iterator

Qt 5.9.0 Reference Documentation

iterator Class

(QMap::iterator)

The QMap::iterator class provides an STL-style non-const iterator for QMap and QMultiMap. 更多...

头文件:	#include <iterator>
qmake:	QT += core

所有的成员列表, 包括继承的成员

公有类型

typedef iterator_category

公有函数

	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

详细描述

The QMap::iterator 为 QMap 和 QMultiMap提供 STL 风格的非常量迭代器.

QMap 同时提供 STL 风格迭代器和 Java 风格迭代器. STL 风格迭代器更底层, 使用更笨拙; 同时也稍快一些. 对于已经了解 STL 的开发者来说更加熟悉.

QMap<Key, T>::iterator 用来遍历 QMap (或 QMultiMap), 并修改与特定键相关联的值(不能修改键). 如果你想遍历常量 QMap, 你应该使用 QMap::const_iterator. 对于非常量 QMap, 使用 QMap::const_iterator 通常是好的编程实践, 除非需要在遍历时改变 QMap. 常量类型的迭代器稍快一些并可以提高代码可读性.

QMap::iterator 的默认构造函数创建一个未初始化的迭代器. 你必须在开始遍历前使用 QMap::begin(), QMap::end(), 或 QMap::find() 等QMap函数初始化它. 下面是一个典型的循环, 该循环打印出 map 中的所有键值对:


  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() << endl;

与无序存储元素的 QHash 不同, QMap 通过键的大小有序存储元素. 相同键的元素, 将按照从最新到最早插入的顺序连续出现.

让我们通过几个例子来了解哪些情况下可以用 QMap::iterator, 而不能用 QMap::const_iterator. 下面的例子将存储在 QMap 中的每个值增加2:


  QMap<QString, int>::iterator i;
  for (i = map.begin(); i != map.end(); ++i)
      i.value() += 2;

下面的例子移除所有键字符串的首字符是下划线的元素:


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

调用 QMap::erase() 从 map 中移除迭代器所指元素, 返回指向下一个元素的迭代器. 下面是另一个在遍历时移除元素的方法:


  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);
  }

有人会写出像下面这样的代码:


  // WRONG
  while (i != map.end()) {
      if (i.key().startsWith('_'))
          map.erase(i);
      ++i;
  }

然而, 这会导致程序在 ++i 处崩溃, 因为调用完 erase()后, i 成为悬空迭代器.

同一 map 可以使用多个迭代器. 如果向 map 添加元素, 已有的迭代器仍然有效. 如果从 map 移除元素, 指向被移除元素的迭代器会变成悬空迭代器.

警告: 隐式共享容器迭代器的工作方式和 STL 迭代器不完全相同. 当容器的迭代器还处于活动状态时, 应该避免拷贝容器. 详见隐式共享迭代器问题.

参见 QMap::const_iterator, QMap::key_iterator, QMutableMapIterator.

成员类型

typedef iterator::iterator_category

A synonym for std::bidirectional_iterator_tag indicating this iterator is a bidirectional iterator.

成员函数

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.

参见 QMap::begin() and 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() or QMap::insertMulti().

参见 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";

参见 key() and operator*().

bool iterator::operator!=(const iterator &other) const

Returns true if other points to a different item than this iterator; otherwise returns false.

参见 operator==().

bool iterator::operator!=(const const_iterator &other) const

Returns true if other points to a different item than this iterator; otherwise returns false.

参见 operator==().

T &iterator::operator*() const

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

Same as value().

参见 key().

iterator iterator::operator+(int j) const

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 values.

参见 operator-().

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.

参见 operator--().

iterator iterator::operator++(int)

This is an overloaded function.

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+=(int j)

Advances the iterator by j items. (If j is negative, the iterator goes backward.)

参见 operator-=() and operator+().

iterator iterator::operator-(int j) const

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 values.

参见 operator+().

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.

参见 operator++().

iterator iterator::operator--(int)