Qt5 Tutorial QHash - 2020

In this tutorial, we will learn about QHash.

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

QHash provides very similar functionality to QMap. The differences are:

1. QHash provides faster lookups than QMap.
2. When iterating over a QMap, the items are always sorted by key. With QHash, the items are arbitrarily ordered.
3. The key type of a QMap must provide operator<(). The key type of a QHash must provide operator==() and a global hash function called qHash().

#include <QCoreApplication>
#include <QHash>
#include <QDebug>

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

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

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

    QHashIterator<int, QString> iter(Hash);

    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 hash, we can use insert():

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

or use operator[]():

Hash[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 QHash, we can use an iterator. QHash provides both Java-style iterators (QHashIterator and QMutableHashIterator) and STL-style iterators (QHash::const_iterator and QHash::iterator). The example above shows how to iterate over a QHash<QString, int> using a Java-style iterator:

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

Qt 5 Tutorial

1. Hello World
2. Signals and Slots
3. Q_OBJECT Macro
4. MainWindow and Action
5. MainWindow and ImageViewer using Designer A
6. MainWindow and ImageViewer using Designer B
7. Layouts
8. Layouts without Designer
9. Grid Layouts
10. Splitter
11. QDir
12. QFile (Basic)
13. Resource Files (.qrc)
14. QComboBox
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16. QTreeWidget
17. QAction and Icon Resources
18. QStatusBar
19. QMessageBox
20. QTimer
21. QList
22. QListIterator
23. QMutableListIterator
24. QLinkedList
25. QMap
26. QHash
27. QStringList
28. QTextStream
29. QMimeType and QMimeDatabase
30. QFile (Serialization I)
31. QFile (Serialization II - Class)
32. Tool Tips in HTML Style and with Resource Images
33. QPainter
34. QBrush and QRect
35. QPainterPath and QPolygon
36. QPen and Cap Style
37. QBrush and QGradient
38. QPainter and Transformations
39. QGraphicsView and QGraphicsScene
40. Customizing Items by inheriting QGraphicsItem
41. QGraphicsView Animation
42. FFmpeg Converter using QProcess
43. QProgress Dialog - Modal and Modeless
44. QVariant and QMetaType
45. QtXML - Writing to a file
46. QtXML - QtXML DOM Reading
47. QThreads - Introduction
48. QThreads - Creating Threads
49. Creating QThreads using QtConcurrent
50. QThreads - Priority
51. QThreads - QMutex
52. QThreads - GuiThread
53. QtConcurrent QProgressDialog with QFutureWatcher
54. QSemaphores - Producer and Consumer
55. QThreads - wait()
56. MVC - ModelView with QListView and QStringListModel
57. MVC - ModelView with QTreeView and QDirModel
58. MVC - ModelView with QTreeView and QFileSystemModel
59. MVC - ModelView with QTableView and QItemDelegate
60. QHttp - Downloading Files
61. QNetworkAccessManager and QNetworkRequest - Downloading Files
62. Qt's Network Download Example - Reconstructed
63. QNetworkAccessManager - Downloading Files with UI and QProgressDialog
64. QUdpSocket
65. QTcpSocket
66. QTcpSocket with Signals and Slots
67. QTcpServer - Client and Server
68. QTcpServer - Loopback Dialog
69. QTcpServer - Client and Server using MultiThreading
70. QTcpServer - Client and Server using QThreadPool
71. Asynchronous QTcpServer - Client and Server using QThreadPool
72. Qt Quick2 QML Animation - A
73. Qt Quick2 QML Animation - B
74. Short note on Ubuntu Install
75. OpenGL with QT5
76. Qt5 Webkit : Web Browser with QtCreator using QWebView Part A
77. Qt5 Webkit : Web Browser with QtCreator using QWebView Part B
78. Video Player with HTML5 QWebView and FFmpeg Converter
79. Qt5 Add-in and Visual Studio 2012
80. Qt5.3 Installation on Ubuntu 14.04
81. Qt5.5 Installation on Ubuntu 14.04
82. Short note on deploying to Windows