oop - 为什么 is-a-role-of 是继承错误之一？

Question

下图摘自我的演讲幻灯片。我已经使用组合和继承来编写一个Student - Person类，我认为它们都很合理。

（我知道如果一个人可以有很多职位 - 1:n ，那么继承不起作用，所以我只接受大约 1 人：2 个职位关系）。

使用继承的代码：

class Person
    attr_accessor :name, :gender, :height, :weight
    def initialize(name, gender, height, weight)
        @name = name
        @gender = gender
        @height = height
        @weight = weight
    end
    def cry
        "woooo"
    end
end


class Student < Person
    attr_accessor :student_id
    def initialize(name, gender, height, weight, student_id)
        super(name, gender, height, weight)
        @student_id = student_id
    end
    def study
        "Student #{name} with #{student_id} is studying now"
    end
end

s = Student.new("Lin", "Male", 173, 75, 666777)
puts s.cry()
puts s.study

使用组合的代码：

class Person
    attr_accessor :name, :gender, :height, :weight, :position
    def initialize(name, gender, height, weight, position = nil)
        @name = name
        @gender = gender
        @height = height
        @weight = weight
        @position = position
    end
    def cry
        "woooo"
    end
end


class Student
    attr_accessor :student_id
    def initialize(student_id)

        @student_id = student_id
    end
    def study
        "#{student_id} is studying now"
    end
end
s = Student.new(666777)
p = Person.new("Lin", "Male", 173, 75, s)
puts p.cry()
puts p.position.study

而且我发现使用组合的代码有一个不好的方面，我不能student叫他的名字。我的意思是我不能让 study() 方法返回像继承代码那样的东西：

"Student #{name} with #{student_id} is studying now"