Should I write JavaDoc for *private* or *protected* methods? And what about *private* variables?

I see class examples on my Java book and the *private* variables are JavaDoc&#39;ed.
So I can&#39;t understand if it is a good practice to JavaDoc the *private* (or *protected*) methods.

JavaDoc for private / protected methods?

We all know members specified `protected` from a base class can only be accessed from a derived class own instance. This is a feature from the Standard, and this has been discussed on Stack Overflow multiple times:

 - [Cannot access protected member of another instance from derived type&#39;s scope
](https://stackoverflow.com/q/45146025/5470596 &quot;a similar question on Stack Overflow&quot;);
 - [Why can&#39;t my object access protected members of another object defined in common base class?](https://stackoverflow.com/q/17717027/5470596 &quot;a similar question on Stack Overflow&quot;)
 - And others.

But it seems possible to walk around this restriction with member pointers, as user chtz [has shown me](https://stackoverflow.com/a/49538355/5470596 &quot;their answer to a related question&quot;):

    struct Base { protected: int value; };
    struct Derived : Base
    {
        void f(Base const&amp; other)
        {
            //int n = other.value; // error: &#39;int Base::value&#39; is protected within this context
            int n = other.*(&amp;Derived::value); // ok??? why?
            (void) n;
        }
    };

[Live demo on coliru](http://coliru.stacked-crooked.com/a/7680e07f219a73a7 &quot;(external)&quot;)

Why is this possible, is it a wanted feature or a glitch somewhere in the implementation or the wording of the Standard?

---

From comments emerged another question: [if `Derived::f` is called with an actual `Base`](http://coliru.stacked-crooked.com/a/185e06f44865b243), is it undefined behaviour?

Access to protected member through member-pointer: is it a hack?

When and why should I use `public`, `private`, and `protected` functions and variables inside a class? What is the difference between them?

Examples:

    // Public
    public $variable;
    public function doSomething() {
      // ...
    }

    // Private
    private $variable;
    private function doSomething() {
      // ...
    }

    // Protected
    protected $variable;
    protected function doSomething() {
      // ...
    }

What is the difference between public, private, and protected?

In Java, are there clear rules on when to use each of access modifiers, namely the default (package private), `public`, `protected` and `private`, while making `class` and `interface` and dealing with inheritance?

What is the difference between public, protected, package-private and private in Java?

What&#39;s the best way to unit test protected and private methods in Ruby, using the standard Ruby `Test::Unit` framework?

I&#39;m sure somebody will pipe up and dogmatically assert that &quot;you should only unit test public methods; if it needs unit testing, it shouldn&#39;t be a protected or private method&quot;, but I&#39;m not really interested in debating that.  I&#39;ve got several methods that **are** protected or private for good and valid reasons, these private/protected methods are moderately complex, and the public methods in the class depend upon these protected/private methods functioning correctly, therefore I need a way to test the protected/private methods.

One more thing...  I generally put all the methods for a given class in one file, and the unit tests for that class in another file.  Ideally, I&#39;d like all the magic to implement this &quot;unit test of protected and private methods&quot; functionality into the unit test file, not the main source file, in order to keep the main source file as simple and straightforward as possible.


What&#39;s the best way to unit test protected &amp; private methods in Ruby?

I was going through this question [Is there a way to override class variables in Java?][1]
The first comment with 36 upvotes was:
&gt; If you ever see a `protected static`, run.

Can anyone explain why is a `protected static` frowned upon?

  [1]: https://stackoverflow.com/questions/685300/is-there-a-way-to-override-class-variables-in-java

Why we should not use protected static in java

Does anyone have any idea, *why* removeRange method in [AbstractList](http://java.sun.com/javase/6/docs/api/java/util/AbstractList.html) (and also in [ArrayList](http://java.sun.com/j2se/1.5.0/docs/api/java/util/ArrayList.html)) is `protected`?  It looks like a quite well-defined and useful operation, but still, to use it, we&#39;re forced to subclass the List implementation.

Is there some hidden rationale? Seems quite inexplicable to me.

