Question

I’m facing some memory leaks using QIcon (Qt 4.8.1 with MSVC 2008).
This happens with simple QAction (like menu entries, even those generated automatically by Qt Designer) or container items (like QTreeWidgetItem).
For example:

QTreeWidgetItem *newItem = new QTreeWidgetItem();
newItem->setText(0, "Item");

// This causes a memory leak!
newItem->setIcon(0, QIcon("D:\\Dnl\\QtSandBoxApp\\Resources\\dataset2.png"));

treeWidget->addTopLevelItem(newItem);

After a lot of debugging I discovered that QIcon internally uses a QImage, which seems to be correctly destroyed (the reference counter drops to zero).
The simplest example I could produce is the following:

#include "stdafx.h"
#include <QtGui/QApplication>
#include <crtdbg.h>

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

    QApplication a(argc, argv);
    //QPixmapCache::setCacheLimit(0);

    // MEMORY LEAK!
    // Internally uses QImageReader
    QImage image("D:\\Dnl\\QtSandBoxApp\\Resources\\dataset2.png");

    // NO MEMORY LEAK!
    //QImage image(QSize(16, 16), QImage::Format_ARGB32);
    return 0;
}

Am I missing something?
This is a very annoying problem, because it prevents proper memory leak tracking (i.e. caused by the application, not Qt).

