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

Question

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

User · Accepted Answer

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. This question about protected variables might also help.

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

Private   Accessible by class member functions  amp  friend function or friend class  For C   class this is default access specifier   Protected  Accessible by class member functions  friend function or friend class  amp  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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

Private   Accessible by class member functions  amp  friend function or friend class  For C   class this is default access specifier   Protected  Accessible by class member functions  friend function or friend class  amp  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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

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