# QHash Class

template <typename Key, typename T> class QHashThe QHash class is a template class that provides a hash-table-based dictionary. More...

Header: | #include <QHash> |

CMake: | find_package(Qt6 COMPONENTS Core REQUIRED) target_link_libraries(mytarget PRIVATE Qt6::Core) |

qmake: | QT += core |

**Note:** All functions in this class are reentrant.

## Public Types

class | const_iterator |

class | iterator |

class | key_iterator |

ConstIterator | |

Iterator | |

const_key_value_iterator | |

difference_type | |

key_type | |

key_value_iterator | |

mapped_type | |

size_type |

## Public Functions

QHash(InputIterator begin, InputIterator end) | |

QHash(QHash<Key, T> &&other) | |

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

QHash(std::initializer_list<std::pair<Key, T> > list) | |

QHash() | |

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

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

~QHash() | |

QHash::iterator | begin() |

QHash::const_iterator | begin() const |

qsizetype | capacity() const |

QHash::const_iterator | cbegin() const |

QHash::const_iterator | cend() const |

void | clear() |

QHash::const_iterator | constBegin() const |

QHash::const_iterator | constEnd() const |

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

QHash::const_key_value_iterator | constKeyValueBegin() const |

QHash::const_key_value_iterator | constKeyValueEnd() const |

bool | contains(const Key &key) const |

qsizetype | count(const Key &key) const |

qsizetype | count() const |

QHash::iterator | emplace(const Key &key, Args &&... args) |

QHash::iterator | emplace(Key &&key, Args &&... args) |

bool | empty() const |

QHash::iterator | end() |

QHash::const_iterator | end() const |

QHash::iterator | erase(QHash::const_iterator pos) |

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

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

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

void | insert(const QHash<Key, T> &other) |

bool | isEmpty() const |

Key | key(const T &value, const Key &defaultKey = Key()) const |

QHash::key_iterator | keyBegin() const |

QHash::key_iterator | keyEnd() const |

QHash::key_value_iterator | keyValueBegin() |

QHash::const_key_value_iterator | keyValueBegin() const |

QHash::key_value_iterator | keyValueEnd() |

QHash::const_key_value_iterator | keyValueEnd() const |

QList<Key> | keys() const |

QList<Key> | keys(const T &value) const |

float | load_factor() const |

bool | remove(const Key &key) |

qsizetype | removeIf(Predicate pred) |

void | reserve(qsizetype size) |

qsizetype | size() const |

void | squeeze() |

void | swap(QHash<Key, T> &other) |

T | take(const Key &key) |

T | value(const Key &key, const T &defaultValue = T()) const |

QList<T> | values() const |

QTypeTraits::compare_eq_result<U> | operator!=(const QHash<Key, T> &other) const |

QTypeTraits::compare_eq_result<U> | operator==(const QHash<Key, T> &other) const |

