Difference between private, public, and protected inheritance


What is the difference between public, private, and protected inheritance in C++? All of the questions I've found on SO deal with specific cases.

8/22/2018 1:52:27 AM

Accepted Answer

To answer that question, I'd like to describe member's accessors first in my own words. If you already know this, skip to the heading "next:".

There are three accessors that I'm aware of: public, protected and private.


class Base {
        int publicMember;
        int protectedMember;
        int privateMember;
  • Everything that is aware of Base is also aware that Base contains publicMember.
  • Only the children (and their children) are aware that Base contains protectedMember.
  • No one but Base is aware of privateMember.

By "is aware of", I mean "acknowledge the existence of, and thus be able to access".


The same happens with public, private and protected inheritance. Let's consider a class Base and a class Child that inherits from Base.

  • If the inheritance is public, everything that is aware of Base and Child is also aware that Child inherits from Base.
  • If the inheritance is protected, only Child, and its children, are aware that they inherit from Base.
  • If the inheritance is private, no one other than Child is aware of the inheritance.
9/1/2015 8:08:18 PM

class A 
    int x;
    int y;
    int z;

class B : public A
    // x is public
    // y is protected
    // z is not accessible from B

class C : protected A
    // x is protected
    // y is protected
    // z is not accessible from C

class D : private A    // 'private' is default for classes
    // x is private
    // y is private
    // z is not accessible from D

IMPORTANT NOTE: Classes B, C and D all contain the variables x, y and z. It is just question of access.

About usage of protected and private inheritance you could read here.

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