Qt's 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.
foreach keyword is a Qt-specific addition to the C++ language, and is implemented using the preprocessor.
foreach code is significantly shorter than the equivalent code that uses iterators:
QList<QString> values; ... QListIterator<QString> i(values); while (i.hasNext()) qDebug() << i.next();
Unless the data type contains a comma (e.g.,
QPair<int, int>), the variable used for iteration can be defined within the
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:
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:
For a multi-valued map:
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.
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