protected

I recently discovered that a method in a derived class can only access the base class's protected instance members through an instance of the derived class (or one of its subclasses): class Base { protected virtual void Member() { } } class MyDerived : Base { // error CS1540 void Test(Base b) { b.Member(); } // error CS1540 void Test(YourDerived yd) { yd.Member(); } // OK void Test(MyDerived md) { md.Member(); } // OK void Test(MySuperDerived msd) { msd.Member(); } } class MySuperDerived : MyDerived { } class YourDerived : Base { } I managed to work around this restriction by adding a static

Below is a subtle example of accessing an instance's protected field x. B is a subclass of A so any variable of type B is also of type A. Why can B::foo() access b's x field, but not a's x field? class A { protected: int x; }; class B : public A { protected: A *a; B *b; public: void foo() { int u = x; // OK : accessing inherited protected field x int v = b->x; // OK : accessing b's protected field x int w = a->x; // ERROR : accessing a's protected field x } }; Here is the error I get with g++ $ g++ -c A.cpp A.cpp: In member function ‘void B::foo()’: A.cpp:3: error: ‘int A::x’ is protected A

I have two classes in two different packages: package package1; public class Class1 { public void tryMePublic() { } protected void tryMeProtected() { } } package package2; import package1.Class1; public class Class2 extends Class1 { doNow() { Class1 c = new Class1(); c.tryMeProtected(); // ERROR: tryMeProtected() has protected access in Class1 tryMeProtected(); // No error } } I can understand why there is no error in calling tryMeProtected() since Class2 sees this method as it inherits from Class1 . But why isn't it possible for an object of Class2 to access this method on an object of Class1

Should you ever use protected member variables? What are the the advantages and what issues can this cause? Should you ever use protected member variables? Depends on how picky you are about hiding state. If you don't want any leaking of internal state, then declaring all your member variables private is the way to go. If you don't really care that subclasses can access internal state, then protected is good enough. If a developer comes along and subclasses your class they may mess it up because they don't understand it fully. With private members, other than the public interface, they can't

I was going through this question Is there a way to override class variables in Java? The first comment with 36 upvotes was: If you ever see a protected static , run. Can anyone explain why is a protected static frowned upon? Tim B It's more a stylistic thing than a direct problem. It suggests that you haven't properly thought through what is going on with the class. Think about what static means: This variable exists at class level, it does not exist separately for each instance and it does not have an independent existence in classes which extend me . Think about what protected means: This

Method visibility in Ruby (public, protected, and private methods) has been well explained in places like this blog post . But in Ruby on Rails it seems slightly different than it would be in a regular Ruby application because of the way the framework is set up. So, in Rails models, controllers, helpers, tests, etc., when is/isn't it appropriate to use protected or private methods? Edit : Thanks for the answers so far. I understand the concept of protected and private in Ruby, but I'm looking more for an explanation of the typical way those types of visibility are used within the context of