Private and Protected Members : C++

Question

Can someone enlighten me as to the difference between private and protected members in classes? I understand from best practice conventions that variables and functions which are not called outside the class should be made private - but looking at my MFC project, MFC seems to favour protected.

Whats the difference and which should I use?

Thanks.

Answer 1

Private members are only accessible within the class defining them.

Protected members are accessible in the class that defines them and in classes that inherit from that class.

Edit: Both are also accessible by friends of their class, and in the case of protected members, by friends of their derived classes.

Edit 2: Use whatever makes sense in the context of your problem. You should try to make members private whenever you can to reduce coupling and protect the implementation of the base class, but if that's not possible then use protected members. Check C++ FAQ Lite for a better understanding of the issue. This question about protected variables might also help.

Answer 2

Attributes and methods marked as protected are -- unlike private ones -- still visible in subclasses.

Unless you don't want to use or provide the possibility to override the method in possible subclasses, I'd make them private.

Answer 3

Protected members can only be accessed by descendants of the class, and by code in the same module. Private members can only be accessed by the class they're declared in, and by code in the same module.

Of course friend functions throw this out the window, but oh well.

Answer 4

private members are only accessible from within the class, protected members are accessible in the class and derived classes. It's a feature of inheritance in OO languages.

You can have private, protected and public inheritance in C++, which will determine what derived classes can access in the inheritance hierarchy. C# for example only has public inheritance.

Answer 5

Protected members can be accessed from derived classes. Private ones can't.

class Base {

private: 
  int myPrivateInt;
protected: 
  int myProtectedInt;
public:
  int myPublicInt;
}

class Derived : Base
{
public:
  int foo1()  { return MyPrivateInt;} // Won't compile!
  int foo2()  { return MyProtectedInt;} // OK  
  int foo3()  { return MyPublicInt;} // OK
};

class Unrelated 
{
private:
  Base B;
public:
  int foo1()  { return B.MyPrivateInt;} // Won't compile!
  int foo2()  { return B.MyProtectedInt;} // Won't compile
  int foo3()  { return B.MyPublicInt;} // OK
};

In terms of "best practice", it depends. If there's even a faint possibility that someone might want to derive a new class from your existing one and need access to internal members, make them Protected, not Private. If they're private, your class may become difficult to inherit from easily.

Answer 6

The reason that MFC favors protected, is because it is a framework. You probably want to subclass the MFC classes and in that case a protected interface is needed to access methods that are not visible to general use of the class.

Answer 7

Public members of a class A are accessible for all and everyone.

Protected members of a class A are not accessible outside of A's code, but is accessible from the code of any class derived from A.

Private members of a class A are not accessible outside of A's code, or from the code of any class derived from A.

So, in the end, choosing between protected or private is answering the following questions: How much trust are you willing to put into the programmer of the derived class?

By default, assume the derived class is not to be trusted, and make your members private. If you have a very good reason to give free access of the mother class' internals to its derived classes, then you can make them protected.

Answer 8

It all depends on what you want to do, and what you want the derived classes to be able to see.

class A
{
private:
    int _privInt = 0;
    int privFunc(){return 0;}
    virtual int privVirtFunc(){return 0;}
protected:
    int _protInt = 0;
    int protFunc(){return 0;}
public:
    int _publInt = 0;
    int publFunc()
    {
         return privVirtFunc();
    }
};

class B : public A
{
private:
    virtual int privVirtFunc(){return 1;}
public:
    void func()
    {
        _privInt = 1; // wont work
        _protInt = 1; // will work
        _publInt = 1; // will work
        privFunc(); // wont work
        privVirtFunc(); // wont work
        protFunc(); // will work
        publFunc(); // will return 1 since it's overridden in this class
    }
}

Answer 9

Sure take a look at the Protected Member Variables question. It is recommended to use private as a default (just like C++ classses do) to reduce coupling. Protected member variables are most always a bad idea, protected member functions can be used for e.g. the Template Method pattern.