QtGui实现计算圆的面积

#ifndef DIALOG_H
#define DIALOG_H

#include <QtWidgets/QDialog>
#include <QtWidgets/QLabel>
#include <QtWidgets//QLineEdit>
#include <QtWidgets/QPushButton>
#include <QtWidgets/QGridLayout>
const static double PI = 3.1416;
class Dialog : public QDialog
{
Q_OBJECT//启动Qt元对象系统（比如支持SIGNAL/SLOT）

public:
Dialog(QWidget *parent = 0);
~Dialog();

private:
QLabel *label1, *label2;
QLineEdit *lineEdit;
QPushButton *button;

private slots:
void showArea();
};

#endif // DIALOG_H

#include "dialog.h"

Dialog::Dialog(QWidget *parent)
: QDialog(parent)
{
label1 = new QLabel(this);
label1->setText(QStringLiteral("请输入圆的半径："));

lineEdit = new QLineEdit(this);

label2 = new QLabel(this);
label2->setFrameStyle(QFrame::Panel | QFrame::Sunken);

button = new QPushButton(this);
button->setText(QStringLiteral("显示对应圆的面积："));

QGridLayout *mainLayout = new QGridLayout(this);
mainLayout->addWidget(label1, 0, 0);
mainLayout->addWidget(lineEdit, 0, 1);
mainLayout->addWidget(label2, 1, 0);
mainLayout->addWidget(button, 1, 1);

connect(button, SIGNAL(clicked()), this, SLOT(showArea()));
//connect(lineEdit, SIGNAL(textChanged(QString)), this, SLOT(showArea()));
}

Dialog::~Dialog()
{

}

void Dialog::showArea()
{
bool ok;
QString tempStr;
QString valueStr = lineEdit->text();
int valueInt = valueStr.toInt(&ok);
double area = valueInt * valueInt * PI;
label2->setText(tempStr.setNum(area));
}

QStringLiteral