EDIT: Here’s the memory leak report

Detected memory leaks!
Dumping objects ->
{5637} normal block at 0x00A46020, 76 bytes long.
 Data: <d ,g ^      L Bg> 64 D2 2C 67 E8 5E A4 00 00 00 00 00 4C B9 42 67 
{5636} normal block at 0x00A45EE8, 12 bytes long.
 Data: < z   `   z  > 14 7A D3 00 20 60 A4 00 00 7A D3 00 
{5634} normal block at 0x00A46148, 128 bytes long.
 Data: <    6   6   Za  > 01 00 00 00 36 00 00 00 36 00 00 00 5A 61 A4 00 
{5614} normal block at 0x00A45DC8, 76 bytes long.
 Data: <d ,g x      L Bg> 64 D2 2C 67 E8 78 A4 00 00 00 00 00 4C B9 42 67 
{5613} normal block at 0x00A478E8, 12 bytes long.
 Data: <     ]      > A4 96 CA 00 C8 5D A4 00 90 96 CA 00 
{5611} normal block at 0x00A45C48, 128 bytes long.
 Data: <    6   5   Z\  > 01 00 00 00 36 00 00 00 35 00 00 00 5A 5C A4 00 
{5591} normal block at 0x00A477C8, 76 bytes long.
 Data: <d ,g u      L Bg> 64 D2 2C 67 10 75 A4 00 00 00 00 00 4C B9 42 67 
{5590} normal block at 0x00A47510, 12 bytes long.
 Data: <     w      > B8 96 C6 00 C8 77 A4 00 A4 96 C6 00 
{5588} normal block at 0x00A45B88, 128 bytes long.
 Data: <    6   5    [  > 01 00 00 00 36 00 00 00 35 00 00 00 9A 5B A4 00 
{5566} normal block at 0x00A473F0, 76 bytes long.
 Data: <d ,g s      L Bg> 64 D2 2C 67 A8 73 A4 00 00 00 00 00 4C B9 42 67 
{5565} normal block at 0x00A473A8, 12 bytes long.
 Data: <     s      > E0 D7 C3 00 F0 73 A4 00 CC D7 C3 00 
{5563} normal block at 0x00A471E0, 128 bytes long.
 Data: <    6   5    q  > 01 00 00 00 36 00 00 00 35 00 00 00 F2 71 A4 00 
{5543} normal block at 0x00A47008, 76 bytes long.
 Data: <d ,g o      L Bg> 64 D2 2C 67 C0 6F A4 00 00 00 00 00 4C B9 42 67 
{5542} normal block at 0x00A46FC0, 12 bytes long.
 Data: <La   p  8a  > 4C 61 BB 00 08 70 A4 00 38 61 BB 00 
{5540} normal block at 0x00A46DF8, 128 bytes long.
 Data: <    6   6    n  > 01 00 00 00 36 00 00 00 36 00 00 00 0A 6E A4 00 
{5520} normal block at 0x003ED9A8, 76 bytes long.
 Data: <d ,g` >     L Bg> 64 D2 2C 67 60 D9 3E 00 00 00 00 00 4C B9 42 67 
{5519} normal block at 0x003ED960, 12 bytes long.
 Data: <      >     > A4 C6 B4 00 A8 D9 3E 00 90 C6 B4 00 
{5517} normal block at 0x00A46D38, 128 bytes long.
 Data: <    6   5   Jm  > 01 00 00 00 36 00 00 00 35 00 00 00 4A 6D A4 00 
{5496} normal block at 0x003ED8D8, 76 bytes long.
 Data: <d ,g  >     L Bg> 64 D2 2C 67 80 D7 3E 00 00 00 00 00 4C B9 42 67 
{5495} normal block at 0x003ED780, 12 bytes long.
 Data: <<     > (   > 3C CA 00 10 D8 D8 3E 00 28 CA 00 10 
{5491} normal block at 0x00A412A0, 128 bytes long.
 Data: <    6   5       > 01 00 00 00 36 00 00 00 35 00 00 00 B2 12 A4 00 
{5260} normal block at 0x00A48448, 64 bytes long.
 Data: <            Z   > 01 00 00 00 16 00 00 00 13 00 00 00 5A 84 A4 00 
{5233} normal block at 0x00A48380, 56 bytes long.
 Data: <        Ha    Bg> 00 00 CD 00 C0 82 A4 00 48 61 A4 00 04 BA 42 67 
{5231} normal block at 0x00A482C0, 128 bytes long.
 Data: <    6   6       > 01 00 00 00 36 00 00 00 36 00 00 00 D2 82 A4 00 
{4681} normal block at 0x00A48150, 64 bytes long.
 Data: <            b   > 01 00 00 00 16 00 00 00 13 00 00 00 62 81 A4 00 
{4655} normal block at 0x00A47FB8, 56 bytes long.
 Data: <     ~  H\    Bg> 00 00 CA 00 F8 7E A4 00 48 5C A4 00 04 BA 42 67 
{4653} normal block at 0x00A47EF8, 126 bytes long.
 Data: <    5   5       > 01 00 00 00 35 00 00 00 35 00 00 00 0A 7F A4 00 
{4102} normal block at 0x00A47DB8, 64 bytes long.
 Data: <             }  > 01 00 00 00 16 00 00 00 13 00 00 00 CA 7D A4 00 
{4075} normal block at 0x00A47D40, 56 bytes long.
 Data: <     {   [    Bg> 00 00 C6 00 D8 7B A4 00 88 5B A4 00 04 BA 42 67 
{4073} normal block at 0x00A47BD8, 126 bytes long.
 Data: <    5   5    {  > 01 00 00 00 35 00 00 00 35 00 00 00 EA 7B A4 00 
{3522} normal block at 0x00A479F8, 64 bytes long.
 Data: <             z  > 01 00 00 00 16 00 00 00 13 00 00 00 0A 7A A4 00 
{3496} normal block at 0x00A47AA0, 56 bytes long.
 Data: <    8y   q    Bg> 00 00 BD 00 38 79 A4 00 E0 71 A4 00 04 BA 42 67 
{3494} normal block at 0x00A47938, 126 bytes long.
 Data: <    5   5   Jy  > 01 00 00 00 35 00 00 00 35 00 00 00 4A 79 A4 00 
{2943} normal block at 0x00A41220, 64 bytes long.
 Data: <            2   > 01 00 00 00 16 00 00 00 13 00 00 00 32 12 A4 00 
{2917} normal block at 0x00A40418, 56 bytes long.
 Data: <    X    m    Bg> 00 00 B6 00 58 03 A4 00 F8 6D A4 00 04 BA 42 67 
{2915} normal block at 0x00A40358, 128 bytes long.
 Data: <    6   6   j   > 01 00 00 00 36 00 00 00 36 00 00 00 6A 03 A4 00 
{2367} normal block at 0x00A40760, 64 bytes long.
 Data: <            r   > 01 00 00 00 16 00 00 00 13 00 00 00 72 07 A4 00 
{2341} normal block at 0x00A401D0, 56 bytes long.
 Data: <    h   8m    Bg> 00 00 B4 00 68 00 A4 00 38 6D A4 00 04 BA 42 67 
{2339} normal block at 0x00A40068, 126 bytes long.
 Data: <    5   5   z   > 01 00 00 00 35 00 00 00 35 00 00 00 7A 00 A4 00 
{1788} normal block at 0x00A40820, 64 bytes long.
 Data: <            2   > 01 00 00 00 16 00 00 00 13 00 00 00 32 08 A4 00 
{1762} normal block at 0x00A417B0, 56 bytes long.
 Data: <              Bg> 00 00 00 10 B0 18 A4 00 A0 12 A4 00 04 BA 42 67 
{1760} normal block at 0x00A418B0, 126 bytes long.
 Data: <    5   5       > 01 00 00 00 35 00 00 00 35 00 00 00 C2 18 A4 00 
{252} normal block at 0x003E8A08, 40 bytes long.
 Data: <        @B      > 00 00 00 00 CC CD CD CD 40 42 0F 00 00 00 00 00 
{251} normal block at 0x003E8950, 120 bytes long.
 Data: <Lc-g  >     L Bg> 4C 63 2D 67 08 89 3E 00 00 00 00 00 4C B9 42 67 
{250} normal block at 0x003E8908, 8 bytes long.
 Data: < c-gP > > 0C 63 2D 67 50 89 3E 00 
{249} normal block at 0x003E88A0, 40 bytes long.
 Data: <        @B      > 00 00 00 00 CC CD CD CD 40 42 0F 00 00 00 00 00 
{248} normal block at 0x003E8828, 60 bytes long.
 Data: <      >         > 07 00 00 00 08 89 3E 00 80 06 00 00 00 CD CD CD 
Object dump complete.
The program '[2808] QtSandBoxApp.exe: Native' has exited with code 0 (0x0).

