How to access private data members outside the cla

问题:

This question already has an answer here:

• Can I access private members from outside the class without using friends? 24 answers

I have a class A as mentioned below:-

class A{
     int iData;
};

I neither want to create member function nor inherit the above class A nor change the specifier of iData.

My doubts:-

• How to access iData of an object say obj1 which is an instance of class A?
• How to change or manipulate the iData of an object obj1?

Note: Don't use friend.

回答1:

You can't. That member is private, it's not visible outside the class. That's the whole point of the public/protected/private modifiers.

(You could probably use dirty pointer tricks though, but my guess is that you'd enter undefined behavior territory pretty fast.)

回答2:

Here's a way, not recommended though

class Weak {
private:
    string name;

public:
    void setName(const string& name) {
        this->name = name;
    }

    string getName()const {
        return this->name;
    }

};

struct Hacker {
    string name;
};

int main(int argc, char** argv) {

    Weak w;
    w.setName("Jon");
    cout << w.getName() << endl;
    Hacker *hackit = reinterpret_cast<Hacker *>(&w);
    hackit->name = "Jack";
    cout << w.getName() << endl;

}

回答3:

---

EDIT:
Just saw you edited the question to say that you don't want to use friend.
Then the answer is:

NO you can't, atleast not in a portable way approved by the C++ standard.

---

The later part of the Answer, was previous to the Q edit & I leave it here for benefit of >those who would want to understand a few concepts & not just looking an Answer to the >Question.

---

If you have members under a Private access specifier then those members are only accessible from within the class. No outside Access is allowed.

An Source Code Example:

class MyClass
{
    private:
        int c;
    public:
    void doSomething()
    {
        c = 10;    //Allowed 
    }
};

int main()
{
    MyClass obj;
    obj.c = 30;     //Not Allowed, gives compiler error
    obj.doSomething();  //Allowed
}

A Workaround: friend to rescue
To access the private member, you can declare a function/class as friend of that particular class, and then the member will be accessible inside that function or class object without access specifier check.

Modified Code Sample:

class MyClass
{
    private:
        int c;

    public:
    void doSomething()
    {
        c = 10;    //Allowed 
    }

    friend void MytrustedFriend();    
};

void MytrustedFriend()
{
        MyClass obj;
        obj.c = 10; //Allowed
}

int main()
{
    MyClass obj;
    obj.c = 30;     //Not Allowed, gives compiler error
    obj.doSomething();  //Allowed
    //Call the friend function
    MytrustedFriend();
    return 0;
}

回答4:

Bad idea, don't do it ever - but here it is how it can be done:

int main()
{
   A aObj;
   int* ptr;

   ptr = (int*)&aObj;

   // MODIFY!
   *ptr = 100;
}

回答5:

http://bloglitb.blogspot.com/2010/07/access-to-private-members-thats-easy.html

this guy's blog shows you how to do it using templates. With some modifications, you can adapt this method to access a private data member, although I found it tricky despite having 10+ years experience.

I wanted to point out like everyone else, that there is an extremely few number of cases where doing this is legitimate. However, I want to point out one: I was writing unit tests for a software suite. A federal regulatory agency requires every single line of code to be exercised and tested, without modifying the original code. Due to (IMHO) poor design, a static constant was in the 'private' section, but I needed to use it in the unit test. So the method seemed to me like the best way to do it.

I'm sure the way could be simplified, and I'm sure there are other ways. I'm not posting this for the OP, since it's been 5 months, but hopefully this will be useful to some future googler.

回答6:

In C++, almost everything is possible! If you have no way to get private data, then you have to hack. Do it only for testing!

class A {
     int iData;
};

int main ()
{
    A a;
    struct ATwin { int pubData; }; // define a twin class with public members
    reinterpret_cast<ATwin*>( &a )->pubData = 42; // set or get value

    return 0;
}

回答7:

friend is your friend.

class A{
    friend void foo(A arg);
    int iData;
};

void foo(A arg){
     // can access a.iData here
}

If you're doing this regularly you should probably reconsider your design though.

回答8:

Start making friends of class A. e.g.

void foo ();

class A{
  int iData;
  friend void foo ();
};

Edit:

If you can't change class A body then A::iData is not accessible with the given conditions in your question.

回答9:

iData is a private member of the class. Now, the word private have a very definite meaning, in C++ as well as in real life. It means you can't touch it. It's not a recommendation, it's the law. If you don't change the class declaration, you are not allowed to manipulate that member in any way, shape or form.

回答10:

There's no legitimate way you can do it.

回答11:

It's possible to access the private data of class directly in main and other's function...

here is a small code...

class GIFT
{
    int i,j,k;

public:
    void Fun() 
    {
        cout<< i<<" "<< j<<" "<< k;
    }

};

int main()
{
     GIFT *obj=new GIFT(); // the value of i,j,k is 0
     int *ptr=(int *)obj;
     *ptr=10;
     cout<<*ptr;      // you also print value of I
     ptr++;
     *ptr=15;
     cout<<*ptr;      // you also print value of J
     ptr++;
     *ptr=20; 
     cout<<*ptr;      // you also print value of K
     obj->Fun();
}

回答12:

access private members outside class ....only for study purpose .... This program accepts all the below conditions "I dont want to create member function for above class A. And also i dont want to inherit the above class A. I dont want to change the specifier of iData."

//here member function is used only to input and output the private values ... //void hack() is defined outside the class...

//GEEK MODE....;)
#include<iostream.h>
#include<conio.h>

    class A
    {
    private :int iData,x;
    public: void get()             //enter the values
        {cout<<"Enter iData : ";
            cin>>iData;cout<<"Enter x : ";cin>>x;}

        void put()                               //displaying values
    {cout<<endl<<"sum = "<<iData+x;}
};

void hack();        //hacking function

void main()
{A obj;clrscr();
obj.get();obj.put();hack();obj.put();getch();
}

void hack()         //hack begins
{int hck,*ptr=&hck;
cout<<endl<<"Enter value of private data (iData or x) : ";
cin>>hck;     //enter the value assigned for iData or x
for(int i=0;i<5;i++)
{ptr++;
if(*ptr==hck)
{cout<<"Private data hacked...!!!\nChange the value : ";
cin>>*ptr;cout<<hck<<" Is chaged to : "<<*ptr;
return;}
}cout<<"Sorry value not found.....";
}