHash(const QPair<T1, T2> &key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

Types `T1` and `T2` must be supported by qHash().

This function was introduced in Qt 5.0.

### uint qHash(uint key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### template <typename T1, typename T2> uint qHash(const std::pair<T1, T2> &key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

Types `T1` and `T2` must be supported by qHash().

Note: The return type of this function is not the same as that of

     qHash(qMakePair(key.first, key.second), seed);

The two functions use different hashing algorithms; due to binary compatibility constraints, we cannot change the QPair algorithm to match the std::pair one before Qt 6.

This function was introduced in Qt 5.7.

### uint qHash(int key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(const QVersionNumber &key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.6.

### uint qHash(ulong key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(long key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(quint64 key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(qint64 key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(float key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.3.

### uint qHash(double key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.3.

### uint qHash(long double key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.3.

### uint qHash(const QChar key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(const QByteArray &key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(const QString &key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHash(const QStringRef &key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.0.

### uint qHashBits(const void *p, size_t len, uint seed = 0)

Returns the hash value for the memory block of size len pointed to by p, using seed to seed the calculation.

Use this function only to implement qHash() for your own custom types. For example, here's how you could implement a qHash() overload for std::vector<int>:

     inline uint qHash(const std::vector<int> &key, uint seed = 0)
     {
         if (key.empty())
             return seed;
         else
             return qHashBits(&key.front(), key.size() * sizeof(int), seed);
     }

This takes advantage of the fact that std::vector lays out its data contiguously. If that is not the case, or the contained type has padding, you should use qHashRange() instead.

It bears repeating that the implementation of qHashBits() - like the qHash() overloads offered by Qt - may change at any time. You must not rely on the fact that qHashBits() will give the same results (for the same inputs) across different Qt versions.

This function was introduced in Qt 5.4.

See also qHashRange() and qHashRangeCommutative().

### template <typename InputIterator> uint qHashRange(InputIterator first, InputIterator last, uint seed = 0)

Returns the hash value for the range [first,last), using seed to seed the calculation, by successively applying qHash() to each element and combining the hash values into a single one.

The return value of this function depends on the order of elements in the range. That means that

     {0, 1, 2}

and

     {1, 2, 0}

hash to different values. If order does not matter, for example for hash tables, use qHashRangeCommutative() instead. If you are hashing raw memory, use qHashBits().

Use this function only to implement qHash() for your own custom types. For example, here's how you could implement a qHash() overload for std::vector<int>:

     inline uint qHash(const std::vector<int> &key, uint seed = 0)
     {
         return qHashRange(key.begin(), key.end(), seed);
     }

It bears repeating that the implementation of qHashRange() - like the qHash() overloads offered by Qt - may change at any time. You must not rely on the fact that qHashRange() will give the same results (for the same inputs) across different Qt versions, even if qHash() for the element type would.

This function was introduced in Qt 5.5.

See also qHashBits() and qHashRangeCommutative().

### template <typename InputIterator> uint qHashRangeCommutative(InputIterator first, InputIterator last, uint seed = 0)

Returns the hash value for the range [first,last), using seed to seed the calculation, by successively applying qHash() to each element and combining the hash values into a single one.

The return value of this function does not depend on the order of elements in the range. That means that

     {0, 1, 2}

and

     {1, 2, 0}

hash to the same values. If order matters, for example, for vectors and arrays, use qHashRange() instead. If you are hashing raw memory, use qHashBits().

Use this function only to implement qHash() for your own custom types. For example, here's how you could implement a qHash() overload for std::unordered_set<int>:

     inline uint qHash(const std::unordered_set<int> &key, uint seed = 0)
     {
         return qHashRangeCommutative(key.begin(), key.end(), seed);
     }

It bears repeating that the implementation of qHashRangeCommutative() - like the qHash() overloads offered by Qt - may change at any time. You must not rely on the fact that qHashRangeCommutative() will give the same results (for the same inputs) across different Qt versions, even if qHash() for the element type would.

This function was introduced in Qt 5.5.

See also qHashBits() and qHashRange().

### void qSetGlobalQHashSeed(int newSeed)

Sets the global QHash seed to newSeed.

Manually setting the global QHash seed value should be done only for testing and debugging purposes, when deterministic and reproducible behavior on a QHash is needed. We discourage to do it in production code as it can make your application susceptible to algorithmic complexity attacks.

From Qt 5.10 and onwards, the only allowed values are 0 and -1. Passing the value -1 will reinitialize the global QHash seed to a random value, while the value of 0 is used to request a stable algorithm for C++ primitive types types (like `int`) and string types (QString, QByteArray).

The seed is set in any newly created QHash. See qHash about how this seed is being used by QHash.

If the environment variable `QT_HASH_SEED` is set, calling this function will result in a no-op.

This function was introduced in Qt 5.6.

See also qGlobalQHashSeed.

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

Writes the hash hash to stream out.

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

See also Serializing Qt Data Types.

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

Reads a hash from stream in into hash.

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

See also Serializing Qt Data Types.