私有成员只能从类内部访问，受保护成员可以在类和派生类中访问。这是面向对象语言中继承的一个特性。

在c++中可以有私有、受保护和公共继承，这将决定哪些派生类可以在继承层次结构中访问。例如，c#只有公共继承。

MFC偏爱protected的原因是因为它是一个框架。您可能想要子类化MFC类，在这种情况下，需要一个受保护的接口来访问类的一般用途不可见的方法。

受保护的非静态基类成员可以被派生自该基类的任何类的成员和友元访问，方法是使用以下方法之一:

指向直接或间接派生类的指针 对直接或间接派生类的引用 直接或间接派生类的对象

The protected keyword specifies access to class members in the member-list up to the next access specifier (public or private) or the end of the class definition. Class members declared as protected can be used only by the following: Member functions of the class that originally declared these members. Friends of the class that originally declared these members. Classes derived with public or protected access from the class that originally declared these members. Direct privately derived classes that also have private access to protected members. When preceding the name of a base class, the protected keyword specifies that the public and protected members of the base class are protected members of its derived classes. Protected members are not as private as private members, which are accessible only to members of the class in which they are declared, but they are not as public as public members, which are accessible in any function. Protected members that are also declared as static are accessible to any friend or member function of a derived class. Protected members that are not declared as static are accessible to friends and member functions in a derived class only through a pointer to, reference to, or object of the derived class.

保护(c++)

Private =只能由母类(基类)访问 (只有我父母才能进我父母的卧室)

Protected =母类(基类)和她的子类可以访问 (只有父母可以进父母的卧室，但允许儿子/女儿进父母的卧室)

Public =母类(基类)、子类和其他所有人都可以访问 (只有我父母才能进我父母的卧室，但那是家庭聚会——我的家是我的家)

可以从派生类访问受保护的成员。私立学校则不行。

class Base {

private: 
  int MyPrivateInt;
protected: 
  int MyProtectedInt;
public:
  int MyPublicInt;
};

class Derived : Base
{
public:
  int foo1()  { return MyPrivateInt;} // Won't compile!
  int foo2()  { return MyProtectedInt;} // OK  
  int foo3()  { return MyPublicInt;} // OK
};‌‌

class Unrelated 
{
private:
  Base B;
public:
  int foo1()  { return B.MyPrivateInt;} // Won't compile!
  int foo2()  { return B.MyProtectedInt;} // Won't compile
  int foo3()  { return B.MyPublicInt;} // OK
};

至于“最佳实践”，则视情况而定。如果有人想要从您现有的类中派生一个新类，并且需要访问内部成员，那么将它们设置为Protected，而不是Private。如果它们是私有的，那么您的类就很难轻易继承。