我应该如何声明这个朋友功能

我有一个程序是这样的:

class Vehicle
{
private:
      int km;
      double hours;
      string model;
public:
       Vehicle() {}
       [...]
       const void show()
       {
           cout << model << " (speed: " << km/hours << "km/h)\n";
       }
};  
int main ()
{
      Vehicle v1;
      [...]
      v1.show();
      return 0;
}

我想更改它,以便show()是朋友函数而不是const。但是我该怎么做呢?我不明白

香港专业教育学院迄今尝试过的工作以及我得到的错误:

friend void show() //replaced const with friend
        {
            cout << model << " (speed: " << km/hours << "km/h)\n";
        }

//errors: invalid use of non-staticdata member 'Vehicle::(model, km, hours)'

class Vehicle {    
[...]            
friend void show();
};

void show()
{
    cout << model << " (speed: " << km/hours << "km/h)\n";
}

//error: '(model, km, hours)' was not declared in this scope
//in function 'int main()': error: 'class Vehicle' has no member named 'show'

并且

class Vehicle {    
[...]
friend void show();
};

void Vehicle::show()
{
    cout << model << " (speed: " << km/hours << "km/h)\n";
        }
//error: no 'void Vehicle::show()' member function declared in class 'Vehicle'