c++类中的私有成员和受保护成员有什么区别?

我从最佳实践惯例中了解到,没有在类外部调用的变量和函数应该是私有的,但看看我的MFC项目,MFC似乎更倾向于受保护。

有什么区别,我应该用哪个?


当前回答

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。

其他回答

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

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。

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。

Private: It is an access specifier. By default the instance (member) variables or the methods of a class in c++/java are private. During inheritance, the code and the data are always inherited but is not accessible outside the class. We can declare our data members as private so that no one can make direct changes to our member variables and we can provide public getters and setters in order to change our private members. And this concept is always applied in the business rule. Protected: It is also an access specifier. In C++, the protected members are accessible within the class and to the inherited class but not outside the class. In Java, the protected members are accessible within the class, to the inherited class as well as to all the classes within the same package.

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。