The QHash::key_iterator 类提供 STL 样式 const 迭代器为 QHash and QMultiHash keys. 更多...
| 头： | #include <key_iterator> |
| qmake: | QT += core |
| Since: | Qt 5.6 |
| key_iterator () | |
| key_iterator (const_iterator o ) | |
| const_iterator | base () const |
| bool | operator!= (key_iterator other ) const |
| const Key & | operator* () const |
| key_iterator & | operator++ () |
| key_iterator | operator++ ( int ) |
| key_iterator & | operator-- () |
| key_iterator | operator-- ( int ) |
| const Key * | operator-> () const |
| bool | operator== (key_iterator other ) const |
The QHash::key_iterator 类提供 STL 样式 const 迭代器为 QHash and QMultiHash keys.
QHash::key_iterator is essentially the same as QHash::const_iterator with the difference that operator*() and operator->() return a key instead of a value.
For most uses QHash::iterator and QHash::const_iterator should be used, you can easily access the key by calling QHash::iterator::key ():
for (QHash<int, QString>::const_iterator it = hash.cbegin(), end = hash.cend(); it != end; ++it) { cout << "The key: " << it.key() << endl cout << "The value: " << it.value() << endl; cout << "Also the value: " << (*it) << endl; }
However, to have interoperability between QHash 's keys and STL-style algorithms we need an iterator that dereferences to a key instead of a value. With QHash::key_iterator we can apply an algorithm to a range of keys without having to call QHash::keys (), which is inefficient as it costs one QHash iteration and memory allocation to create a temporary QList .
// Inefficient, keys() is expensive QList<int> keys = hash.keys(); int numPrimes = std::count_if(keys.cbegin(), keys.cend(), isPrimeNumber); qDeleteAll(hash2.keys()); // Efficient, no memory allocation needed int numPrimes = std::count_if(hash.keyBegin(), hash.keyEnd(), isPrimeNumber); qDeleteAll(hash2.keyBegin(), hash2.keyEnd());
QHash::key_iterator is const, it's not possible to modify the key.
默认 QHash::key_iterator constructor creates an uninitialized iterator. You must initialize it using a QHash function like QHash::keyBegin () 或 QHash::keyEnd ().
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 QHash::const_iterator and QHash::iterator .
Default constructs an instance of key_iterator.
Default constructs an instance of key_iterator.
Returns the underlying const_iterator this key_iterator is based on.
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== ().
Returns the current item's key.
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 QHash::keyEnd () leads to undefined results.
另请参阅 operator-- ().
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 previous item.
The prefix -- operator (
--i
) makes the preceding item current and returns an iterator pointing to the new current item.
Calling this function on QHash::keyBegin () leads to undefined results.
另请参阅 operator++ ().
这是重载函数。
The postfix -- operator (
i--
) makes the preceding item current and returns an iterator pointing to the previous item.
Returns a pointer to the current item's key.
返回
true
if
other
points to the same item as this iterator; otherwise returns
false
.
另请参阅 operator!= ().
