language-agnostic - 是否存在具有基于对象访问级别的语言？

Question

关于 Java、C#、C++ 和 PHP 中访问级别的一个常见误解是它适用于对象而不是类。也就是说，(比如说)X 类的一个对象不能看到另一个 X 的私有(private)成员。事实上，当然，访问级别是基于类的，一个 X 对象可以毫不费力地引用另一个对象的私有(private)成员。

是否存在具有基于对象访问级别的语言？它们是替代或补充基于类的访问吗？这个特性对程序设计有什么影响？

Answer 1

Ruby 具有基于对象的访问级别。这是来自 Programming Ruby 的引用:

The difference between "protected" and "private" is fairly subtle, and is different in Ruby than in most common OO languages. If a method is protected, it may be called by any instance of the defining class or its subclasses. If a method is private, it may be called only within the context of the calling object---it is never possible to access another object's private methods directly, even if the object is of the same class as the caller.

来源: http://whytheluckystiff.net/ruby/pickaxe/html/tut_classes.html#S4

Java 和 Ruby 之间的示例差异

java

public class Main {
    public static void main(String[] args) {
        Main.A a1 = new A();
        Main.A a2 = new A();

        System.out.println(a1.foo(a2));
    }

    static class A
    {
        public String foo(A other_a)
        {
            return other_a.bar();
        }

        private String bar()
        {
            return "bar is private";
        }
    }
}

// Outputs
// "bar is private"

ruby

class A
  def foo other_a
    other_a.bar
  end

  private
  def bar
    "bar is private"
  end
end

a1 = A.new
a2 = A.new

puts a1.foo(a2)

# outputs something like
# in `foo': private method `bar' called for #<A:0x2ce9f44> (NoMethodError)