17.5 Inheritance and access specifiers

https://www.learncpp.com/cpp-tutorial/inheritance-and-access-specifiers/
このセクションではpublic、private、protectedについて詳しく説明します.
パブリックおよびプライベートアクセス記述子を見たことがあります
共通メンバーはどこからでもアクセスでき、プライベートメンバーは同じクラスのメンバー関数からのみアクセスできます.
したがって、派生クラスからベースのプライベートメンバーに直接アクセスすることはできません.

The protected access specifier

protected access speciferはfriendクラスとderivedクラスでアクセス可能なメンバーです
でも外では近づけない

class Base
{
public:
    int m_public {}; // can be accessed by anybody
protected:
    int m_protected {}; // can be accessed by Base members, friends, and derived classes
private:
    int m_private {}; // can only be accessed by Base members and friends (but not derived classes)
};

class Derived: public Base
{
public:
    Derived()
    {
        m_public = 1; // allowed: can access public base members from derived class
        m_protected = 2; // allowed: can access protected base members from derived class
        m_private = 3; // not allowed: can not access private base members from derived class
    }
};

int main()
{
    Base base;
    base.m_public = 1; // allowed: can access public members from outside class
    base.m_protected = 2; // not allowed: can not access protected members from outside class
    base.m_private = 3; // not allowed: can not access private members from outside class

    return 0;
}

上記の例から、保護されたメンバーはderivedからアクセスできますが、mainなどの外部からアクセスできません.

So when should I use the protected access specifier?

protectedのプロパティは派生クラスから直接アクセスできます
baseだけでなくderivedでも変更が可能な場合はprotectedに設定します.

So when should I use the protected access specifier?

継承にも種類があります
パブリック、保護済み、プライベート継承
継承タイプを指定しない場合は、デフォルトでプライベート継承を行います.

Public inheritance

class Base
{
public:
    int m_public {};
protected:
    int m_protected {};
private:
    int m_private {};
};

class Pub: public Base // note: public inheritance
{
    // Public inheritance means:
    // Public inherited members stay public (so m_public is treated as public)
    // Protected inherited members stay protected (so m_protected is treated as protected)
    // Private inherited members stay inaccessible (so m_private is inaccessible)
public:
    Pub()
    {
        m_public = 1; // okay: m_public was inherited as public
        m_protected = 2; // okay: m_protected was inherited as protected
        m_private = 3; // not okay: m_private is inaccessible from derived class
    }
};

int main()
{
    // Outside access uses the access specifiers of the class being accessed.
    Base base;
    base.m_public = 1; // okay: m_public is public in Base
    base.m_protected = 2; // not okay: m_protected is protected in Base
    base.m_private = 3; // not okay: m_private is private in Base

    Pub pub;
    pub.m_public = 1; // okay: m_public is public in Pub
    pub.m_protected = 2; // not okay: m_protected is protected in Pub
    pub.m_private = 3; // not okay: m_private is inaccessible in Pub

    return 0;
}

Protected inheritance

Private inheritance

class Base
{
public:
    int m_public {};
protected:
    int m_protected {};
private:
    int m_private {};
};

class Pri: private Base // note: private inheritance
{
    // Private inheritance means:
    // Public inherited members become private (so m_public is treated as private)
    // Protected inherited members become private (so m_protected is treated as private)
    // Private inherited members stay inaccessible (so m_private is inaccessible)
public:
    Pri()
    {
        m_public = 1; // okay: m_public is now private in Pri
        m_protected = 2; // okay: m_protected is now private in Pri
        m_private = 3; // not okay: derived classes can't access private members in the base class
    }
};

int main()
{
    // Outside access uses the access specifiers of the class being accessed.
    // In this case, the access specifiers of base.
    Base base;
    base.m_public = 1; // okay: m_public is public in Base
    base.m_protected = 2; // not okay: m_protected is protected in Base
    base.m_private = 3; // not okay: m_private is private in Base

    Pri pri;
    pri.m_public = 1; // not okay: m_public is now private in Pri
    pri.m_protected = 2; // not okay: m_protected is now private in Pri
    pri.m_private = 3; // not okay: m_private is inaccessible in Pri

    return 0;
}

A final example

class Base
{
public:
	int m_public {};
protected:
	int m_protected {};
private:
	int m_private {};
};

class D2 : private Base // note: private inheritance
{
	// Private inheritance means:
	// Public inherited members become private
	// Protected inherited members become private
	// Private inherited members stay inaccessible
public:
	int m_public2 {};
protected:
	int m_protected2 {};
private:
	int m_private2 {};
};

class D3 : public D2
{
	// Public inheritance means:
	// Public inherited members stay public
	// Protected inherited members stay protected
	// Private inherited members stay inaccessible
public:
	int m_public3 {};
protected:
	int m_protected3 {};
private:
	int m_private3 {};
};

上の例を見てよく知っておく