g=x+y;

程序代码：

#include<iostream>
using namespace std;
int g;
double (*add)(double x,double y){
    g=x+y;
    return g;
}
double calculate(double x,double y,double *ar){
    double *p=new double;
    p=(*ar)(x,y);
    return *p;
}
int main(void){
    double j,k;
    while(1){
        cout<<"请输入j,k的值：";
        while(!(cin>>j)||!(cin>>k)){
            cin.clear();
            while(cin.get()!='\n');
                continue;    
            cout<<"错误，输入的是字母，请输入数字：\n";        
        } 
    (*add)(j,k);
    double h=calculate(j,k,add);
    cout<<"calculate函数的返回值为："<<h<<endl;    
    delete p;
    }
    return 0;
}

[此贴子已经被作者于2020-12-10 19:30编辑过]

程序代码：

#include <iostream>
#include <limits>
using namespace std;

double add( double x,double y )
{
    return x + y;
}

double calculate( double x, double y, double (*pfun)(double,double) )
{
    return (*pfun)( x, y );
}

int main(void)
{
    double a, b;
    for( ; ; )
    {
        cout << "请输入a,b的值：";
        cin >> a >> b;
        if( cin.eof() || cin.bad() )
            return 1;
        if( cin.fail() )
        {
            cin.clear();
            cin.ignore( numeric_limits<streamsize>::max(), '\n' );
            cout << "格式错误，";
        }
        else
            break;
    }

    double result = calculate( a, b, &add );
    cout << "calculate函数的返回值为：" << result << endl;    
}

double (*add)( double x,double y )
{
    return x + y;
}