What is the difference between private and protected members of C++ classes?

What is the difference between private and protected members in C++ 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 favor protected.

What's the difference and which should I use?

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 for a better understanding of the issue. https://stackoverflow.com/questions/37011/protected-member-variables">This question about protected variables might also help.

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.

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.

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.

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(); // will work, simply calls the derived version.
        protFunc(); // will work
        publFunc(); // will return 1 since it's overridden in this class
    }
}

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.

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.

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.

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.

private = accessible by the mothership (base class) only (ie only my parent can go into my parent's bedroom)

protected = accessible by mothership (base class), and her daughters (ie only my parent can go into my parent's bedroom, but gave son/daughter permission to walk into parent's bedroom)

public = accessible by mothership (base class), daughter, and everyone else (ie only my parent can go into my parent's bedroom, but it's a house party - mi casa su casa)

Since no public member function is needed to fetch and update protected members in the derived class, this increases the efficiency of code and reduces the amount of code we need to write. However, programmer of the derived class is supposed to be aware of what he is doing.

private is preferred for member data. Members in C++ classes are private by default.

public is preferred for member functions, though it is a matter of opinion. At least some methods must be accessible. public is accessible to all. It is the most flexible option and least safe. Anybody can use them, and anybody can misuse them.

private is not accessible at all. Nobody can use them outside the class, and nobody can misuse them. Not even in derived classes.

protected is a compromise because it can be used in derived classes. When you derive from a class, you have a good understanding of the base class, and you are careful not to misuse these members.

MFC is a C++ wrapper for Windows API, it prefers public and protected. Classes generated by Visual Studio wizard have an ugly mix of protected, public, and private members. But there is some logic to MFC classes themselves.

Members such as SetWindowText are public because you often need to access these members.

Members such as OnLButtonDown, handle notifications received by the window. They should not be accessed, therefore they are protected. You can still access them in the derived class to override these functions.

Some members have to do threads and message loops, they should not be accessed or override, so they are declared as private

In C++ structures, members are public by default. Structures are usually used for data only, not methods, therefore public declaration is considered safe.

Private : Accessible by class member functions & friend function or friend class. For C++ class this is default access specifier.

Protected: Accessible by class member functions, friend function or friend class & derived classes.

• You can keep class member variable or function (even typedefs or inner classes) as private or protected as per your requirement.
• Most of the time you keep class member as a private and add get/set functions to encapsulate. This helps in maintenance of code.
• Generally private function is used when you want to keep your public functions modular or to eliminate repeated code instead of writing whole code in to single function. This helps in maintenance of code.

Refer this link for more detail.

• Private: It is an access specifier. By default the instance (member) variables or the methods of a class in c++/java are private. During inheritance, the code and the data are always inherited but is not accessible outside the class. We can declare our data members as private so that no one can make direct changes to our member variables and we can provide public getters and setters in order to change our private members. And this concept is always applied in the business rule.

• Protected: It is also an access specifier. In C++, the protected members are accessible within the class and to the inherited class but not outside the class. In Java, the protected members are accessible within the class, to the inherited class as well as to all the classes within the same package.

Private member can be accessed only in same class where it has declared where as protected member can be accessed in class where it is declared along with the classes which are inherited by it .

A protected nonstatic base class member can be accessed by members and friends of any classes derived from that base class by using one of the following:

• A pointer to a directly or indirectly derived class
• A reference to a directly or indirectly derived class
• An object of a directly or indirectly derived class

> The protected keyword specifies access to class members in the > member-list up to the next access specifier (public or private) or the > end of the class definition. Class members declared as protected can > be used only by the following: > > - Member functions of the class that originally declared these members. > - Friends of the class that originally declared these members. > - Classes derived with public or protected access from the class that originally declared these members. > - Direct privately derived classes that also have private access to protected members. > > > When preceding the name of a base class, the protected keyword > specifies that the public and protected members of the base class are > protected members of its derived classes. > > Protected members are not as private as private members, which are > accessible only to members of the class in which they are declared, > but they are not as public as public members, which are accessible in > any function. > > Protected members that are also declared as static are accessible to > any friend or member function of a derived class. Protected members > that are not declared as static are accessible to friends and member > functions in a derived class only through a pointer to, reference to, > or object of the derived class.

protected (C++)

> What is the difference between private and protected members in C++ classes?

Other answers have stated:

• public - accessible by all.
• protected - accessible by derived classes (and friends).
• private - restricted.

> What's the difference and which should I use?

The C++ core guidelines gives the advice that data should always be private. I think this is good advice as it makes for 'data spaghetti' when you have derived classes that can access protected data. It makes much more sense for functions to be protected, but it depends on the use case.

For functions you have a choice. For data, you should make it private and provide protected accessor functions if needed. This gives more control over the class data.

private and protected access modifiers are one and same only that protected members of the base class can be accessed outside the scope of the base class in the child(derived)class. It also applies the same to inheritance . But with the private modifier the members of the base class can only be accessed in the scope or code of the base class and its friend functions only''''