源码:

头文件: widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QMouseEvent>

QT_BEGIN_NAMESPACE
namespace Ui {
class Widget;
}
QT_END_NAMESPACE

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();

private slots:
    void on_btn_0_clicked();

    void on_btn_1_clicked();

    void on_btn_2_clicked();

    void on_btn_3_clicked();

    void on_btn_4_clicked();

    void on_btn_5_clicked();

    void on_btn_6_clicked();

    void on_btn_7_clicked();

    void on_btn_8_clicked();

    void on_btn_9_clicked();

    void on_btn_clear_clicked();

    void on_btn_dot_clicked();

    void on_btn_back_clicked();

    void on_btn_add_clicked();

    void on_btn_sub_clicked();

    void on_btn_div_clicked();

    void on_btn_mul_clicked();

    void on_btn_mod_clicked();

    void on_btn_equal_clicked();

    void setText(const QString& str);

    void changeLine(const QChar& c);

private:
    Ui::Widget *ui;
    int flag = 0; //0-聚焦于lineEdit, 1-聚焦于lineEdit_2, 2-计算完毕.需要清空后重新计算
    QChar sign;
};
#endif // WIDGET_H

源文件:widget.cpp

#include "widget.h"
#include "ui_widget.h"

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

Widget::~Widget()
{
    delete ui;
}

void Widget::setText(const QString& str) {
    if (flag == 0) {
        QString s = ui->lineEdit->text();
        ui->lineEdit->setText(s += str);
    } else if (flag == 1) {
        QString s = ui->lineEdit_2->text();
        ui->lineEdit_2->setText(s += str);
    }
}

void Widget::on_btn_0_clicked()
{
    setText("0");
}


void Widget::on_btn_1_clicked()
{
    setText("1");
}


void Widget::on_btn_2_clicked()
{
    setText("2");
}


void Widget::on_btn_3_clicked()
{
    setText("3");
}


void Widget::on_btn_4_clicked()
{
    setText("4");
}

void Widget::on_btn_5_clicked()
{
    setText("5");
}


void Widget::on_btn_6_clicked()
{
    setText("6");
}


void Widget::on_btn_7_clicked()
{
    setText("7");
}


void Widget::on_btn_8_clicked()
{
    setText("8");
}


void Widget::on_btn_9_clicked()
{
    setText("9");
}


void Widget::on_btn_clear_clicked()
{
    ui->lineEdit->setText("");
    ui->lineEdit_2->setText("");
    ui->lineEdit_3->setText("");
    this->flag = 0;
    this->sign = '\0';
}


void Widget::on_btn_dot_clicked()
{
    QString s;
    if (flag == 0) {
        s = ui->lineEdit->text();
    } else if (flag == 1){
        s = ui->lineEdit_2->text();
    }
    if (s == nullptr || s == " ") {
        s += "0.";
        if (flag == 0) {
            ui->lineEdit->setText(s);
        } else if (flag == 1){
            ui->lineEdit_2->setText(s);
        }
    }
    if (!s.contains(".")) {
        if (flag == 0) {
            ui->lineEdit->setText(s += '.');
        } else if (flag == 1){
            ui->lineEdit_2->setText(s += '.');
        }
    }
}

void Widget::on_btn_back_clicked()
{
    QString s;
    if (flag == 0) {
        s = ui->lineEdit->text();
        ui->lineEdit->setText(s.removeLast());
    } else if (flag == 1){
        s = ui->lineEdit_2->text();
        ui->lineEdit_2->setText(s.removeLast());
    }
}

void Widget::changeLine(const QChar &c)
{
    if (this->sign.isNull() && c == '-') {
        //1.若第一行为空,则第一个数为负数, 否则做为减号运算符
        QString s = ui->lineEdit->text();
        if (s == "") {
            ui->lineEdit->setText(s += c);
        } else {
            this->sign = c;
            this->flag = 1;
        }
    } else if (!this->sign.isNull() && c == '-') {
        //2.若第二行为空，则第二个数是负数,否则不做处理
        QString s = ui->lineEdit_2->text();
        if (s == "") {
            ui->lineEdit_2->setText(s += c);
        }
    } else if (this->sign.isNull()) { //若还没做过运算,且第一行非空,则转到第二行,若为空则不做处理
        QString s = ui->lineEdit->text();
        if (s != "") {
            this->sign = c;
            this->flag = 1;
        }
    }
}

void Widget::on_btn_add_clicked()
{
    changeLine('+');
}


void Widget::on_btn_sub_clicked()
{
    changeLine('-');
}


void Widget::on_btn_div_clicked()
{
    changeLine('/');
}


void Widget::on_btn_mul_clicked()
{
    changeLine('*');
}


void Widget::on_btn_mod_clicked()
{
    changeLine('%');
}


void Widget::on_btn_equal_clicked()
{
    QString s1 = ui->lineEdit->text().trimmed();
    QString s2 = ui->lineEdit_2->text().trimmed();
    double a, b, res;
    if (flag == 1 && s1 != "" && s2 !="") {//1.两行数都正常输入
        a = s1.toDouble();
        b = s2.toDouble();
        switch(sign.unicode()) {
        case '+':
            res = a + b;
            break;
        case '-':
            res = a - b;
            break;
        case '*':
            res = a * b;
            break;
        case '/':
            res = a / b;
            break;
        case '%':
            res = static_cast<int>(a) % static_cast<int>(b);
            break;
        default:
            throw std::runtime_error("未知运算符异常!");
        }
        ui->lineEdit_3->setText(QString::number(res));
        flag = 2;
    } else if (flag == 0 && s1 != "") {//2.只有第一行有输入,则直接取第一行为结果
        res = s1.toDouble();
        ui->lineEdit_3->setText(QString::number(res));
    } else if (flag == 0 && s1 == "") {//3.两行都为空输入0
        ui->lineEdit_3->setText(QString::number(0));
    }
}

测试计算器功能: