compilation error due to private variable declaration inehritance - c++

Question

I get the following error compiling the code below:
Student.cpp:20:9: error: ‘std::string Student::name’ is private within this context
20 | name = _name;
| ^~~~
Student.cpp:7:12: note: declared private here
7 | string name;
#include <iostream>
using namespace std;
class Student
{
private:
string name;
};
class UndergraduateStudent : public Student
{
};
class GraduateStudent : public Student
{
};
class Freshman : public UndergraduateStudent
{
Freshman(string _name)
{
name = _name;
}
};
What could be the reason for this?
I want to keep UndergraduateStudent class as abstract

Answer 1

I had a hard time understanding public, private, and protected when I was learning object-oriented programming in C++, and thought that I would share a few thoughts to hopefully clear some of this up.
First of all, public, private, and protected are used to determine the level of accessibility of a variable or method in a class. If you declare a variable as private, you can access that variable inside of the class that it was declared in, but nowhere else. With protected, you may access that variable in the child classes of the class it was declared in. And with public, you can access that variable anywhere.
So what does this mean?
Using your code, we start off with this:
#include <iostream>
using namespace std;
class Student
{
private:
string name;
};
class UndergraduateStudent : public Student
{
};
class GraduateStudent : public Student
{
};
class Freshman : public UndergraduateStudent
{
Freshman(string _name)
{
name = _name;
}
};
The reason that you got a compile error is that you tried to access the variable "name" in the Freshman class. Freshman is a child class of Student (well, it's actually a child class of Undergraduate Student which is a child of student, but the concept still applies). Because of this, any method in the Freshman class will only be able to access protected and public variables and methods in the Student class. Therefore, if you want to access "name" in the Freshman class, you must change it from private to either protected or public.
However, after doing this, you might wonder how to access private and protected variables from outside of the class? In main() how would you get the value of a freshman's name if it is private or protected? The answer to this is getters and setters. Modifying your code a little bit, you get something like this:
#include <iostream>
using namespace std;
class Student
{
protected:
string name;
public:
string getName(){
return name;
}
void setName(string _name){
name = _name;
}
};
class UndergraduateStudent : public Student
{
};
class GraduateStudent : public Student
{
};
class Freshman : public UndergraduateStudent
{
public:
Freshman(){}
Freshman(string _name)
{
name = _name;
}
};
As you can see, I added a couple of public methods in Student, getName() and setName(). I also added some constructors in the Freshman class. The important part about these Student methods is that they are public, and can be accessed anywhere. It's not difficult code to understand, essentially getName() returns the value of the private variable "name", and setName() is able to change that value.
So lets say that in main() I wanted to create a Freshman object, and declare her name as Sarah, and print out that result.
int main(){
Freshman freshman("Sarah");
cout << freshman.name;
}
If I did this, I would get a compile error, because I am trying to access the protected variable "name". If I wanted to access the value, I would have to access it through the public method getName(). Therefore:
int main(){
Freshman freshman("Sarah");
cout << freshman.getName();
}
This would print out the correct result of "Sarah". Now, let's say that I spelled her name wrong, and it turns out there's no 'h' in her name. Then, I would have to call the public method of setName in order to change her name to something else.
int main(){
Freshman freshman("Sarah");
freshman.setName("Sara");
cout << freshman.getName();
}
This would now output the value of "Sara". These are just some basic examples of how to use private, protected, and public, sorry if it was a little bit lengthy. I hope that it can help you if you don't quite understand exactly how to use them yet. Just practice, it comes after doing it a few times.