Help me why this below code shows error !!

Question

This code belongs to use of friend function :

This is the code :

#include <iostream>
using namespace std;
class bike;
class cycle;

class car{

int score;
public :
car (int a){
score=a;
}
friend void max(car,bike,cycle);
};
class bike{
int score;
public :
bike (int b){
score=b;
}
friend void max(bike,cycle,car);
};
class cycle{
int score;
public :
cycle (int c){
score=c;
}
friend void max(cycle,bike,car);
};
void max(car c,bike b,cycle cy){
if(c.score>b.score && c.score>cy.score)
{
cout<<"Car wins"<<endl;

}
}

int main() {

bike b(10);
cycle c(5);
car cr(15);

return 0;
}
https://code.sololearn.com/cAQNO4NVo7hl/?ref=app

Answer 1

Friend function declaration arguments are not matching with definition

friend void max(car,bike,cycle); // use this in all classes..

max(cr,b,c); // call this in main..

Answer 2

Perhaps it would be better that way (less likely to make a mistake).😎:
#include <iostream>
using namespace std;

class Transport{
public:
int s;
void score(int arg){
s = arg;
}
};
class Car: public Transport {};
class Bike: public Transport {};
class Cycle: public Transport {};

void finish(Car c, Bike b, Cycle cy){
if(c.s>b.s && c.s>cy.s)
cout<<"Car wins!"<<endl;
else cout<<"Car loses."<<endl;
}

int main() {
Bike b;
b.score(10);
Cycle cy;
cy.score(5);
Car c;
c.score(15);
finish(c,b,cy);
return 0;
}