Answer 1

I solved the problem.
It seems that using CrtDbg library APIs is not a good idea with Qt.

Quote from http://winfig.com/?p=154:

Qt allocates memory in it’s DLLs and this memory is released when the
DLLs are unloaded. Unfortunately this is after Visual Studio reports
the supposed leaks.

This is the case, infact when I request the QImage from disk, the following modules are loaded at runtime:

'QtSandBoxApp.exe': Loaded 'C:\Libraries\Qt\4.8.1\plugins\imageformats\qgifd4.dll'
'QtSandBoxApp.exe': Loaded 'C:\Libraries\Qt\4.8.1\plugins\imageformats\qicod4.dll'
'QtSandBoxApp.exe': Loaded 'C:\Libraries\Qt\4.8.1\plugins\imageformats\qjpegd4.dll'
'QtSandBoxApp.exe': Loaded 'C:\Libraries\Qt\4.8.1\plugins\imageformats\qmngd4.dll'
'QtSandBoxApp.exe': Loaded 'C:\Libraries\Qt\4.8.1\plugins\imageformats\qsvgd4.dll'
'QtSandBoxApp.exe': Loaded 'C:\Libraries\Qt\4.8.1\bin\QtSvgd4.dll', Symbols loaded.
'QtSandBoxApp.exe': Loaded 'C:\Libraries\Qt\4.8.1\bin\QtXmld4.dll', Symbols loaded.
'QtSandBoxApp.exe': Loaded 'C:\Libraries\Qt\4.8.1\plugins\imageformats\qtgad4.dll'
'QtSandBoxApp.exe': Loaded 'C:\Libraries\Qt\4.8.1\plugins\imageformats\qtiffd4.dll'

Visual Leak Detector correctly reports no memory leaks.

P.S: This wiki entry from QtCentre is misleading 😛