Ruby (2.1+) protected method weirdness. Fails with "NoMethodError", accessing protected method. See Ruby code in action: https://eval.in/209730 See Java code in action: http://ideone.com/2B3CEq See C# code in action: http://ideone.com/RJjn4f See C++ code in action: http://ideone.com/7Ihvw0 See Ruby alternative code in action: https://eval.in/209934

protected_method.rb

class A

  def testme(other)
    puts "value from this: #{ value } value from other: #{ other.value }"
  end

  protected def value
    raise NotImplementedError
  end

end

class B < A

  protected def value
    "B class"
  end
  
end

class C < A

  protected def value
    "C class"
  end
  
end

b = B.new
c = C.new

b.testme(c)

# /tmp/execpad-e832b1b53f4e/source-e832b1b53f4e:4:in `testme': protected method `value' called for #<C:0x4105c724> (NoMethodError)
#	from /tmp/execpad-e832b1b53f4e/source-e832b1b53f4e:32:in `<main>'

protected_method_alternative.rb

class A

  def testme(other)
    puts "value from this: #{ value } value from other: #{ other.value }"
  end

  protected def value
    _value
  end

  private def _value
    raise NotImplementedError
  end

end

class B < A

  private def _value
    "B class"
  end
  
end

class C < A

  private def _value
    "C class"
  end
  
end

b = B.new
c = C.new

b.testme(c)

# value from this: B class value from other: C class

ProtectedMethod.cpp

#include <iostream>
using namespace std;

class A;
class B;
class C;

class A
{
	public:
		void Testme(A& other)
		{
			cout << "value from this: " << Value() << " value from other: " << other.Value();
		}
	
	protected:
		virtual char const* Value() = 0;
};

class B : public A
{
	protected:
		char const* Value()
		{
			return "B class";
		}
};

class C : public A
{
	protected:
		char const* Value()
		{
			return "C class";
		}
};

int main() {
	B b = B();
	C c = C();
	b.Testme(c);
	return 0;
}

// value from this: B class value from other: C class

ProtectedMethod.cs

using System.IO;
using System;

abstract class A
{
    public void Testme(A other)
    {
        Console.WriteLine("value from this: " + Value() + " value from other: " + other.Value());
    }
    
    protected abstract string Value();
}

class B : A
{
    protected override string Value()
    {
        return "B class";
    }
}

class C : A
{
    protected override string Value()
    {
        return "C class";
    }
}



class Program
{
    static void Main()
    {
        B b = new B();
        C c = new C();
        
        b.Testme(c);
    }
}

// value from this: B class value from other: C class

ProtectedMethod.java

/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

abstract class A
{
    public void Testme(A other)
    {
        System.out.println("value from this: " + Value() + " value from other: " + other.Value());
    }
    
    abstract protected String Value();
}
 
class B extends A
{
    protected String Value()
    {
        return "B class";
    }
}
 
class C extends A
{
    protected String Value()
    {
        return "C class";
    }
}

/* Name of the class has to be "Main" only if the class is public. */
class Program
{
	public static void main (String[] args) throws java.lang.Exception
	{
		B b = new B();
		C c = new C();
		b.Testme(c);
	}
}

/* value from this: B class value from other: C class */