Explanation of QList in Qt with C++

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QList is one of the most important data storing structure in Qt Framework. QList helps to manage and store data in efficient way as a data sequence with a dynamic array template class in it.
Qlist is coming with Qt Core modules and QList store data like std::vector and std::list containers in C++ but QList are optimised for Qt applications and offers more additional features than standard lists in C++.

• Dynamic Size: QList automatically update the size of container when new values are added or removed.
• Iterators: Iterating a QList is very easy and efficient with Qt built-in mechanism.
• Pre-Allocation of memory: QList performs insertion and removal very fast thanks to memory preallocation of QList.
• Implicitly Shared: QList works as copy-on-write when copied. When it is copied initially, it is not being copied, after some element is modified then it is copied.

To use QList we need to import QList library and we should initialise the list.

// import the library
#include <qlist>

int main() {
    // initialise the QLists with template classes, so we can store in these data types
    QList<double> numbers;
    QList<qstring> colors;

    return 0;
}
</qstring></double></qlist>

Now lets see how to organise information within QLists.

• We can add new data with insert, append, prepend methods and `<<` operator.
• We can remove data with the methods which are starting with remove... and take...
• We can clear all datas of QList with clear, removeAll.
• We can access the data with at, value and `[]` operator.

Below lets make an example with above methods. For more details please check official documentation of Qt.

#include <qlist>

int main() {
    QList<qstring> colors;
    colors.append("red"); // Add value to end
    colors.prepend("blue"); // Add value to beginning
    colors.insert("green", 1); // Inserts value to index 1 and slide the rest
    colors &lt;&lt; "yellow"; // Add value to end
    
    colors.removeAt(0); // Removes the element at specific index
    colors.removeOne("yellow"); // Removes the first occurrence of value
    
    QString str = colors.takeAt(0); // Removes element at zero position from list and returns removed element
    
    QString val = colors.at(0); // Gets element at specified index
    QString val2 = colors[0]; // Gets element at specified index
    
    colors.clear(); // Removes all elements of list.

    return 0;
}
</qstring></qlist>

We can iterate a QList in very easy and efficient way. Below you can see the example for this.

#include <qlist>

int main() {
    QList<qstring> colors = {"red", "green", "blue"};
    for (const QString&amp; color : colors) {
        qDebug() &lt;&lt; colors;
    }
    return 0;
}
</qstring></qlist>

We also have some some algorithms like sort, filter, contains, join.... Qt framework provides some algorithms for QList also to use it easier and optimised way. Below you can see some examples for these algorithms.

#include <qlist>

int main() {
    // sort methods sorts the list in ascending order
    QList<qstring> colors = {"red", "green", "blue"};
    colors.sort(); 
    
    // filter returns a QStringList which fits the condition string
    QStringList res;
    res = colors.filter("r"); // red, green
    
    bool exist = colors.contains("red"); // true
    bool exist2 = colors.contains("RED", Qt::CaseInsensitive); //true
    bool exist3 = colors.contains("RED", Qt::CaseSensitive); //false
    
    // combines the elements with separator
    QString colorsStr = colors.join(", "); //red, green, blue

    return 0;
}
</qstring></qlist>

Conclusion

QList is very important container in Qt framework which makes easier to work with data collections. In Qt we can use QList directly instead of std::list to also have the optimisations for Qt Framework. In this article we have seen some basic operations of QList to manage the data collections in Qt.

That is all.

Burak Hamdi TUFAN