Fixing memory leak with temporary data table (heap vs stack) - c++

Question

I have a snippet of code that is used to populate a table. It opens a file on disk and throws the data into a table.
if (file.open(QIODevice::ReadOnly))
{
QDataStream stream(&file);
qint32 numRows, numColumns;
stream >> numRows >> numColumns;
QStandardItemModel* tempModel = new QStandardItemModel;
tempModel->setRowCount(numRows);
tempModel->setColumnCount(numColumns);
for (int i = 0; i < numRows ; ++i) {
for (int j = 0; j < numColumns; j++) {
QStandardItem* tempItem = new QStandardItem; // stored in heap
tempItem->read(stream);
tempModel->setItem(i, j, tempItem);
}
}
file.close();
tableView->setModel(tempModel);
...
}
This code works. But the issue I'm having is that, the more files I open, the more memory is being used and it never goes down. If I add a second file, for example, I don't need to store the previous file's model anymore. I want to delete it.
I'm guessing the memory is not being freed up because it never gets deleted since I'm using the new keyword and a pointer.
If we take the tempItem for loop as an example, I imagine I'd have to do something similar to this to fix it:
for (int i = 0; i < numRows ; ++i) {
for (int j = 0; j < numColumns; j++) {
//QStandardItem* tempItem = new QStandardItem;
QStandardItem tempItem; // store on stack and delete at end of scope
//tempItem->read(stream);
tempItem.read(stream);
tempModel->setItem(i, j, tempItem);
}
But even then, it throws an error because QStandardItemModel's setItem (seen here) takes a QStandardItem pointer.
I'd like to fix this for both tempModel and tempItem if possible. What am I doing wrong here?

Answer 1

The memory leak is not due to QStandardItem ownership. The setItem() method takes ownership of the QStandardItem objects, which will be freed automatically when the QStandardItemModel object is freed.
Your memory leak is due to the tableView->setModel(tempModel);statement, because that method does not take ownership. When you change the model, or free the view, you are responsible for freeing the model.
See this document for details.
For example:
QItemSelectionModel *m = tableView->selectionModel();
tableView->setModel(tempModel);
delete m;

Answer 2

I wound up setting a single QStandardItemModel in my header file and my main window's initialization list.
myapp.h
...
QStandardItemModel* mainModel;
...
myapp.cpp
...
MyApp::MyApp(QWidget* parent)
: QMainWindow(parent),
mainModel(new QStandardItemModel(tableView)),
...
I changed the way the application works so that I only ever need one model for my table view. Whenever I need to add a new data set to populate the table with, I clear the current table entirely, create a new, temporary model, then set the temp model to the main model.
mainTableModel->clear();
QStandardItemModel* tempModel = new QStandardItemModel;
tempModel->setRowCount(numRows);
tempModel->setColumnCount(numColumns);
for (int i = 0; i < numRows ; ++i) {
for (int j = 0; j < numColumns; j++) {
QStandardItem* tempItem = new QStandardItem();
tempItem->read(stream);
tempModel->setItem(i, j, tempItem);
}
}
file.close();
mainModel = tempModel;
tableView->setModel(mainModel);
Clearing the model and reusing it lessens, but does not fix, the memory leak.