CODEWITHSUNDEEP
c++classpointers
Saurabh Saluja
Saurabh Saluja

Reputation: 290

Accessing private member of class in C++

I have following code that can access and change private member outside the class

#include<iostream>
#include<conio.h>
using namespace std;

class Test
{
private:
    int data;
public:
    Test() { data = 0; }
    int getData() { return data; }
};

int main()
{
    Test t;
    int* ptr = (int*)&t;
    *ptr = 10;
    cout << t.getData();
    getch();
    return 0;
}

It is basically against encapsulation .. I wish to know why such a technique exist to access private members? how this is happening beneath .. please explain???

If such a technique is there what's the use of making a member private??

Upvotes: 1

Views: 208

Answers (2)

user2709465
user2709465

Reputation:

Short answer: It doesn't work.


Why "works"?

The size of your object is 4 bytes (cout << size(t);), which is the size of the integer. This is why this appears to work, when you say this int* ptr = (int*)&t;, you are saying that this 4 bytes of t will be 4 bytes of int.

Beneath the code

This class is a POD (ignoring the constructor) (see more)

The function int getData() { return data; }, is inlined by the compiler (I'm not sure) so return data returns the value of *ptr not data.

Test it with int getData() { cout << data; } and you will that ptr never touch the value data. This is called undefined behavior.

If your class has this private members (example):

int foobar;
int data;

Then your t class has the size of 8 bytes, and when converting t to int, you're getting the first 4 bytes, which is the variable foobar (see more). And this line cout << t.getData(); will return 0, which is the value specific in the constructor.

Upvotes: 0

Mantosh Kumar
Mantosh Kumar

Reputation: 5741

Because C++ protects against accident rather than deliberate circumvention (fraud). Only hardware can protect against malicious use of a general-purpose language, and even that is hard to do in realistic systems.

Mentioned by Bjarne Stroustrup in his book "The C++ Programming Language".

Upvotes: 3

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