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iterator 类

class QMultiHash ::iterator

The QMultiHash::iterator 类提供 STL 样式非常量迭代器为 QMultiHash . 更多...

公共函数

iterator ()
const Key & key () const
T & value () const
bool operator!= (const QMultiHash<Key, T>::iterator & other ) const
bool operator!= (const QMultiHash<Key, T>::const_iterator & other ) const
T & operator* () const
QMultiHash<Key, T>::iterator & operator++ ()
QMultiHash<Key, T>::iterator operator++ (int)
T * operator-> () const
bool operator== (const QMultiHash<Key, T>::iterator & other ) const
bool operator== (const QMultiHash<Key, T>::const_iterator & other ) const

详细描述

QMultiHash <Key, T>::iterator allows you to iterate over a QMultiHash and to modify the value (but not the key) associated with a particular key. If you want to iterate over a const QMultiHash , you should use QMultiHash::const_iterator . It is generally good practice to use QMultiHash::const_iterator on a non-const QMultiHash as well, unless you need to change the QMultiHash through the iterator. Const iterators are slightly faster, and can improve code readability.

默认 QMultiHash::iterator constructor creates an uninitialized iterator. You must initialize it using a QMultiHash function like QMultiHash::begin (), QMultiHash::end ()，或 QMultiHash::find () before you can start iterating. Here's a typical loop that prints all the (key, value) pairs stored in a hash: 

QHash<QString, int> hash;
hash.insert("January", 1);
hash.insert("February", 2);
...
hash.insert("December", 12);
for (auto i = hash.cbegin(), end = hash.cend(); i != end; ++i)
    cout << qPrintable(key()) << ": " << i.value() << endl;

不像 QMap , which orders its items by key, QMultiHash stores its items in an arbitrary order.

Here's an example that increments every value stored in the QMultiHash by 2: 

for (auto i = hash.begin(), end = hash.end(); i != end; ++i)
    i.value() += 2;

To remove elements from a QMultiHash you can use erase_if ( QMultiHash <Key, T> &map, Predicate pred): 

erase_if(hash, [](const QHash<QString, int>::iterator it) { return it.value() > 10; });

Multiple iterators can be used on the same hash. However, be aware that any modification performed directly on the QHash (inserting and removing items) can cause the iterators to become invalid.

Inserting items into the hash or calling methods such as QHash::reserve () 或 QHash::squeeze () can invalidate all iterators pointing into the hash. Iterators are guaranteed to stay valid only as long as the QHash doesn't have to grow/shrink its internal hash table. Using any iterator after a rehashing operation has occurred will lead to undefined behavior.

If you need to keep iterators over a long period of time, we recommend that you use QMultiMap 而不是 QHash .

警告： Iterators on implicitly shared containers do not work exactly like STL-iterators. You should avoid copying a container while iterators are active on that container. For more information, read 隐式共享迭代器问题 .

另请参阅 QMultiHash::const_iterator , QMultiHash::key_iterator ，和 QMultiHash::key_value_iterator .

成员函数文档编制

[noexcept] bool iterator:: operator== (const QMultiHash < Key , T > ::const_iterator & other ) const

[noexcept] bool iterator:: operator== (const QMultiHash < Key , T > ::iterator & other ) const

返回 true if other points to the same item as this iterator; otherwise returns false .

另请参阅 operator!= ().

[noexcept] bool iterator:: operator!= (const QMultiHash < Key , T > ::const_iterator & other ) const

[noexcept] bool iterator:: operator!= (const QMultiHash < Key , T > ::iterator & other ) const

返回 true if other 指向与此迭代器不同的项；否则返回 false .

另请参阅 operator== ().

[constexpr noexcept] iterator:: iterator ()

Constructs an uninitialized iterator.

Functions like key (), value (), and operator++() must not be called on an uninitialized iterator. Use operator=() to assign a value to it before using it.

另请参阅 QMultiHash::begin () 和 QMultiHash::end ().

[noexcept] const Key &iterator:: key () const

Returns the current item's key as a const reference.

There is no direct way of changing an item's key through an iterator, although it can be done by calling QMultiHash::erase() followed by QMultiHash::insert ().

另请参阅 value ().

[noexcept] T &iterator:: value () const

Returns a modifiable reference to the current item's value.

You can change the value of an item by using value() on the left side of an assignment, for example: 

if (i.key() == "Hello")
    i.value() = "Bonjour";

另请参阅 key () 和 operator* ().

[noexcept] T &iterator:: operator* () const

Returns a modifiable reference to the current item's value.

如同 value ().

另请参阅 key ().

[noexcept] QMultiHash < Key , T > ::iterator &iterator:: operator++ ()

The prefix ++ operator ( ++i ) advances the iterator to the next item in the hash and returns an iterator to the new current item.

Calling this function on QMultiHash::end () leads to undefined results.

[noexcept] QMultiHash < Key , T > ::iterator iterator:: operator++ ( int )

这是重载函数。

The postfix ++ operator ( i++ ) advances the iterator to the next item in the hash and returns an iterator to the previously current item.

[noexcept] T *iterator:: operator-> () const

Returns a pointer to the current item's value.

另请参阅 value ().

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