Friend Function Concept in C++

friend_function.cpp

/*
 * Create 3 classes which takes the data for 3 semesters containing info of subject and marks
 * Calculate the % for all semester
 * finally calculate CGPA with a function program
 */

#include<iostream>
#include<conio.h>
using namespace std;

class Semester1;
class Semester2;
class Semester3{
    int marks[5];
    float perc;
    public:
        void get(){
            perc=0;
            cout<<"\nEnter marks of Semester 3\n\n";
            cout<<"Marks of 5 Subjects : ";
            for(int i=0; i<5; i++){
                do{
                    cin>>marks[i];
                    if(marks[i]<0 || marks[i]>100)
                        cout<<"Invalid Marks\n";
                }while(marks[i]<0 || marks[i]>100);
                perc+=marks[i];
            }
            perc/=5;
            cout<<"\nPercentage of Semester 3 : "<<perc<<"%"<<endl;
        }
        friend void CGPA(Semester1, Semester2, Semester3);

};
class Semester1{
    int marks[5];
    float perc;
    public:
        void get(){
            perc=0;
            cout<<"\nEnter marks of Semester 1\n\n";
            cout<<"Marks of 5 Subjects : ";
            for(int i=0; i<5; i++){
                do{
                    cin>>marks[i];
                    if(marks[i]<0 || marks[i]>100)
                        cout<<"Invalid Marks\n";
                }while(marks[i]<0 || marks[i]>100);
                perc+=marks[i];
            }
            perc/=5;
            cout<<"\nPercentage of Semester 1 : "<<perc<<"%"<<endl;
        }
        friend void CGPA(Semester1, Semester2, Semester3);

};
class Semester2{
    int marks[5];
    float perc;
    public:
        void get(){
            perc=0;
            cout<<"\nEnter marks of Semester 2\n\n";
            cout<<"Marks of 5 Subjects : ";
            for(int i=0; i<5; i++){
                do{
                    cin>>marks[i];
                    if(marks[i]<0 || marks[i]>100)
                        cout<<"Invalid Marks\n";
                }while(marks[i]<0 || marks[i]>100);
                perc+=marks[i];
            }
            perc/=5;
            cout<<"\nPercentage of Semester 2 : "<<perc<<"%"<<endl;
        }
        friend void CGPA(Semester1, Semester2, Semester3);

};
void CGPA(Semester1 s1, Semester2 s2, Semester3 s3){
    cout<<"\n\nCGPA : "<<(( s1.perc/10.0)+(s2.perc/10.0)+(s3.perc/10.0))/3.0<<endl;
}
int main(){
    Semester1 s1;
    Semester2 s2;
    Semester3 s3;
    s1.get();
    s2.get();
    s3.get();
    CGPA(s1,s2,s3);
    return getch();
}