Question

I’m try to use tr1‘s shared_ptr and Qt 4.8.2 but I have some troubles.
Here my code :

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <string>
#include <tr1/memory>
using namespace std::tr1;

#include <QHBoxLayout>
#include <QVBoxLayout>
#include <QTreeView>
#include <QListView>
#include <QWidget>

#include <iostream>
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{

    shared_ptr<QHBoxLayout> mainLayout(new QHBoxLayout);

    shared_ptr<QTreeView> mainFeeds(new QTreeView);

    mainLayout->addWidget(mainFeeds.get());
    shared_ptr<QWidget> mainWidget (new QWidget);
    mainWidget->setLayout(mainLayout.get()); // <--- this line

    shared_ptr<QWidget> rightWidget(new QWidget);
    shared_ptr<QVBoxLayout> rightLayout(new QVBoxLayout);

    shared_ptr<QListView> rightItems(new QListView);
    rightLayout->addWidget(rightItems.get());

    shared_ptr<QListView> rightPreview(new QListView);
    rightLayout->addWidget(rightPreview.get());

    rightWidget->setLayout(rightLayout.get());
    mainLayout->addWidget(rightWidget.get());

    this->setCentralWidget(mainWidget.get());
}

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

And the output (I use Qt Creator) :

Starting /path/myproject-build-desktop-Qt_4_8_2_in_PATH_local_Release/myproject…
The program has unexpectedly finished.
/path/myproject-build-desktop-Qt_4_8_2_in_PATH_local_Release/myproject exited with code 0

When I comment the marked line, the program runs but I have an empty window.

I have two questions :

1. Why this line makes an error?
2. Is it the right way to use smart pointers (in fact to build robust C++ code) with Qt?

For your help,
In advance,
Thanks.

Answer 1

Qt manages objects lifetime in its own manner. When you make A child of B, then A will be delete‘d when B destroyed.

So, in this line

mainWidget->setLayout(mainLayout.get());

you make mainLayout child of mainWidget.

As mainWidget declared after mainLayout, it will be removed first. And mainLayout will be removed also. But then shared_ptr will try to remove mainLayout again.

In Qt you should be very carefull using smart pointers. Qt memeory management often take ownership of object, (but not always). Also you may prefer using native Qt smart pointers

• QPointer as a week pointer to QObject
• QScopedPointer as replacement of std::unique_ptr etc.
• QSharedPointer / QExplicitlySharedPointer for shred pointers.