#include <iostream>
using namespace std;
double area(int x)
{
    int area;
    area=x*x;
    return area;
}
double area(int x,int y)
{
    int area;
    area=x*y;
    return area;
}
double area(double c,double d)
{
    double area;
    area=0.5*c*d;
    return area;
}
double area(double c)
{
    double area;
    area=3.14*c*c;
    return area;
}


int main()
{
   char pic;
   int x,y;
   double c ,d,s;
   cout<<"正方形为1，长方形为2,三角形为3，圆形为4"<<endl;
   cout<<"输入所求图形是什么用数字表示"<<endl;
   cin>>pic;
   switch(pic)
   {
   case '1':
       cout<<"输入正方形边长以整数形式输入"<<endl;
       cin>>x;
        s=double area(int x);
        cout<<"正方形面积为="<<s<<endl;
        break;
   case '2':
        cout<<"输入长方形边长以整数形式输入"<<endl;
        cin>>x>>y;
        double area(int x,int y);
        cout<<"长方形面积为="<<endl;
        break;
   case '3':
        cout<<"输入三角形的底和高以小数形式输入"<<endl;
        cin>>c>>d;
        double area(double c,double d);
        cout<<"三角形面积为="<<endl;
        break;
   case '4':
        cout<<"输入圆形半径以小数形式输入"<<endl;
        cin>>c;
        double area(double c);
        cout<<"圆形形面积为="<<endl;
        break;
        return 0;
   }
}

thanks

   case '3':
        cout<<"输入三角形的底和高以小数形式输入"<<endl;
        cin>>c>>d;
        cout<<"三角形面积为="<<area(c,d)<<endl;
        break;
   case '4':
        cout<<"输入圆形半径以小数形式输入"<<endl;
        cin>>c;
        cout<<"圆形形面积为="<<area(c)<<endl;
        break;
        return 0;//这个结束语句要放到SWITCH快外面去

OK,thanks