修改ui时出现Qt segfaults（C ++）

Question

I want a button I press to run a command, and then add widgets to the UI based on the command's output. However, because the UI is protected in Qt and can only be modified from the main thread, I get a segmentation fault when I try to add a widget from inside a slot activated by the command button. I tried creating a loop in the main thread checking the state of a variable that would be modified from the slot's thread, but the loop prevented the window from even being rendered, even if I forced a UI refresh for each loop cycle. I've done lots of Googling, but all I can find are people who are either doing the same thing and not getting a segfault, or people who are using the slot 's thread to modify existing widgets instead of creating a new one.

The code that causes the segfault:

void MainWindow::on_myButton_clicked() {
    ...
    QFrame *myFrame = new QFrame;
    ...
    ui->myContainer->layout()->addWidget(myFrame);
    [SEGFAULT]
    ...
}

EDIT: I fixed the original segfault (apparently QScrollAreas are made of two different components and cry if you use the wrong one). But now I'm getting just about the same problem again, but for a different reason:

void MainWindow::on_myButton_clicked() { //from QPushButton
    //run command, parse output
    QFrame *myFrame = new QFrame;
    ui->myContainer->layout()->addWidget(myFrame); //myContainer is the QWidget 'contents' child of a QScrollArea
    QLayout *myFrameLayout = myFrame->layout();
    if (myFrameLayout == NULL) { //this is what's causing the segfaults
        exit(6); //random number to prove this is the cause
    }
}

and the code always exits with code 6 when I press myButton. How can myFrameLayout be NULL if I just added myFrame to the UI? (also, I don't see why I'm getting segfaults instead of null-pointers, but that's a different question and not one I care too much about).

Answer 1

myFrame->layout() is NULL (as a side-note, use nullptr instead of NULL ) because it doesn't have a layout. It's in a layout, but it doesn't have one itself.

ui->myContainer has a layout. That layout manages myFrame . However, that doesn't mean that myFrame has a layout.

If you want myFrame to have a layout, you need to create one. Layouts are managing the children of widgets. If you wanted to add some widgets that are children of myFrame and have a layout managing them, you'd have to create a new layout, install it on myFrame , at which point myFrame->layout() would return it, and then you'd create widgets and add them to myFrame->layout() , at which point they automatically become children of myFrame .

Right now, you're one level above. You're adding a widget that is a child of ui->myContainer and is managed by ui->myContainer 's layout. Note than even though you didn't explicitly make myFrame a child of ui->myContainer , it is made a child when you call ui->myContainer->layout()->addWidget(myFrame) .