From: Brian Candler <b.candler@...>
Date: 2011-02-04T17:47:45+09:00
Subject: Re: problem with inheritance

Lawrence Maina wrote in post #979566:
>   def prompt_input
>
>
>     print ("a = ")
>     a = gets
>     print ("b = ")
>     b = gets
>     print ("c = ")
>     c = gets
>     print ("d = ")
>     d = gets
>     print ("e = ")
>     e = gets
>     print ("f = ")
>     f = gets
>     print ("g = ")
>     g = gets
>     print ("h = ")
>     h = gets
>     print ("i = ")
>     i = gets
>
>
>   end

In this code you are reading these values into local variables, which 
get destroyed as soon as the method returns, so the values are lost.

Either:

(1) Return an array of values. Put this as the last line before 'end':

    [a,b,c,d,e,f,g,h,i]

Then the caller copies this array into its own local variable, or splats 
them into separate variables:

    a,b,c,d,e,f,g,h,i = *prompt_input

(2) Read the values into instance variables: @a, @b, @c etc. They will 
then be available to other methods on the same object (including an 
instance of a subclass, which is what you were trying to do)

As a hint: subclassing is almost always the wrong way to construct 
systems, as you can only have one parent class, and hence only inherit 
one behaviour. This means you have to decide which is the most important 
behaviour to inherit.

You can use mixins (put your prompt_input method into a Module, and then 
include that Module into your class), but I have found that composition 
is normally the best approach ("has-a" instead of "is-a", that is, 
create an instance of a separate class, keep hold of a reference to that 
object, and invoke it as required)

class KeyboardUserInterface
  attr_reader :a, :b, :c, :d, :e, :f, :g, :h, :i
  def prompt_input
    ...
  end
end

Then create an instance of KeyboardUserInterface and call its methods 
when you need to read values or display them.

This makes it much easier to unit-test the interface separately from the 
matrix logic, and also you can replace the interface with a different 
one (say a GTk one)

HTH,

Brian.

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