Qt5 Tutorial QMap - 2020

In this tutorial, we will learn about QMap.

The QMap class is a template class that provides a red-black-tree-based dictionary.

QMap<Key, T> is one of Qt's generic container classes. It stores (key, value) pairs and provides fast lookup of the value associated with a key.

#include <QCoreApplication>
#include <QMap>
#include <QDebug>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    
    QMap<int, QString> Map;

    Map.insert(1,"A");
    Map.insert(2,"B");
    Map[3] = "C";

    foreach(int i, Map.keys()) qDebug() << Map[i];

    QMapIterator<int, QString> iter(Map);

    while(iter.hasNext())
    {
        iter.next();
        qDebug() << iter.key() << " : " << iter.value();
    }

    return a.exec();
}

Output should look like this:

"A"
"B"
"C"
1  :  "A"
2  :  "B"
3  :  "C"

To insert a (key, value) pair into the map, we can use insert():

Map.insert(1,"A");
Map.insert(2,"B");

or use operator[]():

Map[3] = "C";

We can store multiple values per key by using insertMulti() instead of insert().

If we want to navigate through all the (key, value) pairs stored in a QMap, we can use an iterator. QMap provides both Java-style iterators (QMapIterator and QMutableMapIterator) and STL-style iterators (QMap::const_iterator and QMap::iterator). The example above shows how to iterate over a QMap<QString, int> using a Java-style iterator:

QMapIterator<int, QString> iter(Map);
while(iter.hasNext())
{
    iter.next();
    qDebug() << iter.key() << " : " << iter.value();
}