Qt Memory leaks again, Qt LTS Versions 6.2.4 and 5.15.2

JWSchuetz

I am just starting to develop my first Qt-application (Windows 11 host / C++ ) and I can't believe it.

Building a minimal Widget-application with Qt-Creator 8.01 and running an analysis with the build-in tool "heob", I see about 100 memory leaks!

The same problem using VS 2022 together with CRTDBG.

Is Qt such a mess? Or where is my error?

mrjj

hi
Often such tools do not understand the Qt ownership system.

https://doc.qt.io/qt-6/objecttrees.html

Chris Kawa

It might be a problem with your testing methodology. For example if you use crtdbg and call _CrtDumpMemoryLeaks() at the end of main() you'll see leaks, but that's just because there are global objects allocated that clean up after main exits.
I just created a widgets application in VS2022 and Qt 6.4 with the wizard. Setting the flags for automatic leak reporting with _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);reports no leaks for me.

Can you provide example code that reports leaks for you?

JWSchuetz

Shure Chris, I have tree files, main.cpp, mainwindow.h and mainwindow.cpp.

First "main.cpp":

#include "mainwindow.h"

#include <QApplication>

#define WITH_MEM_LEAK_TESTING
#ifdef WITH_MEM_LEAK_TESTING
    #define _CRTDBG_MAP_ALLOC
    #include <stdlib.h>
    #include <crtdbg.h>
#endif


int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    int ret = a.exec();

#ifdef WITH_MEM_LEAK_TESTING
    bool leak = _CrtDumpMemoryLeaks();
    return leak;
#endif

    return ret;
}

Then "mainwindow.h":

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

and the last file is "mainwindow.cpp":

#include "mainwindow.h"
#include "ui_mainwindow.h"

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

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

JWSchuetz

This post is deleted!

Chris Kawa

@JWSchuetz Yeah, so like I said, main is not the end of an app, so placing _CrtDumpMemoryLeaks(); there is not gonna do what you want. Remove it and instead set automatic leak detection crt flags at the beginning of main, before you create any Qt stuff.

int main(int argc, char *argv[])
{
   _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
   ...

This will do a dump automatically at the crt shutdown, so after everything else cleans up.

JWSchuetz

This post is deleted!

JWSchuetz

@Chris-Kawa First, thanks for your answer.

You are right, during the return-statement of main() some Qt-code is executed, for example the destructor of class MainWindow.

So I did as you suggested: I removed the call to

_CrtDumpMemoryLeaks()

at the end and added the call to

_CrtSetDbgFlag( _CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF );

at the beginning of main().

But still 10 leaked memory blocks are now detected (which is much better than about 100 :-)).

I tried my two Qt LTS versions with the same result. Do you have a suggestion?

In worst case I can hope that the remaining leaks are in some static context and do not increase during the runtime of my program :-)

JWSchuetz

I found out that ignoring "block type identifiers" using only

    _CrtSetDbgFlag( _CRTDBG_LEAK_CHECK_DF );

lead to zero leaked blocks. For me that's good enough. Thanks to all responders.