内容

公共类型
公共函数
相关非成员
详细描述
qHash() 哈希函数
算法复杂性攻击

QHash 类

The QHash 类是提供基于哈希表的字典的模板类。更多...

头：	#include <QHash>
qmake:	QT += core
继承者：	QMultiHash

所有成员列表，包括继承成员

注意： 此类的所有函数可重入 .

公共类型

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

公共函数

	QHash ()
	QHash (std::initializer_list<std::pair<Key, T> > list )
	QHash (const QHash<K, V> & other )
	QHash (QHash<K, V> && other )
	~QHash ()
QHash::iterator	begin ()
QHash::const_iterator	begin () const
int	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
int	count (const Key & key ) const
int	count () const
bool	empty () const
QHash::iterator	end ()
QHash::const_iterator	end () const
QPair<QHash::iterator, QHash::iterator>	equal_range (const Key & key )
QPair<QHash::const_iterator, QHash::const_iterator>	equal_range (const Key & key ) const
QHash::iterator	erase (QHash::const_iterator pos )
QHash::iterator	erase (QHash::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 )
QHash::iterator	insertMulti (const Key & key , const T & value )
bool	isEmpty () const
const Key	key (const T & value ) const
const Key	key (const T & value , const Key & defaultKey ) 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
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 )
QList<Key>	uniqueKeys () const
QHash<K, V> &	unite (const QHash<K, V> & other )
const T	value (const Key & key ) const
const T	value (const Key & key , const T & defaultValue ) const
QList<T>	values () const
QList<T>	values (const Key & key ) const
bool	operator!= (const QHash<K, V> & other ) const
QHash<K, V> &	operator= (const QHash<K, V> & other )
QHash<K, V> &	operator= (QHash<K, V> && other )
bool	operator== (const QHash<K, V> & other ) const
T &	operator[] (const Key & key )
const T	operator[] (const Key & key ) const

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

详细描述

The QHash 类是提供基于哈希表的字典的模板类。

QHash <Key, T> 是一种 Qt 一般容器类。它存储 (键，值) 对，并提供键关联值的非常快速查找。

QHash 提供的功能非常类似于 QMap 。差异：

QHash 提供更快的查找相比 QMap 。(见算法的复杂性了解细节。)
当遍历 QMap ，项始终按键排序。采用 QHash ，项是任意次序。
Key 类型对于 QMap must provide operator<(). The key type of a QHash must provide operator==() and a global hash function called qHash () (见 qHash ).

这里是范例 QHash with QString 键和 int 值：

QHash<QString, int> hash;

要将 (键，值) 对插入哈希，可以使用 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);

要查找值，使用 operator[]() 或 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 默认构造值 .

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

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

要避免此问题，替换 hash[i] with hash.value(i) 在以上代码中。

在内部， 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 风格迭代器 ( QHashIterator and QMutableHashIterator ) 和 STL 样式迭代器 ( QHash::const_iterator and QHash::iterator ). Here's how to iterate over a QHash < QString , int> using a Java-style iterator:

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

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

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

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

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

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

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

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

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

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

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

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

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

QHash 's key and value data types must be 可赋值数据类型。例如，无法存储 QWidget 作为值；取而代之，存储 QWidget *.

qHash() 哈希函数

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 ，那么 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 ， 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 ，和 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 () 实现。

范例：

#ifndef EMPLOYEE_H
#define EMPLOYEE_H
class Employee
{
public:
    Employee() {}
    Employee(const QString &name, const QDate &dateOfBirth);
    ...
private:
    QString myName;
    QDate myDateOfBirth;
};
inline bool operator==(const Employee &e1, const Employee &e2)
{
    return e1.name() == e2.name()
           && e1.dateOfBirth() == e2.dateOfBirth();
}
inline uint qHash(const Employee &key, 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 不必 rely on the fact that qHash () will give the same results (for the same inputs) across different Qt versions.

算法复杂性攻击

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 算法的复杂性 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 () 函数。

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.

另请参阅 QHashIterator , QMutableHashIterator , QMap ，和 QSet .

成员类型文档编制

typedef QHash:: ConstIterator

Qt 样式同义词 QHash::const_iterator .

typedef QHash:: Iterator

Qt 样式同义词 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.

该 typedef 在 Qt 5.10 引入。

另请参阅 QKeyValueIterator .

typedef QHash:: difference_type

typedef 对于 ptrdiff_t。为兼容 STL 提供。

typedef QHash:: key_type

typedef 对于 Key。为兼容 STL 提供。

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.

