Qt/C++ - Tutorial 085. Work with QJsonObject, QJsonArray, QJsonDocument. Save and load JSON from files

Qt5, QJsonArray, QJsonObject, QJsonDocument, JSON, Qt

Consider a small example of the formation of a JSON document from, for example, the text and the title of this text.

For example, the text has:

• Title - First Title
• Content - First Content

And so on

We will add this text to QJsonObject, which we will add to the QJsonArray text array. The array of objects will be in the general working QJsonObject, which we will save to a file.

We will do all this through a graphical interface in which we have:

• QLineEdit - titleLineEdit - contains the title of the text to add
• QTextEdit - contentTextEdit - contains the contents of the text to add
• QTextEdit - jsonDocumentTextEdit - JSON document preview
• QPushButton - addButton - button to add new text in JSON
• QPushButton - clearButton - button to remove all texts from the current QJsonObject
• QPushButton - saveButton - button to save JSON document to file
• QPushButton - loadButton - button to read JSON from file

In this case, we will be able to read the JSON file and add additional texts to the read file.

The application will look like this.

Implementation

widget.h

#ifndef WIDGET_H
#define WIDGET_H
 
#include <QWidget>
#include <QJsonObject>
 
namespace Ui {
class Widget;
}
 
class Widget : public QWidget
{
    Q_OBJECT
 
public:
    explicit Widget(QWidget *parent = nullptr);
    ~Widget();
 
private:
    // Slot of adding new text to the array of texts
    void onAddButtonClicked();
    // Слот удаления всей информации о текстах в текущем объекте
    void onClearButtonClicked();
    // Slot save text in json file
    void onSaveButtonClicked();
    // Slot of loading texts into the program from json file
    void onLoadButtonClicked();
 
    Ui::Widget *ui;
 
    // The current json object you are working with
    QJsonObject m_currentJsonObject;
};
 
#endif // WIDGET_H

widget.cpp

#include "widget.h"
#include "ui_widget.h"
 
#include <QFileDialog>
#include <QFile>
#include <QDir>
#include <QJsonObject>
#include <QJsonArray>
#include <QJsonDocument>
 
Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);
 
    // Connecting handler slots to button click signals
    connect(ui->addButton, &QPushButton::clicked, this, &Widget::onAddButtonClicked);
    connect(ui->clearButton, &QPushButton::clicked, this, &Widget::onClearButtonClicked);
    connect(ui->saveButton, &QPushButton::clicked, this, &Widget::onSaveButtonClicked);
    connect(ui->loadButton, &QPushButton::clicked, this, &Widget::onLoadButtonClicked);
}
 
Widget::~Widget()
{
    delete ui;
}
 
void Widget::onAddButtonClicked()
{
    // Create a text object
    QJsonObject textObject;
    textObject["title"] = ui->titleLineEdit->text();                // Set the text header
    textObject["content"] = ui->contentTextEdit->toPlainText();     // Set the content of the text
    QJsonArray textsArray = m_currentJsonObject["texts"].toArray(); // We take the current array of texts, even if it does not exist, it will be created automatically
    textsArray.append(textObject);                                  // Add a text object to the array
    m_currentJsonObject["texts"] = textsArray;                      // Save the array back to the current object.
 
    // Set the text of the entire Json object in the text field for verification
    ui->jsonDocumentTextEdit->setText(QJsonDocument(m_currentJsonObject).toJson(QJsonDocument::Indented));
}
 
void Widget::onClearButtonClicked()
{
    m_currentJsonObject = QJsonObject();    // Re-create the new current QJsonObject
    ui->jsonDocumentTextEdit->clear();      // Clear the text field
    // Set the text of the entire Json object in the text field to see that it is an empty object.
    // See the following -> {}
    ui->jsonDocumentTextEdit->setText(QJsonDocument(m_currentJsonObject).toJson(QJsonDocument::Indented));
}
 
void Widget::onSaveButtonClicked()
{
    // Using the dialog box we get the file name with the absolute path.
    QString saveFileName = QFileDialog::getSaveFileName(this,
                                                        tr("Save Json File"),
                                                        QString(),
                                                        tr("JSON (*.json)"));
    QFileInfo fileInfo(saveFileName);   // Using QFileInfo
    QDir::setCurrent(fileInfo.path());  // set the current working directory where the file will be, otherwise it may not work
    // Create a file object and open it for writing.
    QFile jsonFile(saveFileName);
    if (!jsonFile.open(QIODevice::WriteOnly))
    {
        return;
    }
 
    // Write the current Json object to a file.
    jsonFile.write(QJsonDocument(m_currentJsonObject).toJson(QJsonDocument::Indented));
    jsonFile.close();   // Close file
}
 
void Widget::onLoadButtonClicked()
{
    // Choose a file
    QString openFileName = QFileDialog::getOpenFileName(this,
                                                        tr("Open Json File"),
                                                        QString(),
                                                        tr("JSON (*.json)"));
    QFileInfo fileInfo(openFileName);   // Using QFileInfo
    QDir::setCurrent(fileInfo.path());  // set the current working directory where the file will be
    // Create a file object and open it for reading.
    QFile jsonFile(openFileName);
    if (!jsonFile.open(QIODevice::ReadOnly))
    {
        return;
    }
 
    // Read the entire file
    QByteArray saveData = jsonFile.readAll();
    // Create QJsonDocument
    QJsonDocument jsonDocument(QJsonDocument::fromJson(saveData));
    // From which we select an object into the current working QJsonObject
    m_currentJsonObject = jsonDocument.object();
    // Clear the text field
    ui->jsonDocumentTextEdit->clear();
    // And set the contents of the Json object in the verification text field.
    ui->jsonDocumentTextEdit->setText(QJsonDocument(m_currentJsonObject).toJson(QJsonDocument::Indented));
}

Conclusion

In this particular case, QFileDialog is used to get the path to the recordable file and the file being opened. Of course, from manipulating an existing file, only adding new texts is available and you can open any correct JSON document and then add text to it. But for example, I believe that more is not needed.

Example on GitHub repository of EVILEG