T & | operator[](const Key &key) |

const T | operator[](const Key &key) const |

## Related Non-Members

qsizetype | erase_if(QHash<Key, T> &hash, Predicate pred) |

int | qGlobalQHashSeed() |

size_t | qHash(const QOcspResponse &response, size_t seed) |

size_t | qHash(const QMqttTopicFilter &filter, size_t seed) |

size_t | qHash(const QStringRef &key, size_t seed) |

size_t | qHash(const QUrl &url, size_t seed = 0) |

size_t | qHash(const QSslCertificate &key, size_t seed) |

size_t | qHash(const QMqttTopicName &name, size_t seed) |

size_t | qHash(const QDateTime &key, size_t seed = 0) |

size_t | qHash(QSslEllipticCurve curve, size_t seed) |

size_t | qHash(QDate key, size_t seed = 0) |

size_t | qHash(const QSslError &key, size_t seed) |

size_t | qHash(QTime key, size_t seed = 0) |

size_t | qHash(const std::pair<T1, T2> &key, size_t seed = 0) |

size_t | qHash(char key, size_t seed = 0) |

size_t | qHash(uchar key, size_t seed = 0) |

size_t | qHash(signed char key, size_t seed = 0) |

size_t | qHash(ushort key, size_t seed = 0) |

size_t | qHash(short key, size_t seed = 0) |

size_t | qHash(uint key, size_t seed = 0) |

size_t | qHash(int key, size_t seed = 0) |

size_t | qHash(ulong key, size_t seed = 0) |

size_t | qHash(long key, size_t seed = 0) |

size_t | qHash(quint64 key, size_t seed = 0) |

size_t | qHash(qint64 key, size_t seed = 0) |

size_t | qHash(char8_t key, size_t seed = 0) |

size_t | qHash(char16_t key, size_t seed = 0) |

size_t | qHash(char32_t key, size_t seed = 0) |

size_t | qHash(wchar_t key, size_t seed = 0) |

size_t | qHash(float key, size_t seed = 0) |

size_t | qHash(double key, size_t seed = 0) |

size_t | qHash(long double key, size_t seed = 0) |

size_t | qHash(const QChar key, size_t seed = 0) |

size_t | qHash(const QByteArray &key, size_t seed = 0) |

size_t | qHash(const QByteArrayView &key, size_t seed = 0) |

size_t | qHash(const QBitArray &key, size_t seed = 0) |

size_t | qHash(const QString &key, size_t seed = 0) |

size_t | qHash(QLatin1String key, size_t seed = 0) |

size_t | qHash(const T *key, size_t seed = 0) |

size_t | qHash(std::nullptr_t key, size_t seed = 0) |

size_t | qHash(const QHash<Key, T> &key, size_t seed = 0) |

size_t | qHash(QPoint key, size_t seed = 0) |

size_t | qHash(const QSet<T> &key, size_t seed = 0) |

size_t | qHash(const QVersionNumber &key, size_t seed = 0) |

size_t | qHash(const QTypeRevision &key, size_t seed = 0) |

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

size_t | qHashMulti(size_t seed, const T &... args) |

size_t | qHashMultiCommutative(size_t seed, const T &... args) |

size_t | qHashRange(InputIterator first, InputIterator last, size_t seed = 0) |

size_t | qHashRangeCommutative(InputIterator first, InputIterator last, size_t seed = 0) |

void | qSetGlobalQHashSeed(int newSeed) |

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

QDataStreamIfHasIStreamOperators<Key, T> | 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:

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 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.

QHash will not shrink automatically if items are removed from the table. To minimize the memory used by the hash, call squeeze().

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.

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 need to store multiple entries for the same key in the hash table, use QMultiHash.

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

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 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 hashing function that returns a hash value for an argument of the key's type.

The hashing 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 `hash(e1) == hash(e2)`

must hold as well. However, to obtain good performance, the hashing function should attempt to return different hash values for different keys to the largest extent possible.

A hashing function for a key type `K`

may be provided in two different ways.

The first way is by having an overload of `qHash()`

in `K`

's namespace. The `qHash()`

function must have one of these signatures:

size_t qHash(K key); size_t qHash(const K &key); size_t qHash(K key, size_t seed); size_t qHash(const K &key, size_t 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).

The second way to provide a hashing function is by specializing the `std::hash`

class for the key type `K`

, and providing a suitable function call operator for it:

namespace std { template <> struct hash<K> { // seed is optional size_t operator()(const K &key, size_t seed = 0) const; }; }

The seed argument has the same meaning as for `qHash()`

, and may be left out.

This second way allows to reuse the same hash function between QHash and the C++ Standard Library unordered associative containers. If both a `qHash()`

overload and a `std::hash`

specializations are provided for a type, then the `qHash()`

overload is preferred.

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 hash implementation.

The convenience qHashMulti() function can be used to implement qHash() for a custom type, where one usually wants to produce a hash value from multiple fields:

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 size_t qHash(const Employee &key, size_t seed) { return qHashMulti(seed, key.name(), key.dateOfBirth()); } #endif // EMPLOYEE_H

In the example above, we've relied on Qt's own implementation of qHash() for QString and QDate to give us a hash value for the employee's name and date of birth respectively.

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

### QHash::ConstIterator

Qt-style synonym for QHash::const_iterator.

### QHash::Iterator

Qt-style synonym for QHash::iterator.

`[since 5.10] `

QHash::const_key_value_iterator

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

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.

`[alias] `

QHash::difference_type

Typedef for ptrdiff_t. Provided for STL compatibility.

`[alias] `

QHash::key_type

Typedef for Key. Provided for STL compatibility.

`[since 5.10] `

QHash::key_value_iterator

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

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.

`[alias] `

QHash::mapped_type

Typedef for T. Provided for STL compatibility.

`[alias] `

QHash::size_type

Typedef for int. Provided for STL compatibility.

## Member Function Documentation

### template <typename Args> QHash::iterator QHash::emplace(Key &&*key*, Args &&... *args*)

### template <typename Args> QHash::iterator QHash::emplace(const Key &*key*, Args &&... *args*)

Inserts a new element into the container. This new element is constructed in-place using *args* as the arguments for its construction.

