Qt debugger does different than simple running - c++

Question

void MainWindow::edit()
{
//Check if item is selected, if not return
const int row = list->currentRow();
if( row == -1 )
return;
EditWindow w( this, currentCategory() );
switch( currentCategory() )
{
case cApp:
{
App old = appList.at( row );
w.setApp( old );
if( w.exec() == QDialog::Rejected )
return;
if( old == w.app() ) return;
else old = w.app();
if( dm->updateApp( old ) ){
appList.replace( row, old );
list->item(row)->setText( old.name() );
}
break;
}
case cFilm:
{
Film old = filmList.at( row );
w.setFilm( old );
if( w.exec() == QDialog::Rejected )
return;
if( old == w.film() ) return;
else old = w.film();
if( dm->updateFilm( old ) ){
filmList.replace( row, old );
list->item(row)->setText( old.name() );
}
break;
}
case cSong:
{
Song old = songList.at( row );
w.setSong( old );
if( w.exec() == QDialog::Rejected )
return;
if( old == w.song() ) return;
else old = w.song();
if( dm->updateSong(old) ){
songList.replace( row, old );
list->item(row)->setText( old.name() );
}
break;
}
}
displayItem(row);
}
This code checks which category we have at the moment. Then it edits an app, a movie or a song (depends on the category). Then when I change some informations it writes these changes to the database and the global QLists, change the name in the QListWidget and displays the current item's information in a few labels (it's the last function).
As you can see it's the same code except that the QLists are different. Movies and songs work perfectly. I can change every detail and it automatically updates the name in the list.
But when I change the name of an app and click ok it doesn't change the name in the list and doesn't change the apps information in the QList.
However, when I go through it with a debugger and click on every single step the list changes the name and the information.
So basically what I am trying to ask is why does this code above work when using the debugger going through each step but not when just running it?

Answer 1

I guess that what you call "simple running" is actually using your IDE in "build to run" mode and you are in fact comparing a debug build with an optimised release build. The debugger properly initialises all memory to zero which doesn't happen in the release build.
Regardless of what you are doing, this behaviour is indicative of illegal memory access.
From your code everytime you use at() you are at risk of going out of bounds and I guess that is what is happening. You should check all instances of at() and also list->item(row) are not going out of bounds.
Furthermore using list is at risk of being confused with std::list, therefore this variable should be renamed.