The QHash::iterator 类提供 STL 样式非常量迭代器为 QHash and QMultiHash . 更多...
| 头： | #include <iterator> |
| qmake: | QT += core |
| iterator () | |
| const Key & | key () const |
| T & | value () const |
| bool | operator!= (const iterator & other ) const |
| bool | operator!= (const const_iterator & other ) const |
| T & | operator* () const |
| iterator | operator+ (int j ) const |
| iterator & | operator++ () |
| iterator | operator++ ( int ) |
| iterator & | operator+= (int j ) |
| iterator | operator- (int j ) const |
| iterator & | operator-- () |
| iterator | operator-- ( int ) |
| iterator & | operator-= (int j ) |
| T * | operator-> () const |
| bool | operator== (const iterator & other ) const |
| bool | operator== (const const_iterator & other ) const |
The QHash::iterator 类提供 STL 样式非常量迭代器为 QHash and QMultiHash .
QHash features both STL 样式迭代器 and Java 风格迭代器 . The STL-style iterators are more low-level and more cumbersome to use; on the other hand, they are slightly faster and, for developers who already know STL, have the advantage of familiarity.
QHash <Key, T>::iterator allows you to iterate over a QHash (或 QMultiHash ) and to modify the value (but not the key) associated with a particular key. If you want to iterate over a const QHash , you should use QHash::const_iterator . It is generally good practice to use QHash::const_iterator on a non-const QHash as well, unless you need to change the QHash through the iterator. Const iterators are slightly faster, and can improve code readability.
默认 QHash::iterator constructor creates an uninitialized iterator. You must initialize it using a QHash function like QHash::begin (), QHash::end ()，或 QHash::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); QHash<QString, int>::iterator i; for (i = hash.begin(); i != hash.end(); ++i) cout << i.key() << ": " << i.value() << endl;
不像 QMap , which orders its items by key, QHash stores its items in an arbitrary order. The only guarantee is that items that share the same key (because they were inserted using QHash::insertMulti ()) will appear consecutively, from the most recently to the least recently inserted value.
Let's see a few examples of things we can do with a QHash::iterator that we cannot do with a QHash::const_iterator . Here's an example that increments every value stored in the QHash by 2:
QHash<QString, int>::iterator i; for (i = hash.begin(); i != hash.end(); ++i) i.value() += 2;
Here's an example that removes all the items whose key is a string that starts with an underscore character:
QHash<QString, int>::iterator i = hash.begin(); while (i != hash.end()) { if (i.key().startsWith('_')) i = hash.erase(i); else ++i; }
The call to QHash::erase () removes the item pointed to by the iterator from the hash, and returns an iterator to the next item. Here's another way of removing an item while iterating:
QHash<QString, int>::iterator i = hash.begin(); while (i != hash.end()) { QHash<QString, int>::iterator prev = i; ++i; if (prev.key().startsWith('_')) hash.erase(prev); }
It might be tempting to write code like this:
// WRONG while (i != hash.end()) { if (i.key().startsWith('_')) hash.erase(i); ++i; }
However, this will potentially crash in
++i
，因为
i
is a dangling iterator after the call to
erase
().
Multiple iterators can be used on the same hash. However, be aware that any modification performed directly on the QHash has the potential of dramatically changing the order in which the items are stored in the hash, as they might cause QHash to rehash its internal data structure. There is one notable exception: QHash::erase (). This function can safely be called while iterating, and won't affect the order of items in the hash. If you need to keep iterators over a long period of time, we recommend that you use QMap 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 QHash::const_iterator , QHash::key_iterator ，和 QMutableHashIterator .
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 QHash::begin () 和 QHash::end ().
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 QHash::erase () followed by QHash::insert () 或 QHash::insertMulti ().
另请参阅 value ().
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";
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== ().
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 a modifiable reference to the current item's value.
如同 value ().
另请参阅 key ().
Returns an iterator to the item at j positions forward from this iterator. (If j is negative, the iterator goes backward.)
This operation can be slow for large j 值。
另请参阅 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 QHash::end () 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 previously current item.
Advances the iterator by j items. (If j is negative, the iterator goes backward.)
另请参阅 operator-= () 和 operator+ ().
Returns an iterator to the item at j positions backward from this iterator. (If j is negative, the iterator goes forward.)
This operation can be slow for large j 值。
另请参阅 operator+ ().
The prefix -- operator (
--i
) makes the preceding item current and returns an iterator pointing to the new current item.
Calling this function on QHash::begin () 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 previously current item.
Makes the iterator go back by j items. (If j is negative, the iterator goes forward.)
另请参阅 operator+= () 和 operator- ().
Returns a pointer to the current item's value.
另请参阅 value ().
返回
true
if
other
points to the same item as this iterator; otherwise returns
false
.
另请参阅 operator!= ().
返回
true
if
other
points to the same item as this iterator; otherwise returns
false
.
另请参阅 operator!= ().
