Qt操作将无法连接到插槽

Question

Writing a school project in c++ and qt. It is supposed to be a block editor (like draw.io). I generate blocks as a buttons and setting them to a grid. Each button is supposed to have own menu to be able to get edited, deleted, etc. (image example:

We are encountering a problem, that our action won't connect to a slot. Function akceAkce is supposed only to print 1 onto output (via qInfo). But when I click on the menu button, it does nothing. Any suggestions appreciated. Thanks!

void BlockItem::createButton() {

    this->button = new QPushButton("+");

    this->buttonMenu = new QMenu(this->button);

    this->connectBlocks = new QAction(tr("Connect"), this->buttonMenu);

    connect(this->connectBlocks, &QAction::triggered, this, &BlockItem::akceAkce);

    this->buttonMenu->addAction(this->connectBlocks);


    this->button->setMenu(this->buttonMenu);
}

void BlockItem::akceAkce() { 
    qInfo("1"); 
}

Answer 1

I would suggest a possible explanation: you put the BlockItem instance on the stack, and it's going out of scope somewhere in your code, and destroyed consequently, while the push button, menu and action all survive it, since they're instantiated with new , thus they live on the heap.

Something like this:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    BlockItem blockitem;
    blockitem.createButton();

    ui->gridLayout->addWidget(blockitem.getButton());
}

In the sample code above, blockitem goes out of scope at the end of the method, so it is destroyed and the connection loses its receiver side (ie breaks).

The same would happen if you instantiate a BlockItem object on the heap, using new , but accidentally delete it somewhere. Again, the signal sender (the QAction object) survives the receiver and the connection is broken.