Returns an iterator pointing to the new element.

`[since 5.14] `

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.

`[since 5.2] `

QHash::QHash(QHash<Key, T> &&*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<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=().

`[since 5.1] `

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 was introduced in Qt 5.1.

### QHash::QHash()

Constructs an empty hash.

**See also **clear().

`[since 5.2] `

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

Move-assigns *other* to this QHash instance.

This function was introduced in Qt 5.2.

### QHash<Key, T> &QHash::operator=(const QHash<Key, T> &*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.

### QHash::iterator QHash::begin()

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

**See also **constBegin() and end().

### QHash::const_iterator QHash::begin() const

This is an overloaded function.

### qsizetype 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().

`[since 5.0] `

QHash::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.

`[since 5.0] `

QHash::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.

### void QHash::clear()

Removes all items from the hash and frees up all memory used by it.

**See also **remove().

### QHash::const_iterator QHash::constBegin() const

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

**See also **begin() and constEnd().

### QHash::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().

### QHash::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().

**See also **find().

`[since 5.10] `

QHash::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().

`[since 5.10] `

QHash::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().

### qsizetype QHash::count(const Key &*key*) const

Returns the number of items associated with the *key*.

**See also **contains().

### qsizetype 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`

.

### QHash::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().

### QHash::const_iterator QHash::end() const

This is an overloaded function.

`[since 5.7] `

QHash::iterator QHash::erase(QHash::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.

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().

### QHash::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() and values().

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

This is an overloaded function.

### QHash::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*.

`[since 5.15] `

void QHash::insert(const QHash<Key, T> &*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().

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

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.

`[since 5.6] `

QHash::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().

`[since 5.6] `

QHash::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().

`[since 5.10] `

QHash::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().

`[since 5.10] `

QHash::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().

`[since 5.10] `

QHash::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().

`[since 5.10] `

QHash::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.

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

This function creates a new list, in linear time. The time and memory use that entails can be avoided by iterating from keyBegin() to keyEnd().

### 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.

### float QHash::load_factor() const

Returns the current load factor of the QHash's internal hash table. This is the same as capacity()/size(). The implementation used will aim to keep the load factor between 0.25 and 0.5. This avoids having too many hash table collisions that would degrade performance.

Even with a low load factor, the implementation of the hash table has a very low memory overhead.

This method purely exists for diagnostic purposes and you should rarely need to call it yourself.

**See also **reserve() and squeeze().

### bool QHash::remove(const Key &*key*)

Removes the item that has the *key* from the hash. Returns true if the key exists in the hash and the item has been removed, and false otherwise.

`[since 6.1] `

template <typename Predicate> qsizetype QHash::removeIf(Predicate *pred*)

Removes all elements for which the predicate *pred* returns true from the hash.

The function supports predicates which take either an argument of type `QHash<Key, T>::iterator`

, or an argument of type `std::pair<const Key &, T &>`

.

Returns the number of elements removed, if any.

This function was introduced in Qt 6.1.

### void QHash::reserve(qsizetype *size*)

Ensures that the QHash's internal hash table has space to store at least *size* items without having to grow the hash table.

This implies that the hash table will contain at least 2 * *size* buckets to ensure good performance

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 the maximum number of items expected in the hash. QHash will then choose the smallest possible number of buckets that will allow storing *size* items in the table without having to grow the internal hash table. 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 grows to provide good performance without wasting too much memory.

**See also **squeeze() and capacity().

### qsizetype 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<Key, T> &*other*)

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

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

**See also **remove().

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

This is an overloaded function.

Returns the value associated with the *key*.

If the hash contains no item with the *key*, the function returns *defaultValue*, which is a default-constructed value if the parameter has not been specified.

### QList<T> QHash::values() const

Returns a list containing all the values in the hash, in an arbitrary order.

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

This function creates a new list, in linear time. The time and memory use that entails can be avoided by iterating from keyValueBegin() to keyValueEnd().

### template <typename U> QTypeTraits::compare_eq_result<U> 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==().

### template <typename U> QTypeTraits::compare_eq_result<U> 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* 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.

**See also **insert() and value().

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

This is an overloaded function.

Same as value().

## Related Non-Members

`[since 6.1] `

template <typename Key, typename T, typename Predicate> qsizetype erase_if(QHash<Key, T> &*hash*, Predicate *pred*)

Removes all elements for which the predicate *pred* returns true from the hash *hash*.

The function supports predicates which take either an argument of type `QHash<Key, T>::iterator`

, or an argument of type `std::pair<const Key &, T &>`

.

