参考：

C++中的private, public, protected_c++ private-CSDN博客
https://www.cnblogs.com/corineru/p/11001242.html

C++ 中 Private、Public 和 Protected 的区别

Private	Public	Protected
声明为private类成员只能由基类内部的函数访问。	可以从任何地方访问声明为public的类成员。	声明为protected类成员可以在基类及其派生类中访问。
private成员使用关键字private后跟冒号 (:) 字符来声明。	public成员使用关键字public后跟冒号 (:) 字符来声明。	protected成员使用关键字protected后跟冒号 (:) 字符来声明。
private成员能被子类继承，但不能被子类的对象直接访问。	public成员可以在子类中被继承并被直接访问。	protected成员可以在子类中被继承并访问。
它为其数据成员提供高安全性。	它不为其数据提供任何类型的安全性。	它提供的安全性低于private访问说明符。
可以在友元功能的帮助下访问private成员。	public成员可以在有或没有友元功能的帮助下访问。	protected成员不能通过友元函数访问，它们在类之外是不可访问的，但它们可以被该类的任何子类（派生类），包括派生类及派生类的子类访问。

继承方式总结：

class Base {public:int x;protected:int y;private:int z;};class PublicDerived: public Base {// x is public and is accessible in PublicDerived class// y is protected and is accessible in PublicDerived class// z is not accessible from PublicDerived, as it is a private member of the base class};class ProtectedDerived: protected Base {// x is protected and is accessible in ProtectedDerived class// y is protected and is accessible in ProtectedDerived class// z is not accessible from ProtectedDerived, as it is a private member of the base class};class PrivateDerived: private Base {// x is private and is only accessible by the member functions of the PrivateDerived class// y is private and is only accessible by the member functions of the PrivateDerived class// z is not accessible from PrivateDerived, as it is a private member of the base class
};