如何在Qt的CentralWidget中用QLabel替换QPushButton？

Question

I am trying to learn and build a small minesweeper gui app.
Here is how this looks like:

The next thing I want to do is after clicking one button, then the button will be set to hide() and a QLabel will appear in the same place.

My code is like this:
.h

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

    void setUI();
    void clickedBtnInfo();

private:
    Ui::MainWindow *ui;

    QWidget *centralWidget;
    QGridLayout *centralLayout;
    QPushButton *btn[81];
    QPushButton *btnSender;
    QLabel *lbl[81];
    QString clickedBtnName;

private slots:
    void btnClicked();
};

.cpp

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

    centralWidget = new QWidget(this);
    setCentralWidget(centralWidget);

    lbl[81] = new QLabel(centralWidget);
    setUI();

    for(int i = 0; i < 81; i++) {
        connect(btn[i], SIGNAL(clicked(bool)), btn[i], SLOT(hide()));
        connect(btn[i], SIGNAL(clicked(bool)), this, SLOT(btnClicked()));
    }

    centralWidget->setLayout(centralLayout);
}

void MainWindow::setUI()
{
    ...
    centralLayout = new QGridLayout(centralWidget);
    for(int i = 0; i < 9; i++) {
        for(int j = 0; j < 9; j++) {
            centralLayout->addWidget(btn[j + i * 9], 0 + i, j);
            centralLayout->setSpacing(0);
        }
    }
    ...
}

void MainWindow::clickedBtnInfo()
{
    btnSender = qobject_cast<QPushButton*>(sender()); 
    clickedBtnName = btnSender->objectName();
}

void MainWindow::btnClicked()
{
    clickedBtnInfo();
    for(int i = 0; i < 9; i++) {
        for(int j = 0; j < 9; j++) {
            if(btn[j + i * 9]->objectName() == clickedBtnName) {
                centralLayout->addWidget(lbl[j + i * 9], 0 + i, j);
                centralLayout->setSpacing(0);
            }
        }
    }
}

When I ran this and clicked one of the buttons, the app just force quit( The program has unexpectedly finished. )
So how can I solve this problem and replace QPushButton with QLabel after clicking?
Thanks.

Answer 1

The Problem why your code leads to a crash was correctly pointed out by @GM - lbl[81] = new QLabel(centralWidget); will create only 1 label, and place it in the 81st array field. That are 2 errors at once:

1. If your array is 81 elements long, they are numbered: 0, 1, ..., 79, 80 . The last element is 80, because you start counting at 0. So placing something at position 81 is not possible
2. To actually create 81 new labels, you have to create them in a loop:

Sample code:

for(int i = 0; i < 81; i++) { //goes from 0 to 80
    lbl[i] = new QLabel(centralWidget);
    lbl[i]->setObjectName(QStringLiteral("Label %1").arg(i));
}

The second line gives each label a custom name. See QString::arg for details.

---

One more tip: Avoid C-arrays, unless you need high performace/low memory (which is not the case for your example). Instead try to use one of the Qt container classes, eg QList or QVector . (You can use std::vector etc. as well, but when working with Qt, I would recommend to use the Qt containers)

For your case, I would recommend QVector, as it performs best with fixed-sized arrays. With both of these changes, update your code to:

class MainWindow : public QMainWindow
{
    //...
private:
    Ui::MainWindow *ui;

    QWidget *centralWidget;
    QGridLayout *centralLayout;
    QVector<QPushButton> btn;
    QPushButton *btnSender;
    QVector<QLabel> lbl;
    QString clickedBtnName;
};

In your cpp file, update the part where you create the arrays:

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

    centralWidget = new QWidget(this);
    setCentralWidget(centralWidget);

    lbl.resize(81);
    for(int i = 0; i < lbl.size(); i++) { //goes from 0 to 80
        lbl[i] = new QLabel(centralWidget);
        lbl[i]->setObjectName(QStringLiteral("Label %1").arg(i));
    }
    setUI();

    //...
}

void MainWindow::setUI()
{
    //keep your code, but remember to prepare the btn vector with:
    btn.resize(81);

    //then you can fill the vector just like you are used to:
    btn[0] = ui->btn0;
    //...
}

And the rest stays the same, as these classes allow you to keep the standard array access syntax you know.