QHash Class

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

• List of all members, including inherited members

Note: All functions in this class are reentrant.

Public Types

Public Functions

Related Non-Members

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 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:

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:

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).

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. 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

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.

See also begin() and cend().

[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.

See also cbegin() and end().

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().

See also 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.

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.

See also clear() and take().

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().

See also keys() and value().

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().

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);
}

#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

{0, 1, 2}

{1, 2, 0}

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

{0, 1, 2}

{1, 2, 0}

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