Friend function in class.

Example of friend function in class.

simple code:

#include<iostream>
using namespace std;
class integer;//formal diclaration
class complex{
int a,b;
char sign='+';
int sign1=1;
public:
    complex(){a=0,b=0;}
    complex(int a,int b){
        this->a=a;
        this->b=b;
    }
    void show(){
        if(b<0) sign1=-1,sign='-';
        else sign='+';
        cout<<"Complex number : "<<a<<sign<<b*sign1<<'i'<<endl;
    }
    friend complex add(complex ,integer );
    friend complex add(complex ,complex );
};
class integer{
    int c;
    friend complex add(complex ,integer );
    friend complex add(complex ,complex );
public:
    integer(int c){this->c=c;}
};
complex add(complex c,integer i){
    complex temp;
    temp.a=c.a+i.c;
    temp.b=c.b;
    return temp;
}
complex add(complex c1,complex c2){
    complex temp;
    temp.a=c1.a+c2.a;
    temp.b=c1.b+c2.b;
    return temp;
}
int main()
{
    complex com(2,4);
    integer in(5);
    com.show();
    cout<<"After addition with integer : "<<endl;
    com=add(com,in);
    com.show();
    complex com1(2,-7);
    com1.show();
    cout<<"After addition with complex : "<<endl;
    com=add(com,com1);
    com.show();
    return 0;
}

Operator overloading using friend function

code:

#include<iostream>
using namespace std;
class integer;//formal diclaration
class complex{
int a,b;
char sign='+';
int sign1=1;
public:
    complex(){a=0,b=0;}
    complex(int a,int b){
        this->a=a;
        this->b=b;
    }
    void show(){
        if(b<0) sign1=-1,sign='-';
        else sign='+';
        cout<<"Complex number : "<<a<<sign<<b*sign1<<'i'<<endl;
    }
    friend complex operator + (complex ,integer );
    friend complex operator + (complex ,complex );
};
class integer{
    int c;
    friend complex operator + (complex ,integer );
    friend complex operator + (complex ,complex );
public:
    integer(int c){this->c=c;}
};
complex operator + (complex c,integer i){
    complex temp;
    temp.a=c.a+i.c;
    temp.b=c.b;
    return temp;
}
complex operator + (complex c1,complex c2){
    complex temp;
    temp.a=c1.a+c2.a;
    temp.b=c1.b+c2.b;
    return temp;
}
int main()
{
    complex com(2,4);
    integer in(5);
    com.show();
    cout<<"After addition with integer : "<<endl;
    com=com+in;
    com.show();
    complex com1(2,-7);
    com1.show();
    cout<<"After addition with complex : "<<endl;
    com=com+com1;
    com.show();
    return 0;
}

Another example

code:

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

class student;
class information{
    string name,address,grade;
    public:
    information(){
        name="Are not initialised.";
        address="Not found.";
    }
    information(string name,string address){
        this->name=name;
        this->address=address;
    }
    void grade_cal(student A);
    void dis(student B);
};
class student{
    int roll;
    double cgpa;
    public:
    student(int roll,double cgpa){
        this->roll=roll;
        this->cgpa=cgpa;
    }
    student(){
        roll=0;
        cgpa=0.0;
    }
    friend void information::grade_cal(student A);
    friend void information::dis(student B);
};
void information::grade_cal(student A){
    if(A.cgpa>=3.0){
        grade="First class.";
    }
    else if(A.cgpa>=2.5){
        grade="Second class.";
    }
    else if(A.cgpa>=2.0){
        grade="Third class.";
    }
    else{
        grade="Fail.";
    }
}
void information::dis(student B){
        cout<<"Name: "<<name<<endl;
        cout<<"Roll: "<<B.roll<<endl;
        cout<<"Address: "<<address<<endl;
        cout<<"Grade: "<<grade<<endl;
        cout<<"CGPA: "<<B.cgpa<<endl;
    }
int main() {
    system("cls");
    int roll=0.0;
    double cgpa=0.0;
     cout<<"Input Name : ";
    string name="Are not initialised.",address="not found.";
    getline(cin,name);
    cout<<"\nInput Address: ";
    getline(cin,address);
    cout<<"\nInput roll and CGPA: ";
    cin>>roll>>cgpa;
    student s(roll,cgpa);
    information p(name,address);

    p.grade_cal(s);

    cout<<"\n\n\t\t\tStudent Information: \n"<<endl;

    p.dis(s);
    return 0;
}

Input/Output: