+1 from my side. Explicit ownership declaration is the king. always - all other solutions are paid with sweat and blood. but mainly with blood... ;)
Zoltan On Friday, May 3, 2019, Daniel Teske wrote: > Hi, > > a year back I wrote a patch that added unique_ptr apis to most of > qtbase. I still think that would be a good addition to Qt, and thus I've > updated the patch with the feedback gained at the Contributor Summit > last year and taking into account that with Qt6, binary compatibility is > not required. The result is: https://codereview.qt-project.org/#/c/260618/ > > Since not everyone remembers last years mail, I'll explain the problem > that patch is trying to solve, but won't go into all the details: > Consider this small program: > > QWidget widget; > QHBoxLayout *layout = new QHBoxLayout(&widget); > QPushButton *leftButton = new QPushButton("LEFT", &widget); // Owned by > widget > layout->addWidget(leftButton); // No Ownership transfer > > QPushButton* rightButton = new QPushButton("RIGHT"); // No owner > layout->addWidget(rightButton); // Ownership transfer > > QObject::connect(leftButton, &QPushButton::clicked, > [leftButton]{ > QMenu menu; > auto act = new QAction("Action!"); // No Owner > menu.addAction(act); // No Ownership transfer > menu.exec(leftButton->mapToGlobal(leftButton->rect().bottomLeft())); > }); > widget.show(); > > In that program: > > * The left button is owned at construction by the parent. > * The right button is created unowned, but adding a widget to a layout > automatically sets the parent > * The action is created unowned, and is leaked because addAction does > not set the parent. > > That is, even though: > > * layout->addWidget(new QPushButton()) and > * menu->addAction(new QAction()) > > look very similar, the later leaks memory. > > That's a less than ideal api. With Qt6 there's a opportunity to leverage > unique_ptr to ensure that memory is not leaked. > > The patch adds two ways to create new QObjects: > > * std::make_unique<QPushButton>("LEFT"); and > * parent->makeChild<QPushButton>("RIGHT"); > * adds various overloads for ownership transfer > > The first half of the program would thus look like: > > QWidget widget; > QHBoxLayout *layout = widget.makeChild<QHBoxLayout>(); > > QPushButton *leftButton = widget.makeChild<QPushButton>("LEFT"); > layout->addWidget(leftButton); > > std::unique_ptr<QPushButton> rightButton = > std::make_unique<QPushButton>("RIGHT"); > layout->addWidget(std::move(rightButton)); > > For second half, the naive transformation: > > { > QMenu menu; > auto act = std::make_unique<QAction>("Action!"); > menu.addAction(act.get()); > menu.exec(leftButton->mapToGlobal(leftButton->rect().bottomLeft())); > } > > does no longer leak memory, instead the action is freed at the end of > the scope as its still owned by the unique_ptr. > > I believe that Qt6 is a unique opportunity to add apis for better memory > management to Qt. > > So, who thinks that this is something that ought to be a Qt6 feature? > > And who wants to actually help make that happen? > > daniel > > _______________________________________________ Development mailing list Development@qt-project.org https://lists.qt-project.org/listinfo/development
