编程计算图形的面积。程序可计算圆形、长方形、正方形的面积，运行时先提示用户选择图形的类型，然后，对圆形要求用户输入半径值，对长方形要求用户输入长和宽的值，对正方形要求用户输入边长的值，计算出面积的值后将其显示出来。C++的两种方式

第一种方式：使用C++编写一个控制台程序，实现计算图形面积的功能。代码如下：

#include <iostream>
#include <cmath>

using namespace std;

int main()
{
    int choice;
    float radius, length, width, side, area;

    cout << "请选择图形的类型：" << endl;
    cout << "1. 圆形" << endl;
    cout << "2. 长方形" << endl;
    cout << "3. 正方形" << endl;

    cin >> choice;

    switch (choice)
    {
    case 1:
        cout << "请输入圆的半径：";
        cin >> radius;
        area = M_PI * radius * radius;
        cout << "圆的面积为：" << area << endl;
        break;
    case 2:
        cout << "请输入长方形的长度：";
        cin >> length;
        cout << "请输入长方形的宽度：";
        cin >> width;
        area = length * width;
        cout << "长方形的面积为：" << area << endl;
        break;
    case 3:
        cout << "请输入正方形的边长：";
        cin >> side;
        area = side * side;
        cout << "正方形的面积为：" << area << endl;
        break;
    default:
        cout << "输入有误，请重新选择" << endl;
        break;
    }

    return 0;
}

第二种方式：使用C++和Qt编写一个图形界面程序，实现计算图形面积的功能。代码如下：

#include <QApplication>
#include <QWidget>
#include <QGridLayout>
#include <QLabel>
#include <QLineEdit>
#include <QPushButton>
#include <QRadioButton>
#include <cmath>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QWidget window;
    window.setWindowTitle("图形面积计算器");

    QGridLayout *layout = new QGridLayout(&window);

    QLabel *shape_label = new QLabel("请选择图形的类型：", &window);
    layout->addWidget(shape_label, 0, 0, 1, 2, Qt::AlignLeft | Qt::AlignTop);

    QRadioButton *circle_radiobutton = new QRadioButton("圆形", &window);
    layout->addWidget(circle_radiobutton, 1, 0);

    QRadioButton *rectangle_radiobutton = new QRadioButton("长方形", &window);
    layout->addWidget(rectangle_radiobutton, 2, 0);

    QRadioButton *square_radiobutton = new QRadioButton("正方形", &window);
    layout->addWidget(square_radiobutton, 3, 0);

    QLabel *radius_label = new QLabel("请输入圆的半径：", &window);
    layout->addWidget(radius_label, 4, 0);

    QLineEdit *radius_edit = new QLineEdit(&window);
    layout->addWidget(radius_edit, 4, 1);

    QLabel *length_label = new QLabel("请输入长方形的长度：", &window);
    layout->addWidget(length_label, 5, 0);

    QLineEdit *length_edit = new QLineEdit(&window);
    layout->addWidget(length_edit, 5, 1);

    QLabel *width_label = new QLabel("请输入长方形的宽度：", &window);
    layout->addWidget(width_label, 6, 0);

    QLineEdit *width_edit = new QLineEdit(&window);
    layout->addWidget(width_edit, 6, 1);

    QLabel *side_label = new QLabel("请输入正方形的边长：", &window);
    layout->addWidget(side_label, 7, 0);

    QLineEdit *side_edit = new QLineEdit(&window);
    layout->addWidget(side_edit, 7, 1);

    QPushButton *calculate_button = new QPushButton("计算面积", &window);
    layout->addWidget(calculate_button, 8, 0, 1, 2, Qt::AlignCenter);

    QLabel *result_label = new QLabel("", &window);
    layout->addWidget(result_label, 9, 0, 1, 2, Qt::AlignCenter);

    QObject::connect(calculate_button, &QPushButton::clicked, [&](){
        if (circle_radiobutton->isChecked())
        {
            float radius = radius_edit->text().toFloat();
            float area = M_PI * radius * radius;
            result_label->setText(QString("圆的面积为：%1").arg(area));
        }
        else if (rectangle_radiobutton->isChecked())
        {
            float length = length_edit->text().toFloat();
            float width = width_edit->text().toFloat();
            float area = length * width;
            result_label->setText(QString("长方形的面积为：%1").arg(area));
        }
        else if (square_radiobutton->isChecked())
        {
            float side = side_edit->text().toFloat();
            float area = side * side;
            result_label->setText(QString("正方形的面积为：%1").arg(area));
        }
        else
        {
            result_label->setText("请选择图形的类型");
        }
    });

    window.show();

    return app.exec();
}