Modify class private outside class without using public method - c++

Question

I am a C++ newbie and I need help with a strange issue (or at least its strange to me)
I have a class as such:
class Myclass {
private:
int A;
// some other stuff...
public:
// constructor and stuff...
void setA(int a);
int* getA_addr();
};
void Myclass::setA(int a){
A = a;
};
int* Myclass::getA_addr(){
return &A;
};
Now, I want to modify A in main() and I am not using any other methods in the class (I did it by using extra methods and now I want to see how I can do it without using those extras). I have a function as such:
void change(int *ptr, int tmp){
*ptr = tmp;
};
In a call to this function, I do the passing as such: change(obj.getA_addr(), other arguments...) where obj is an instance of Myclass.
When done in this way, I receive no compilation errors but I also can't seem to modify A (of obj). As a debug effort, I tried to print the address of A (of obj) by directly calling getA_addr(). I saw that with every call, the function returns a different address. So I am assuming that I am not passing the intended address into the function.
I have no idea why this is happening and would like to know. Also, the way I'm trying to do this is most likely not at all accurate so please, if you can provide a solution, it would be appreciated. Thanks.
EDIT: Here's the most minimal code I could come up with that reproduces the error
#include <iostream>
#define MAX_SIZE 10
using namespace std;
class Student {
private:
int mt1;
public:
Student();
void setMt1(int in_mt1);
int* getMt1();
};
Student::Student() {};
void Student::setMt1(int in_mt1) { mt1 = in_mt1; };
int* Student::getMt1(){ return &mt1; };
class Course {
private:
Student entries[MAX_SIZE];
int num;
public:
Course();
void addStudent(Student in_student);
Student getStudent(int index);
};
Course::Course(){ num = 0; };
void Course::addStudent(Student in_student){
entries[num] = in_student;
num++;
};
Student Course::getStudent(int index){ return entries[index]; };
int main() {
void updateStudentScore(int *uscore, int newscore);
Course mycourse;
Student tmp_student;
tmp_student.setMt1(60);
mycourse.addStudent(tmp_student);
cout<<mycourse.getStudent(0).getMt1()<<"\t"<<*mycourse.getStudent(0).getMt1()<<endl;
updateStudentScore(mycourse.getStudent(0).getMt1(), 90);
cout<<mycourse.getStudent(0).getMt1()<<"\t"<<*mycourse.getStudent(0).getMt1()<<endl;
return 0;
}
void updateStudentScore(int *uscore, int newscore){
*uscore = newscore;
};
I am fairly certain that my understanding of pointers and passing-by-whatevers is lacking and the way I defined functions here is creating the bug. I am sorry to inconvenience you guys. I would appreciate it if you could take a look.

Answer 1

Looking at your student/course code, i notice that Course::getStudent returns a Student rather than a Student &. Basically, that means each time you call getStudent(x), you get a temporary copy of student x, whose getMt1 function will give you a pointer to a temporary field, and any changes you make won't even survive past that statement.
If you have getStudent return a reference or pointer instead, your changes should persist to the Student contained in the array. (They still won't affect tmp_student, though, because addStudent copied it to add it to the array. If you want that reliably, then you need to redo quite a bit of stuff.)