内容

• 公共类型
• 公共函数
• 相关非成员
• 详细描述

iterator 类

class QMap ::iterator

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

• 所有成员列表，包括继承成员
• 弃用成员

公共类型

iterator_category

公共函数

	iterator ()
const Key &	key () const
T &	value () const
T &	operator* () const
iterator &	operator++ ()
iterator	operator++ (int)
iterator &	operator-- ()
iterator	operator-- (int)
T *	operator-> () const

相关非成员

bool	operator!= (const iterator & lhs , const iterator & rhs )
bool	operator== (const iterator & lhs , const iterator & rhs )

详细描述

QMap 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.

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

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

QMap<QString, int> map;
map.insert("January", 1);
map.insert("February", 2);
...
map.insert("December", 12);
QMap<QString, int>::iterator i;
for (i = map.begin(); i != map.end(); ++i)
    cout << i.key() << ": " << i.value() << Qt::endl;
					

不像 QHash , which stores its items in an arbitrary order, QMap stores its items ordered by key.

Let's see a few examples of things we can do with a QMap::iterator that we cannot do with a QMap::const_iterator . Here's an example that increments every value stored in the QMap by 2:

QMap<QString, int>::iterator i;
for (i = map.begin(); i != map.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:

QMap<QString, int>::iterator i = map.begin();
while (i != map.end()) {
    if (i.key().startsWith('_'))
        i = map.erase(i);
    else
        ++i;
}
					

The call to QMap::erase () removes the item pointed to by the iterator from the map, and returns an iterator to the next item. Here's another way of removing an item while iterating:

QMap<QString, int>::iterator i = map.begin();
while (i != map.end()) {
    QMap<QString, int>::iterator prev = i;
    ++i;
    if (prev.key().startsWith('_'))
        map.erase(prev);
}
					

It might be tempting to write code like this:

// WRONG
while (i != map.end()) {
    if (i.key().startsWith('_'))
        map.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 map. If you add items to the map, existing iterators will remain valid. If you remove items from the map, iterators that point to the removed items will become dangling iterators.

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 QMap::const_iterator , QMap::key_iterator ，和 QMutableMapIterator .

成员类型文档编制

[alias] iterator:: iterator_category

同义词 std::bidirectional_iterator_tag indicating this iterator is a bidirectional iterator.

成员函数文档编制

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 QMap::begin () 和 QMap::end ().

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 QMap::erase () followed by QMap::insert ().

另请参阅 value ().

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* ().

T &iterator:: operator* () const

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

如同 value ().

另请参阅 key ().

iterator &iterator:: operator++ ()

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

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

另请参阅 operator-- ().

iterator iterator:: operator++ ( int )

这是重载函数。

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

iterator &iterator:: operator-- ()

The prefix -- operator ( --i ) makes the preceding item current and returns an iterator pointing to the new current item.

Calling this function on QMap::begin () leads to undefined results.

另请参阅 operator++ ().

iterator iterator:: operator-- ( int )

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.

T *iterator:: operator-> () const

Returns a pointer to the current item's value.

另请参阅 value ().

相关非成员

bool operator!= (const iterator & lhs , const iterator & rhs )

返回 true if lhs points to a different item than the rhs iterator; otherwise returns false .

另请参阅 operator== ().

bool operator== (const iterator & lhs , const iterator & rhs )

返回 true if lhs points to the same item as the rhs iterator; otherwise returns false .

另请参阅 operator!= ().