帮忙用C++ Qt写一个简单的界面，界面是计算器，可以进行简单的加减乘除计算。

好的，下面是一个简单的计算器界面的代码，使用了Qt框架和C++语言。你可以根据自己的需求进行修改和扩展。

// mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QLineEdit>
#include <QPushButton>

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

private:
    QLineEdit *m_lineEdit;
    QPushButton *m_btnPlus;
    QPushButton *m_btnMinus;
    QPushButton *m_btnMultiply;
    QPushButton *m_btnDivide;
    QPushButton *m_btnEqual;
    QPushButton *m_btnClear;

    double m_firstNumber;
    double m_secondNumber;
    QString m_operation;

private slots:
    void onBtnClicked();
    void onClearClicked();
};

#endif // MAINWINDOW_H

// mainwindow.cpp

#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    m_lineEdit = new QLineEdit(this);
    m_lineEdit->setGeometry(10, 10, 280, 30);

    m_btnPlus = new QPushButton("+", this);
    m_btnPlus->setGeometry(10, 50, 60, 30);
    connect(m_btnPlus, &QPushButton::clicked, this, &MainWindow::onBtnClicked);

    m_btnMinus = new QPushButton("-", this);
    m_btnMinus->setGeometry(80, 50, 60, 30);
    connect(m_btnMinus, &QPushButton::clicked, this, &MainWindow::onBtnClicked);

    m_btnMultiply = new QPushButton("*", this);
    m_btnMultiply->setGeometry(150, 50, 60, 30);
    connect(m_btnMultiply, &QPushButton::clicked, this, &MainWindow::onBtnClicked);

    m_btnDivide = new QPushButton("/", this);
    m_btnDivide->setGeometry(220, 50, 60, 30);
    connect(m_btnDivide, &QPushButton::clicked, this, &MainWindow::onBtnClicked);

    m_btnEqual = new QPushButton("=", this);
    m_btnEqual->setGeometry(220, 90, 60, 30);
    connect(m_btnEqual, &QPushButton::clicked, this, &MainWindow::onBtnClicked);

    m_btnClear = new QPushButton("Clear", this);
    m_btnClear->setGeometry(150, 90, 60, 30);
    connect(m_btnClear, &QPushButton::clicked, this, &MainWindow::onClearClicked);

    m_firstNumber = 0;
    m_secondNumber = 0;
    m_operation = "";
}

MainWindow::~MainWindow()
{
}

void MainWindow::onBtnClicked()
{
    QPushButton *btn = qobject_cast<QPushButton*>(sender());
    if (btn == m_btnPlus) {
        m_firstNumber = m_lineEdit->text().toDouble();
        m_operation = "+";
        m_lineEdit->clear();
    } else if (btn == m_btnMinus) {
        m_firstNumber = m_lineEdit->text().toDouble();
        m_operation = "-";
        m_lineEdit->clear();
    } else if (btn == m_btnMultiply) {
        m_firstNumber = m_lineEdit->text().toDouble();
        m_operation = "*";
        m_lineEdit->clear();
    } else if (btn == m_btnDivide) {
        m_firstNumber = m_lineEdit->text().toDouble();
        m_operation = "/";
        m_lineEdit->clear();
    } else if (btn == m_btnEqual) {
        m_secondNumber = m_lineEdit->text().toDouble();
        double result = 0;
        if (m_operation == "+") {
            result = m_firstNumber + m_secondNumber;
        } else if (m_operation == "-") {
            result = m_firstNumber - m_secondNumber;
        } else if (m_operation == "*") {
            result = m_firstNumber * m_secondNumber;
        } else if (m_operation == "/") {
            if (m_secondNumber != 0) {
                result = m_firstNumber / m_secondNumber;
            } else {
                result = 0;
                m_lineEdit->setText("Error: Division by zero!");
            }
        }
        m_lineEdit->setText(QString::number(result));
    }
}

void MainWindow::onClearClicked()
{
    m_lineEdit->clear();
    m_firstNumber = 0;
    m_secondNumber = 0;
    m_operation = "";
}

在这里，我们使用了Qt提供的LineEdit和PushButton控件来实现计算器的界面。LineEdit用于显示计算结果和用户输入的数字，PushButton用于响应用户的操作。在点击加减乘除按钮时，我们通过记录当前的数字和操作符来进行计算，最后将结果显示在LineEdit上。在点击清除按钮时，我们将所有记录的数字和操作符清零，并清空LineEdit。