Returns the number of elements removed, if any.

This function was introduced in Qt 6.1.

`[since 5.6] `

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.

`[since 5.0] `

size_t qHash(const QUrl &*url*, size_t *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.

`[since 5.0] `

size_t qHash(const QDateTime &*key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(QDate *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(QTime *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.7] `

template <typename T1, typename T2> size_t qHash(const std::pair<T1, T2> &*key*, size_t *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 5.7.

`[since 5.0] `

size_t qHash(char *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(uchar *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(signed char *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(ushort *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(short *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(uint *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(int *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(ulong *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(long *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(quint64 *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(qint64 *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 6.0] `

size_t qHash(char8_t *key*, size_t *seed* = 0)

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

This function was introduced in Qt 6.0.

`[since 6.0] `

size_t qHash(char16_t *key*, size_t *seed* = 0)

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

This function was introduced in Qt 6.0.

`[since 6.0] `

size_t qHash(char32_t *key*, size_t *seed* = 0)

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

This function was introduced in Qt 6.0.

`[since 6.0] `

size_t qHash(wchar_t *key*, size_t *seed* = 0)

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

This function was introduced in Qt 6.0.

`[since 5.3] `

size_t qHash(float *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.3.

`[since 5.3] `

size_t qHash(double *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.3.

`[since 5.3] `

size_t qHash(long double *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.3.

`[since 5.0] `

size_t qHash(const QChar *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(const QByteArray &*key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 6.0] `

size_t qHash(const QByteArrayView &*key*, size_t *seed* = 0)

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

This function was introduced in Qt 6.0.

`[since 5.0] `

size_t qHash(const QBitArray &*key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(const QString &*key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

size_t qHash(QLatin1String *key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.0.

`[since 5.0] `

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

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

This function was introduced in Qt 5.0.

`[since 6.0] `

size_t qHash(std::nullptr_t *key*, size_t *seed* = 0)

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

This function was introduced in Qt 6.0.

`[since 5.8] `

template <typename Key, typename T> size_t qHash(const QHash<Key, T> &*key*, size_t *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.

`[since 6.0] `

size_t qHash(QPoint *key*, size_t *seed* = 0)

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

This function was introduced in Qt 6.0.

`[since 5.5] `

template <typename T> size_t qHash(const QSet<T> &*key*, size_t *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.

`[since 5.6] `

size_t qHash(const QVersionNumber &*key*, size_t *seed* = 0)

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

This function was introduced in Qt 5.6.

`[since 6.0] `

size_t qHash(const QTypeRevision &*key*, size_t *seed* = 0)

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

This function was introduced in Qt 6.0.

`[since 5.4] `

size_t qHashBits(const void **p*, size_t *len*, size_t *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 size_t qHash(const std::vector<int> &key, size_t 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().

`[since 6.0] `

template <typename T> size_t qHashMulti(size_t *seed*, const T &... *args*)

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

Note that the order of the arguments is significant. If order does not matter, use qHashMultiCommutative() instead. If you are hashing raw memory, use qHashBits(); if you are hashing a range, use qHashRange().

This function is provided as a convenience to implement qHash() for your own custom types. For example, here's how you could implement a qHash() overload for a class `Employee`

:

#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 size_t qHash(const Employee &key, size_t seed) { return qHashMulti(seed, key.name(), key.dateOfBirth()); } #endif // EMPLOYEE_H

This function was introduced in Qt 6.0.

**See also **qHashMultiCommutative and qHashRange.

`[since 6.0] `

template <typename T> size_t qHashMultiCommutative(size_t *seed*, const T &... *args*)

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

The order of the arguments is insignificant. If order does matter, use qHashMulti() instead, as it may produce better quality hashing. If you are hashing raw memory, use qHashBits(); if you are hashing a range, use qHashRange().

This function is provided as a convenience to implement qHash() for your own custom types.

This function was introduced in Qt 6.0.

**See also **qHashMulti and qHashRange.

`[since 5.5] `

template <typename InputIterator> size_t qHashRange(InputIterator *first*, InputIterator *last*, size_t *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 size_t qHash(const std::vector<int> &key, size_t 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().

`[since 5.5] `

template <typename InputIterator> size_t qHashRangeCommutative(InputIterator *first*, InputIterator *last*, size_t *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 size_t qHash(const std::unordered_set<int> &key, size_t 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().

`[since 5.6] `

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> QDataStreamIfHasOStreamOperators<Key, T> 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> QDataStreamIfHasIStreamOperators<Key, T> 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.

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