Qt's foreach Keyword | Qt Core 6.4.1

- Qt 6.4
- Qt Core
- Qt's foreach Keyword
- Qt 6.4.1 Reference Documentation

# Qt's foreach Keyword

## The foreach Keyword

Note: 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) statement. For example, here's how to use `foreach` to iterate over a 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;
     }

Unless the data type contains a comma (e.g., `QPair<int, int>`), the variable used for iteration can be defined within the `foreach` statement:

     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;
     }

With 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);

For a multi-valued map:

     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 implicit sharing 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` loop is the range-based `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, whereas `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.