Qt foreach 關鍵詞

foreach 關鍵字

注意： The foreach keyword was introduced before the C++11 range-based loops existed. New code should prefer C++11 range-based loops.

The foreach keyword is a Qt-specific addition to the C++ language, and is implemented using the preprocessor.

Its syntax is: foreach ( variable , container ) 語句。例如，這裏是如何使用 foreach 來遍曆 QList < QString >:

QList<QString> values;
...
QString str;
foreach (str, values)
    qDebug() << str;

The foreach code is significantly shorter than the equivalent code that uses iterators:

QList<QString> values;
...
QListIterator<QString> i(values);
while (i.hasNext()) {
    QString s = i.next();
    qDebug() << s;
}

除非數據類型包含逗號 (如 std::pair<int, int> ), the variable used for iteration can be defined within the foreach 語句：

QList<QString> values;
...
foreach (const QString &str, values)
    qDebug() << str;

And like any other C++ loop construct, you can use braces around the body of a foreach loop, and you can use break to leave the loop:

QList<QString> values;
...
foreach (const QString &str, values) {
    if (str.isEmpty())
        break;
    qDebug() << str;
}

采用 QMap and QHash , foreach accesses the value component of the (key, value) pairs automatically, so you should not call values() on the container (it would generate an unnecessary copy, see below). If you want to iterate over both the keys and the values, you can use iterators (which are faster), or you can obtain the keys, and use them to get the values too:

QMap<QString, int> map;
...
foreach (const QString &str, map.keys())
    qDebug() << str << ':' << map.value(str);

對於多值映射：

QMultiMap<QString, int> map;
...
foreach (const QString &str, map.uniqueKeys()) {
    foreach (int i, map.values(str))
        qDebug() << str << ':' << i;
}

Qt automatically takes a copy of the container when it enters a foreach loop. If you modify the container as you are iterating, that won't affect the loop. (If you do not modify the container, the copy still takes place, but thanks to 隱式共享 copying a container is very fast.)

Since foreach creates a copy of the container, using a non-const reference for the variable does not allow you to modify the original container. It only affects the copy, which is probably not what you want.

An alternative to Qt's foreach 循環是基於範圍 for that is part of C++11 and newer. However, keep in mind that the range-based for might force a Qt container to detach ，而 foreach would not. But using foreach always copies the container, which is usually not cheap for STL containers. If in doubt, prefer foreach for Qt containers, and range based for for STL ones.

You can remove the availability of the Qt's foreach 循環通過定義 QT_NO_FOREACH 宏。

創建自定義 Qt 類型 Qt Java 風格迭代器

內容

foreach 關鍵字