Qt/C++ - Lesson 050. Logging Qt application events to a text file

log, qDebug, Логирование, qInfo, Logger, file, qWarning, qFatal

All developers uses macro qDebug() , when debugging an application written in the Qt, but there are also macros qInfo(), qWarning(), qCritical() and qFatal() (which, as of this writing has errors and did not work).

With these events, you can divide errors in the application on the level of significance and use filters.

To redirect error messages to a text file, you need to install the CallBack-function handler into the application. For this purpose qInstalMessageHandler function.

The signature handler must be as follows:

void messageHandler(QtMsgType type, const QMessageLogContext &context, const QString &msg);

Through it, we will receive the following information:

1. QtMsgType type - Message Type
2. QtInfoMsg
3. QtDebugMsg
4. QtWarningMsg
5. QtCriticalMsg
6. QtFatalMsg
7. QMessageLogContext &context - context of a message, the most useful of it - this is the message category. What is important when working with data output can be used, and additional categories, due to which it is possible to improve the localization of posts in the component of the program, if you used the same number of messages.
8. QString &msg - message transmitted in error logging

Additional categories - QLoggingCategory

Additional categories are objects QLoggingCategory class. There may be several approaches to work with the categories, for example, you can create a category object in any class and pass it to an error conclusion:

QLoggingCategory  m_category("Test");
qDebug(m_category) << "Check it";

Then we obtain the following output:

Test: Check it

Inconvenience approach is that each will generate a class data category and some grades they generally can be the same. Therefore, we use a different approach. Namely registration category by the macro.

LoggingCategories.h

We declare four different categories.

#ifndef LOGGER_H
#define LOGGER_H

#include <QLoggingCategory>

Q_DECLARE_LOGGING_CATEGORY(logDebug)
Q_DECLARE_LOGGING_CATEGORY(logInfo)
Q_DECLARE_LOGGING_CATEGORY(logWarning)
Q_DECLARE_LOGGING_CATEGORY(logCritical)

#endif // LOGGER_H

LoggingCategories.cpp

Assign data names of categories.

#include "LoggingCategories.h"

Q_LOGGING_CATEGORY(logDebug,    "Debug")
Q_LOGGING_CATEGORY(logInfo,     "Info")
Q_LOGGING_CATEGORY(logWarning,  "Warning")
Q_LOGGING_CATEGORY(logCritical, "Critical")

Using LoggingCategories

In order to take advantage of these categories, you must include the header file in the file where we will use these categories. And use them as follows:

qDebug(logDebug()) << "Check it";

Then we obtain the following output:

Debug: Check it

Installing the handler

To demonstrate the handler we will use graphics application, which will has four buttons:

• Debug
• Info
• Warning
• Critical

In the handler of each button will cause a corresponding macros logging of events and to apply the appropriate categories listed above. Window application created by a graphic designer and is as follows.

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private slots:
    void on_debugButton_clicked();
    void on_infoButton_clicked();
    void on_warningButton_clicked();
    void on_criticalButton_clicked();

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDebug>

#include "LoggingCategories.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_debugButton_clicked()
{
    qDebug(logDebug()) << "Debug Button";
}

void MainWindow::on_infoButton_clicked()
{
    qInfo(logInfo()) << "Info Button";
}

void MainWindow::on_warningButton_clicked()
{
    qWarning(logWarning()) << "Warning Button";
}

void MainWindow::on_criticalButton_clicked()
{
    qCritical(logCritical()) << "Critical Button";
}

main.cpp

#include "mainwindow.h"
#include <QApplication>
#include <QFile>
#include <QDir>
#include <QScopedPointer>
#include <QTextStream>
#include <QDateTime>
#include <QLoggingCategory>

// Smart pointer to log file
QScopedPointer<QFile>   m_logFile;

// 
void messageHandler(QtMsgType type, const QMessageLogContext &context, const QString &msg);

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    // Set the logging file
    // check which a path to file you use 
    m_logFile.reset(new QFile("C:/example/logFile.txt"));
    // Open the file logging
    m_logFile.data()->open(QFile::Append | QFile::Text);
    // Set handler
    qInstallMessageHandler(messageHandler);

    MainWindow w;
    w.show();

    return a.exec();
}

// The implementation of the handler
void messageHandler(QtMsgType type, const QMessageLogContext &context, const QString &msg)
{
    // Open stream file writes
    QTextStream out(m_logFile.data());
    // Write the date of recording
    out << QDateTime::currentDateTime().toString("yyyy-MM-dd hh:mm:ss.zzz ");
    // By type determine to what level belongs message
    switch (type)
    {
    case QtInfoMsg:     out << "INF "; break;
    case QtDebugMsg:    out << "DBG "; break;
    case QtWarningMsg:  out << "WRN "; break;
    case QtCriticalMsg: out << "CRT "; break;
    case QtFatalMsg:    out << "FTL "; break;
    }
    // Write to the output category of the message and the message itself
    out << context.category << ": "
        << msg << endl;
    out.flush();    // Clear the buffered data
}

Result

As a result, the file of logging events will created in the folliwnf path C:\example\logFile.txt.

By pressing the buttons in the application window, the contents of the file will look like this:

2016-06-04 17:05:57.439 DBG Debug: Debug Button
2016-06-04 17:05:57.903 INF Info: Info Button
2016-06-04 17:05:58.367 WRN Warning: Warning Button
2016-06-04 17:05:58.815 CRT Critical: Critical Button

Download Qt application with logging into text file

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