该 typedef 在 Qt 5.10 引入。

另请参阅 QKeyValueIterator .

typedef QHash:: mapped_type

typedef 对于 T。为兼容 STL 提供。

typedef QHash:: size_type

typedef 对于 int。为兼容 STL 提供。

成员函数文档编制

QHash:: QHash ()

构造空哈希。

另请参阅 clear ().

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.

该函数在 Qt 5.1 引入。

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

构造副本为 other .

This operation occurs in 常量时间，因为 QHash is 隐式共享 . 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 线性时间 .

另请参阅 operator= ().

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

移动构造 QHash 实例，使之指向同一对象如 other 所指向的。

该函数在 Qt 5.2 引入。

QHash:: ~QHash ()

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

QHash::iterator QHash:: begin ()

返回 STL 样式迭代器指向哈希中的首项。

另请参阅 constBegin () 和 end ().

QHash::const_iterator QHash:: begin () const

这是重载函数。

int QHash:: capacity () const

Returns the number of buckets in the QHash 的内部哈希表。

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 ().

另请参阅 reserve () 和 squeeze ().

QHash::const_iterator QHash:: cbegin () const

返回常量 STL 样式迭代器指向哈希中的首项。

该函数在 Qt 5.0 引入。

另请参阅 begin () 和 cend ().

QHash::const_iterator QHash:: cend () const

返回常量 STL 样式迭代器 pointing to the imaginary item after the last item in the hash.

该函数在 Qt 5.0 引入。

另请参阅 cbegin () 和 end ().

void QHash:: clear ()

从哈希移除所有项。

另请参阅 remove ().

QHash::const_iterator QHash:: constBegin () const

返回常量 STL 样式迭代器指向哈希中的首项。

另请参阅 begin () 和 constEnd ().

QHash::const_iterator QHash:: constEnd () const

返回常量 STL 样式迭代器 pointing to the imaginary item after the last item in the hash.

另请参阅 constBegin () 和 end ().

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

返回迭代器指向的项具有 key 在哈希中。

若哈希包含的项不具有 key ，函数返回 constEnd ().

该函数在 Qt 4.1 引入。

另请参阅 find () 和 QMultiHash::constFind ().

QHash::const_key_value_iterator QHash:: constKeyValueBegin () const

返回常量 STL 样式迭代器指向第一哈希条目。

该函数在 Qt 5.10 引入。

另请参阅 keyValueBegin ().

QHash::const_key_value_iterator QHash:: constKeyValueEnd () const

返回常量 STL 样式迭代器 pointing to the imaginary entry after the last entry in the hash.

该函数在 Qt 5.10 引入。

另请参阅 constKeyValueBegin ().

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

返回 true 若哈希包含的项具有 key ；否则返回 false .

另请参阅 count () 和 QMultiHash::contains ().

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

返回项数关联 key .

另请参阅 contains () 和 insertMulti ().

int QHash:: count () const

这是重载函数。

如同 size ().

bool QHash:: empty () const

此函数为兼容 STL (标准模板库) 提供。它相当于 isEmpty (), returning true if the hash is empty; otherwise returns false .

QHash::iterator QHash:: end ()

返回 STL 样式迭代器 pointing to the imaginary item after the last item in the hash.

另请参阅 begin () 和 constEnd ().

QHash::const_iterator QHash:: end () const

这是重载函数。

QPair < QHash::iterator , QHash::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 ().

该函数在 Qt 5.7 引入。

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

这是重载函数。

该函数在 Qt 5.7 引入。

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.

不像 remove () 和 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;
    }
}

该函数在 Qt 5.7 引入。

另请参阅 remove (), take ()，和 find ().

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

这是重载函数。

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

返回迭代器指向的项具有 key 在哈希中。

若哈希包含的项不具有 key ，函数返回 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() << endl;
    ++i;
}

另请参阅 value (), values ()，和 QMultiHash::find ().

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

这是重载函数。

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

插入新项具有 key 和值 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 .

另请参阅 insertMulti ().

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

插入新项具有 key 和值 value .

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

另请参阅 insert () 和 values ().

bool QHash:: isEmpty () const

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

另请参阅 size ().

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

Returns the first key mapped to value .

若哈希包含的项不具有 value , the function returns a default-constructed key .

