Is it possible to access private members of a class?

Question

Is it possible to access private members of a class in c++.

provided you don't have a friend function and You don't have access to the class definition

just some interview question...actually i too agree with no sense in doing that — yesraaj, Apr 08 '09 at 10:25
rajKumar i don't know a solution for your updated question :) — Johannes Schaub - litb, Apr 08 '09 at 10:45
Maybe if you explained why you want to do it. Then we can provide more lucid answers. But if you have an object there is not protection in the language for malicious cast of the pointer to something else. — Martin York, Apr 08 '09 at 15:20

score 8 · Accepted Answer · edited May 23 '17 at 12:10

8

You mean using some pointer arithmetic to gain the access ? It is possible but is definitely dangerous. Take a look at this question also: Accessing private members

edited May 23 '17 at 12:10

Community

1
1

answered Apr 08 '09 at 10:23

Naveen

74,600
47
176
233

1

that answer on the other question was accepted, but it's incorrect. it's not possible to do so, but if it works it's by pure accident: undefined behavior happened. – Johannes Schaub - litb Apr 08 '09 at 10:36
Yes, edited the link so that it points to the 'correct' answer. – Naveen Apr 08 '09 at 10:40
oh, nice :) btw i remembered wrongly about that accepted answer. doesn't look too bad imho. but the now linked to answer is better imho. cheers – Johannes Schaub - litb Apr 08 '09 at 10:43

bernhardrusch · Answer 2 · 2014-12-10T07:37:38.463

6

I think there was some old school trick like this:

#define private public
#include "header.h"
#define private private

But you are strongly discouraged to do this (I've read quickly that the said something about this in the C++ standard) - if you want to read more about this google for "#define private public"

edited Dec 10 '14 at 07:37

answered Apr 08 '09 at 11:08

bernhardrusch

11,670
12
48
59

I've not defined a class in two header files. Is it possible to declare public members in one file and private in another and only deliver the public header? If so, we have absolutely no idea what else is in the class. – Les Apr 08 '09 at 12:23
You could use something like the PIMPL idiom http://www.gotw.ca/gotw/024.htm to hide the private parts – bernhardrusch Apr 08 '09 at 15:27

seq · Answer 3 · 2009-04-08T11:24:16.550

Well I might be talking rubish, but I think you could try to define a "twin" class with same members as the class you want to modify but different public/private modifiers and then use reintepret_cast to cast the original class to yours in which you can access the private members.

Its a bit hacky ;-)

A bit of code to explain the idea:

class ClassWithNoAccess 
{
public:
  someMethod();

private:
  int someVar;
};

class ClassTwin 
{
public:
  someMethod();

public:
  int someVar;
}

and somewhere in the code:

ClassWithNoAccess* noAccess = new ClassWithNoAccess();
ClassTwin* twin = reinterpret_cast<ClassTwin *>(noAccess);
twin->someVar = 1;

edit: so like someone already wrote before, this might work but the standard does not guarantee the order of the variables with public and private modifier will be the same

You must be careful if you intend in going that way. Changing your private to public methods can turn a regular class into an aggregate or a POD, effectively changing how the compiler deals with it. — David Rodríguez - dribeas, Apr 08 '09 at 13:13

score 2 · Answer 4 · answered Apr 08 '09 at 13:19

Even if it were possible through some nasty hack - see earlier posts - you SHOULD not do it.

Encapsulation exists for a very good purpose, and setting class member as private means that the developer did not intend anyone to mess around with that member. That should mean

"You don't have to access this member in order to use the public interface to it's full intended extent"

Is it possible to access private members of a class?

4 Answers4

Linked

Related