Why is Java&#39;s AbstractList&#39;s removeRange() method protected?

Why can we not define a class as `protected`? 

I know that we can&#39;t, but why? There should be some specific reason.

Why can a class not be defined as protected?

Why would anyone declare a constructor protected? I know that constructors are declared private for the purpose of not allowing their creation on stack.

What are practical uses of a protected constructor?

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?

I know, there are no &#39;real&#39; private/protected methods in Python. This approach isn&#39;t meant to hide anything; I just want to understand what Python does.

&lt;!-- language: python --&gt;

    class Parent(object):
        def _protected(self):
            pass

        def __private(self):
            pass

    class Child(Parent):
        def foo(self):
            self._protected()   # This works

        def bar(self):
            self.__private()    # This doesn&#39;t work, I get a AttributeError:
                                # &#39;Child&#39; object has no attribute &#39;_Child__private&#39;


So, does this behaviour mean, that &#39;protected&#39; methods will be inherited but &#39;private&#39; won&#39;t at all? &lt;br&gt;
Or did I miss anything?



Inheritance of private and protected methods in Python

I am a little confused over the term &quot;package private&quot; that some of the documentation uses, along with the usage of &quot;default access.&quot; Aren&#39;t package-private and default access both synonymous with protected?

Isn&#39;t &quot;package private&quot; member access synonymous with the default (no-modifier) access?

I come from the Java EE world but now I&#39;m working on a .Net project. In Java when I wanted to test a protected method it was quite easy, just having the test class with the same package name was enough.

Is there anything similar for C#? Is there any good practice for unit testing the protected methods? I only found frameworks and people saying that I should test only public methods.

It should be possible to do it without any framework…

Unit testing C# protected methods

Method visibility in Ruby (public, protected, and private methods) has been well explained in places like [this blog post](http://weblog.jamisbuck.org/2007/2/23/method-visibility-in-ruby). 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&#39;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&#39;m looking more for an explanation of the typical way those types of visibility are used within the context of the various pieces of a Rails app (models, controllers, helpers, tests). For example, public controller methods are action methods, protected methods in the application controller are used for &quot;helper methods&quot; that need to be accessed by multiple controllers, etc.

Protected and private methods in Rails

What is the argument against declaring protected-access members on interfaces? This, for example, is invalid:

    public interface IOrange
    {
        public OrangePeel Peel { get; }
        protected OrangePips Seeds { get; }
    }

In this example, the interface `IOrange` would guarantee that implementors *at least* provide an `OrangePips` instance to their inheritors. If the implementor wanted to, they could expand the scope to full `public`:

    public class NavelOrange : IOrange
    {
        public OrangePeel Peel { get { return new OrangePeel(); } }
        protected OrangePips Seeds { get { return null; } }
    }

    public class ValenciaOrange : IOrange
    {
        public OrangePeel Peel { get { return new OrangePeel(); } }
        public OrangePips Seeds { get { return new OrangePips(6); } }
    }


The intent of `protected` members on interfaces is to provide a support contract for *inheritors* (sub-classes), for example:

    public class SpecialNavelOrange : NavelOrange
    {
        ...
        // Having a seed value is useful to me.
        OrangePips seeds = this.Seeds; 
        ...
    }

(Admittedly, this wouldn&#39;t work for `struct`s)

I can&#39;t see much of a case for `private` or `internal` modifiers in interfaces, but supporting both `public` and `protected` modifiers seems perfectly reasonable.

&lt;hr/&gt;

I&#39;m going to try explaining the utility of `protected` members on `interface`s by separating them from `interface`s entirely:

Let&#39;s imagine a new C# keyword, `support`, to enforce inheritor contracts, so that we declare things as follows:

    public support IOrangeSupport
    {
        OrangePips Seeds { get; }
    }

This would allows us to contract classes to provide protected members to their inheritors:

    public class NavelOrange : IOrange, IOrangeSupport
    {
        public OrangePeel Peel { get { return new OrangePeel(); } }
        protected OrangePips Seeds { get { return null; } }
    }

This is not particularly useful, because classes would already imply this contract by providing the `protected` members in the first place.