此函数可能很慢 ( 线性时间 )，因为 QHash 's internal data structure is optimized for fast lookup by key, not by value.

另请参阅 value () 和 keys ().

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

这是重载函数。

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

这是重载函数。

Returns a list containing all the keys associated with value value ，按任意次序。

此函数可能很慢 ( 线性时间 )，因为 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 usually 1 but will be 0 if the key isn't in the hash, or greater than 1 if insertMulti () has been used with the key .

另请参阅 clear (), take ()，和 QMultiHash::remove ().

void QHash:: reserve ( int size )

确保 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.

另请参阅 squeeze () 和 capacity ().

int QHash:: size () const

返回哈希中的项数。

另请参阅 isEmpty () 和 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.

另请参阅 reserve () 和 capacity ().

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

交换哈希 other with this hash. This operation is very fast and never fails.

该函数在 Qt 4.8 引入。

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 默认构造值 . If there are multiple items for key in the hash, only the most recently inserted one is removed.

若不使用返回值， remove () 效率更高。

另请参阅 remove ().

QList < Key > QHash:: uniqueKeys () const

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

该函数在 Qt 4.2 引入。

另请参阅 keys () 和 values ().

QHash < K , V > &QHash:: unite (const QHash < K , V > & other )

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

另请参阅 insertMulti ().

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

返回值关联 key .

若哈希包含的项不具有 key , the function returns a 默认构造值 . If there are multiple items for the key in the hash, the value of the most recently inserted one is returned.

另请参阅 key (), values (), contains ()，和 operator[] ().

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

这是重载函数。

If the hash contains no item with the given key ，函数返回 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 ().

另请参阅 keys () 和 value ().

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

这是重载函数。

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

另请参阅 count () 和 insertMulti ().

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

返回 true if other 不等于此哈希；否则返回 false .

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

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

另请参阅 operator== ().

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

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

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

移动赋值 other 到此 QHash 实例。

该函数在 Qt 5.2 引入。

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

返回 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==() .

另请参阅 operator!= ().

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

返回值关联 key 作为可修改引用。

若哈希包含的项不具有 key , the function inserts a 默认构造值 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.

另请参阅 insert () 和 value ().

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

这是重载函数。

如同 value ().

相关非成员

int qGlobalQHashSeed ()

Returns the current global QHash seed.

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

返回哈希值为 key ，使用 seed 做计算种子。

类型 T1 and T2 must be supported by qHash ().

该函数在 Qt 5.0 引入。

uint qHash (const std::pair < T1 , T2 > & key , uint seed = ...)

返回哈希值为 key ，使用 seed 做计算种子。

类型 T1 and T2 must be supported by qHash ().

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

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

uint qHash (const QSet < T > & key , uint seed = ...)

返回哈希值为 key ，使用 seed 做计算种子。

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.

该函数在 Qt 5.5 引入。

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

返回哈希值为 key ，使用 seed 做计算种子。

该函数在 Qt 5.6 引入。

qHash ( QSslEllipticCurve curve , uint seed )

该函数在 Qt 5.5 引入。

qHash (const QSslCertificate & key , uint seed )

该函数在 Qt 5.4 引入。

qHash (const QSslError & key , uint seed )

该函数在 Qt 5.4 引入。

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

Returns the hash value for the memory block of size len pointed to by p ，使用 seed 做计算种子。

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 () 代替。

It bears repeating that the implementation of qHashBits() - like the qHash () overloads offered by Qt - may change at any time. You 不必 rely on the fact that qHashBits() will give the same results (for the same inputs) across different Qt versions.

该函数在 Qt 5.4 引入。

另请参阅 qHashRange () 和 qHashRangeCommutative ().

uint qHashRange ( InputIterator first , InputIterator last , uint seed = ...)

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 不必 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.

该函数在 Qt 5.5 引入。

另请参阅 qHashBits () 和 qHashRangeCommutative ().

uint qHashRangeCommutative ( InputIterator first , InputIterator last , uint seed = ...)

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 不必 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.

该函数在 Qt 5.5 引入。

另请参阅 qHashBits () 和 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 。见 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.

该函数在 Qt 5.6 引入。

另请参阅 qGlobalQHashSeed .

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

写入哈希 hash 到流 out .

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

另请参阅 序列化 Qt 数据类型 .

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

读取哈希从流 in into hash .

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

另请参阅 序列化 Qt 数据类型 .