QT drawing exception using pure code

Project scenario ：

for example ： Project scenario ：Qt Use pure code to draw exceptions

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Unable to draw normally

Qt Inherit MainWindow Class needs Add the following code to the function, otherwise , Unable to draw normally

    QWidget *centerWindow = new QWidget;  Declare a new 
    this->setCentralWidget(centerWindow);
    centerWindow->setLayout(layout);   Start 

Qt Inherit Widget Class needs Add the following code to the function, otherwise , Unable to draw normally

layout = new QGridLayout(this);

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