The inheritance mode sits in the declaration class child : <mode> parent; it controls how the parent’s members are demoted for outsiders.

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#include <iostream>

class Base {
public:
  int pub = 1;

protected:
  int prot = 2;

private:
  int priv = 3;
};

// Whether a super-class member is accessible inside child-class or not is solely
// determined by super-class access level. Therefore, the child-class of any
// inheritance mode can access pub and prot.

// In contrast, the inheritance mode demotes super-class members: the effective
// access is the stricter of the member's level and the mode's level
// (public > protected > private), applied only to outsiders (external code and
// further derived classes).
//
//   Base member level | PUBLIC inherit | PROTECTED inherit | PRIVATE inherit
//   ------------------+----------------+-------------------+----------------
//   public              public          protected          private
//   protected           protected       protected          private
//   private             private         private            private
//
// 1) PUBLIC: access levels pass through unchanged.
class Pub : public Base {
public:
  void inside() {
    std::cout << pub << prot; // OK: pub public, prot protected
    // priv;                    // ERROR: private in Base
  }
};

// 2) PROTECTED: Base's public members demoted to protected.
class Prot : protected Base {
public:
  void inside() {
    std::cout << pub << prot; // OK: both protected now
  }
};

// 3) PRIVATE: everything collapses to private in Priv.
class Priv : private Base {
public:
  void inside() {
    std::cout << pub << prot; // OK: private in Priv, visible here
  }
};

int main() {
  Pub p;
  (void)p.pub;                   // OK: public
  // p.prot;                     // ERROR: protected

  Prot t;
  // t.pub;                      // ERROR: demoted to protected

  Priv v;
  // v.pub;                      // ERROR: demoted to private
}