Programming Forums > Application Development > C++

Fall Back Son · Mar 31st, 2008, 10:49 PM

Is it possible to edit private data members in C++? So if I had code like the following, would it be possible to edit it from another class ?

class myClass {

private:
float myFloat;

};

My initial thought is that yes, it would be possible, because C++ does not offer protection for pointer-related access. So I could use a pointer to change the value stored in myFloat. But I'm not sure about this, I use Java generally, not C++. Hopefully someone can help.

Thanks

Jimbo · Mar 31st, 2008, 11:00 PM

There's two ways to change it, and one is supported by the language. That is to declare a friend class, as such:

(Toggle Plain Text)

class foo {
  float myFloat;
  friend class bar;
};

class bar {
public:
  void DoFoo(foo& f) {
    foo.myFloat = 0.0f;
  }
};

The other way is a complete hack and can't be trusted: just get the memory location of the object and start swapping bits

Jabo · Apr 1st, 2008, 3:19 AM

friend classes can access private data? I thought private data was only accessible to the owning class. I was taught in school to use public methods to access private data as this enforces data hiding.

Jimbo · Apr 1st, 2008, 11:09 AM

Quote:

Originally Posted by Jabo

friend classes can access private data? I thought private data was only accessible to the owning class. I was taught in school to use public methods to access private data as this enforces data hiding.

What you were taught is the recommended practice, but sometimes you want something to access the private data. For instance, if you write a custom string class and you want to overload the input operator, which is a global function, you'll want it to access you private data, but you don't want to make said data available via a public method. You'll have something like this (forgive me if the syntax is off, I haven't actually used C++ for a long time):

 cpp Syntax (Toggle Plain Text) 
class MyString {
  char* underlyingData;
  friend istream& operator>> (istream&, MyString&);
// more implementation here
};
 
istream& operator>> (istream& in, MyString& str) {
  // get input and put it in str.underlyingData; resize as necessary
  // ...
  return in;
}
class MyString {
  char* underlyingData;
  friend istream& operator>> (istream&, MyString&);
// more implementation here
};

istream& operator>> (istream& in, MyString& str) {
  // get input and put it in str.underlyingData; resize as necessary
  // ...
  return in;
}

Just remember, friends can see each other's privates

(note that friendship is a one-way relationship though)

Soulstorm · Apr 3rd, 2008, 9:21 AM

Quote:

Originally Posted by Jabo

friend classes can access private data? I thought private data was only accessible to the owning class. I was taught in school to use public methods to access private data as this enforces data hiding.

Everything exists for a reason. Friend functions can be friends to more than one class, so they can have access to the private members of objects of different classes. That makes the, very useful in situations where accessing private members of many classes are the target.

The alternative would be to use getter methods. However, despite what teach you in school, not always is this the best programming approach. For small classes, where data hiding is not necessary, not making all of the variables public can result in code bloat. For the time being, stick to your school's approach. Later, you will see that not always will you use private members.

Fall Back Son · Apr 3rd, 2008, 2:47 PM

Quote:

Originally Posted by Soulstorm

Everything exists for a reason. Friend functions can be friends to more than one class, so they can have access to the private members of objects of different classes. That makes the, very useful in situations where accessing private members of many classes are the target.

Yes... it seems that a lot of things exist for the purpose of a headache.

peaceofpi · Apr 4th, 2008, 2:30 PM

Quote:

Originally Posted by Jimbo

Just remember, friends can see each other's privates

(note that friendship is a one-way relationship though)

Hahahaha.

Mar 31st, 2008, 10:49 PM	#1
Fall Back Son Professional Programmer Join Date: Oct 2006 Posts: 311 Rep Power: 3	Simple Question about C++ Is it possible to edit private data members in C++? So if I had code like the following, would it be possible to edit it from another class ? class myClass { private: float myFloat; }; My initial thought is that yes, it would be possible, because C++ does not offer protection for pointer-related access. So I could use a pointer to change the value stored in myFloat. But I'm not sure about this, I use Java generally, not C++. Hopefully someone can help. Thanks

Mar 31st, 2008, 11:00 PM	#2
Jimbo Battle Programmer Join Date: Feb 2006 Location: Bellevue, WA, USA Posts: 770 Rep Power: 3	Re: Simple Question about C++ There's two ways to change it, and one is supported by the language. That is to declare a friend class, as such: (Toggle Plain Text) class foo { float myFloat; friend class bar; }; class bar { public: void DoFoo(foo& f) { foo.myFloat = 0.0f; } }; class foo { float myFloat; friend class bar; }; class bar { public: void DoFoo(foo& f) { foo.myFloat = 0.0f; } }; The other way is a complete hack and can't be trusted: just get the memory location of the object and start swapping bits __________________ <insert disclaimer here> <insert shameless plug for Visual Studio here>

Apr 1st, 2008, 3:19 AM	#3
Jabo Not a user? Join Date: Sep 2007 Posts: 308 Rep Power: 2	Re: Simple Question about C++ friend classes can access private data? I thought private data was only accessible to the owning class. I was taught in school to use public methods to access private data as this enforces data hiding.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
very simple question. make button change number in edit box	nickm	Delphi	2	Apr 29th, 2006 11:47 PM
A simple programming question	punter	C#	8	Jan 5th, 2006 5:04 PM
Simple c# question	nez	C#	8	Jul 1st, 2005 7:29 PM
Simple (stupid cause I don't know it) basic question	massive-war	C++	17	Apr 12th, 2005 12:03 AM
Simple Writing to Files. Newbie Question	kiaran	C++	1	Feb 8th, 2005 4:22 AM

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Programming Forums > Application Development > C++

Simple Question about C++