#include <iostream>
using namespace std;

class Figure {
protected:
    double x,y;

public:
    Figure(double a,double b) { x = a; y = b; }

    virtual void showArea() {
        cout<<"No area computation defined";
        cout<<"For this class. \n";
    }
};

class Triangle:public Figure {
public:
    Triangle(double a,double b):Figure(a,b) {};

    void showArea() {
        cout<<"Triangle with height "<<x;
        cout<<" and base "<<y;
        cout<<x*y/2<<endl;
    }
};

class Square:public Figure {
public:
    Square(double a,double b):Figure(a,b) {};

    void showArea() {
        cout<<"Square with dimension "<<x;
        cout<<" * "<<y<<" has an area of ";
        cout<<x*y<<endl;
    }
};

class Circle:public Figure {
public:
    Circle(double a):Figure(a,a) {};

    void showArea() {
        cout<<"Circle with radius "<<x;
        cout<<" has an area of ";
        cout<<x*x*3.1416<<endl;
    }
};

class Trapezoid:public Figure {
protected:
    double z;

public:
    Trapezoid(double a,double b,double c):Figure(a,b) { z = c; };

    void showArea() {
        cout<<"Trapezoid area is: "<<endl;
        cout<<(x + y) * z / 2<<endl;
    }
};

int main() {
    int m;
    double base,high;
    double length,wide;
    double radius;
    double b1,b2,h1;

    for(int j = 0; j < 3; j++) {

    cout<<"         _______________________         "<<endl;
    cout<<"         |     请选择操作      |         "<<endl;
    cout<<"         |    1.求三角形面积   |         "<<endl;
    cout<<"         |    2.求矩形面积     |         "<<endl;
    cout<<"         |    3.求圆面积       |         "<<endl;
    cout<<"         |    4.求梯形面积     |         "<<endl;
    cout<<"         |     请输入操作      |         "<<endl;
    cout<<"         |_____________________|         "<<endl;
    cin>>m;

   
    cout<<"请输入三角形底与高"<<endl;
    cin>>base>>high;
    Triangle t(base , high);

    cout<<"请输入矩形长与宽"<<endl;
    cin>>length>>wide;
    Square s(length , wide);

    cout<<"请输入圆的半径";
    cin>>radius;
    Circle c(radius);

    cout<<"请输入梯形上下底与高"<<endl;
    cin>>b1>>b2>>h1;
    Trapezoid p(b1 , b2 , h1);

    switch(m) {
    case 1:
        t.showArea();
        break;

    case 2:
        s.showArea();
        break;

    case 3:
        c.showArea();
        break;

    case 4:
        p.showArea();
        break;
    }
    }
    return 0;
}
每次出现操作界面，无论按什么操作都会出现“输入三角形底与高”等，这怎么改？

你贴的图跟你说的不一样啊。