friend function in C++ is not accessing private members - c++

Question

I am learning C++ and I have written the below given simple program to understand the working of friend function
(ignore all the complication I made by using complex syntax in the code because I am learning and I practice the syntax that I learn in programs).
The friend function is not accessing the private members of test and stu.
#include<iostream>
#include<conio.h>
class test;
class stu
{
private:
int z;
public:
stu(int z)
{
this->z=z;
}
friend disp(stu,test);
~stu(void)
{
std::cout<<"Destructor of stu class is executed!!"<<std::endl;
}
};
class test{
private:
int x;
public:
test(int a)
{
x=a;
}
friend disp(stu,test);
~test(void)
{
std::cout<<"Destructor is executed!!"<<std::endl;
}
};
class test2:public test
{
private:
int b;
public:
test2(int b)
{
this->b=b;
}
void show(void);
~test2(void)
{
std::cout<<"Destructor of second class executed!!"<<std::endl;
}
};
int main()
{
test t1(3);
test2 t2(5);
t2.show();
stu s1(10);
disp(s1,t1);
return 0;
}
void test2::show(void)
{
std::cout<<"Value of k = "<<b<<std::endl;
}
void disp(stu s2, test t2)
{
int sum;
sum = s2.z + t2.x;
std::cout<<"Sum = "<<sum<<std::endl;
}

Answer 1

Try to define the disp function before the main function :
void disp(stu s2, test t2)
{
int sum;
sum = s2.z + t2.x;
std::cout<<"Sum = "<<sum<<std::endl;
}
int main()
{
test t1(3);
test2 t2(5);
t2.show();
stu s1(10);
disp(s1,t1);
return 0;
}
and change the disp function signature as:
friend void disp(stu,test);