Stack using a fixed size array

gistfile1.rb

class Stack

  def initialize(size=10)
    raise "InvalidSizeInput" unless size.is_a?(Fixnum)
    @store = Array.new(size)
    @size = size 
    @top = 0
  end

  def push(ele)
    if full?
      raise "overflow"
    else
      @store[@top] = ele
      @top += 1
    end 
  end

  def pop
    if empty?
      raise "underflow"
    else 
      @top -= 1
      result = @store[@top] 
      @store[@top] = nil 
      
      result 
    end
  end

  private 

  def empty?
    @top == 0 
  end

  def full? 
    @top == @size 
  end

end