But then we could also do this:

    public interface IOrange : IOrangeSupport
    {
       ...
    }

Thereby applying `IOrangeSupport` to all classes which implement `IOrange` and requiring them to provide particular `protected` members - which is not something we can currently do.

Why can&#39;t I have protected interface members?

I have a object having some protected property that I want to get and set. The object looks like



    Fields_Form_Element_Location Object
    (
    [helper] =&gt; formText
    [_allowEmpty:protected] =&gt; 1
    [_autoInsertNotEmptyValidator:protected] =&gt; 1
    [_belongsTo:protected] =&gt; 
    

    [_description:protected] =&gt; 
    [_disableLoadDefaultDecorators:protected] =&gt; 
    [_errorMessages:protected] =&gt; Array
        (
        )

    [_errors:protected] =&gt; Array
        (
        )
    [_isErrorForced:protected] =&gt; 
    [_label:protected] =&gt; Current City
    

    [_value:protected] =&gt; 93399
    [class] =&gt; field_container field_19 option_1 parent_1
    )

I want to get `value` property of the object. When I try `$obj-&gt;_value` or `$obj-&gt;value` it generates error. I searched and found the solution to use `PHP Reflection Class`. It worked on my local but on server PHP version is `5.2.17` So I cannot use this function there. So any solution how to get such property?

How to get protected property of object in PHP

In C++, I can&#39;t think of a case in which I would like to inherit private/protected from a 
base class:

    class Base;
    class Derived1 : private Base;
    class Derived2 : protected Base;

Is it really useful?



Why do we actually need Private or Protected inheritance in C++?

I have this test case:

    struct A{ protected: A(){} };
    struct B: A{};
    struct C: A{ C(){} };
    struct D: A{ D() = default; };
    
    int main(){
        (void)B{};
        (void)C{};
        (void)D{};
    }

Both gcc and clang compile it in C++11 and C++14 mode. Both fail in C++17 mode:

    $ clang++ -std=c++17 main.cpp 
    main.cpp:7:10: error: base class &#39;A&#39; has protected default constructor
            (void)B{};
                    ^
    main.cpp:1:22: note: declared protected here
    struct A{ protected: A(){} };
                         ^
    main.cpp:9:10: error: base class &#39;A&#39; has protected default constructor
            (void)D{};
                    ^
    main.cpp:1:22: note: declared protected here
    struct A{ protected: A(){} };
                         ^
    2 errors generated.

    $ clang++ --version
    clang version 6.0.0 (http://llvm.org/git/clang.git 96c9689f478d292390b76efcea35d87cbad3f44d) (http://llvm.org/git/llvm.git 360f53a441902d19ce27d070ad028005bc323e61)
    Target: x86_64-unknown-linux-gnu
    Thread model: posix

(clang compiled from master Branch 2017-12-05.)

    $ g++ -std=c++17 main.cpp 
    main.cpp: In function &#39;int main()&#39;:
    main.cpp:7:10: error: &#39;A::A()&#39; is protected within this context
      (void)B{};
              ^
    main.cpp:1:22: note: declared protected here
     struct A{ protected: A(){} };
                          ^
    main.cpp:9:10: error: &#39;A::A()&#39; is protected within this context
      (void)D{};
              ^
    main.cpp:1:22: note: declared protected here
     struct A{ protected: A(){} };
                          ^

    $ g++ --version
    g++ (GCC) 8.0.0 20171201 (experimental)
    Copyright (C) 2017 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.  There is NO
    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Is this change of behavior part of C++17 or is it a bug in both compilers?


Changed rules for protected constructors in C++17?

What is the difference between `private` and `protected` members in C++ classes?
   
I understand from best practice conventions that variables and functions which are not called outside the class should be made `private`&amp;mdash;but looking at my MFC project, MFC seems to favor `protected`.

What&#39;s the difference and which should I use?

All Protected Solutions on Gang of Coders

Total of 19 Protected Solutions