accessing private members without using friend class [duplicate] - c++

Question

This question already has answers here:
Can I access private members from outside the class without using friends?
(27 answers)
Closed 2 years ago.
I have a homework in which:
we have this code:
#include <iostream>
using namespace std;
class Test {
int x;
char y;
public:
Test() :x(0), y(0) { ; }
};
int main() {
Test t;
//Do stuff!
return 0;
}
and without adding getters and setters or using friend class we have to read x and y and also change them.
I searched and found these ways:
if there was a template function in my class I could say:
class Test {
int x;
char y;
public:
Test() :x(0), y(0) { ; }
template<typename T>
void do_something() {//not necessarily void function
//Do some stuff
};
};
class a;
// My specialization.
template <>
void Test::do_something<a>() {
cout << x << endl;
cout << y << endl;
// getting data
x = 5;
y = 'a';
// changing data
cout << x << endl;
cout << y << endl;
// getting data after changes we made
}
int main() {
Test t;
t.do_something<a>();
return 0;
}
and also the method, which I think is this question answer, is using pointers.
like this:
class Test {
int x;
char y;
public:
Test() :x(0), y('0') { ; }
};
int main() {
Test t;
int* ptr = (int*)&t;
cout << "x = " << *ptr << " y = " << (char)*(ptr + 1) << endl;
*ptr--;
//getting data
*ptr = 12;
ptr++;
*ptr = 65;
//changing data
ptr--;
cout << "x = " << *ptr << " y = " << (char)*(ptr + 1) << endl;
//getting data after changes we have made
return 0;
}
or using reinterpret_cast and pointers:
struct pointer {
int x;
char y;
};
class Test {
int x;
char y;
public:
Test() :x(0), y('0') { ; }
};
int main()
{
Test t;
pointer* p = reinterpret_cast<pointer*>(&t);
cout << "X = " << p->x << " Y = " << p->y << endl;
//getting data
p->x = 5;
p->y = 'a';
//changing data
cout << "X = " << p->x << " Y = " << p->y << endl;
//getting data from class after changing them with pointers
return 0;
}
my questions are:
is such thing possible in other object oriented languages?
does this mean access modifiers are useless?
and is there anything we can do to prevent such thing to happen?

Answer 1

(with pointers) why this happen?
I don't understand this one, so I will skip it.
is such thing possible in other object oriented languages?
Consider python. In python making something private is explicitly only an agreement between the author and the user, but nothing prevents a user from accessing private members. Though, they should not. C++ isn't that explicit about saying "if you want you can access private members", but still it is possible with some effort. Nevertheless you should not. C++ does not prevent you from shooting yourself in your foot and accessing private members is one way of doing that. It isn't the case in your example, but typically accessing private members directly will break the object beyond repair.
does this mean access modifiers are useless?
I'll repeat my comment: Is a traffic light useless? I mean when it is red I can still cross the street. Access specifiers are not there to prevent you from doing something wrong by all means, they are to help you to avoid doing something wrong (and if you try hard you can still do something wrong).
and is there anything we can do to prevent such thing to happen?
Declaring a member as private is enough to signal that a user should not access the member directly by any means. If someone wants to break that agreement then they can do it. You cannot prevent a user from doing something wrong. If they want to break your class they can do so. However, it is not your responsibility to guarantee that something broken still works as expected. If a user bypasses access specifiers then they broke the agreement between them and you. Consider you buy a laptop and throw it out of the window from 42th floor. Will you complain to the manufacturer that afterwards the laptop is not working properly anymore? I guess no, instead you will understand that you made something wrong with using your laptop.
PS: Your last two examples are undefined behavior. reinterpret_cast is not a way to cast between arbitrary types magically. The set of allowed casts and what you can do with the results is in fact rather limited (see here). Also a c-style cast enables you do to casts that can be very wrong, without your compiler complaining about it. Thats why they should be avoided in favor of the proper c++ casts (static_cast et al).

Answer 2

Answering your question 4, is there anything we can do to prevent such thing to happen?:
It is indeed a language design problem that code using a class in C++ is typically able to see the inner makeup of a class. A visible, complete class definition is clearly a breach of information hiding. It's necessary though because of the "by-value semantics" of C++ that it inherited from C and which distinguishes it from, say, C# or Java.
One of the consequences is what you describe: That users can easier access object data they are not opposed to. (To be fair, with enough malicious energy that is unpreventable in the general sense no matter the precautions, but knowing the class layout allows you to do so with less "criminal effort", in this case through normal language means. Another, even simpler way which I recall was buried in one large project when it went open source was to simply #define private public before including the header in question.)
A second, more relevant problem is that code which uses objects of that class, or one of its descendants, is too tightly coupled with that class; it knows more than it should or needs to. Any trivial change to the class makes it necessary to recompile all code which includes its definition, directly or indirectly. For large projects with elaborate class hierarchies touching a base class may cause a senseless re-build of the whole project.1
To finally answer your question: The canonical C++ strategy to reduce this coupling are compilation firewalls. You essentially define an interface of pure virtual functions and no data, which should be relatively stable. User code sees only that interface. By that you gain information hiding and the power of polymorphism. But because you cannot directly handle objects any longer but only pointers or references, you lose the advantages of C++'s by-value paradigm (speed, no aliasing).
1 In a job interview in 1998 or so as C++ developer at Star Division, which was developing StarOffice, the original precursor to OpenOffice and LibreOffice, I was asked: "You have a base class, directly or indirectly used throughout the project. Now you would like to add a virtual function to it but avoid recompilation of the whole project, because it would just take too long. Can you do that? How?" The answer is that most implementations probably maintain the virtual functions in a vtable to which you can append without changing the offsets of existing functions (and, of course, without altering the object layout). Obviously, there is no guarantee that the implementation does not generate the vtable backwards, or employs some other mechanism, but in practice that's what you can do.