Access specifiers define the accessibility of class members. There are 3 specifiers:

  • public: members accessible outside the class.
  • private: members only accessible within the class and friends.
  • protected: members only accessible within the class, friends, and its derived classes.

Members written beneath an access specifier have that specifier until the next access specifier or end of class definition:

class Base {
public:
  int a = 1;

protected:
  int b = 2;

private:
  int c = 3;
};

Inheritance

Class inheritance can also have access specifiers which redefines the accessibility of inherited members. The rule-of-thumb is member accessibility is capped at the access specifier of inheritance.

class PublicDerived : public Base {
  // a is public
  // b is protected
  // c is inaccessible to PublicDerived
};

class ProtectedDerived : protected Base {
  // a is protected
  // b is protected
  // c is inaccessible to ProtectedDerived
};

class PrivateDerived : private Base {
  // a is private
  // b is private
  // c is inaccessible to PrivateDerived
};

Class vs Struct

Classes and structs are identical except for their default access specifier.

  • Classes are by default private and structs by default public.
  • Class inheritance is by default private and structs by default public. 
    class ClassDerived : Base;
struct StructDerived : Base;

References