Ruby 技术货: 讲讲 Ruby 中纯数据结构 ( Struct 与 OpenStruct )
我们知道在 C 与 C++ 中，拥有 struct 这种纯粹存储数据类型的写法。我们叫它 结构体.
而且，在有的时候，这种用法非常有用，例如，我们要存储一张名片，最简单的方式就是声明一个 Card 结构体：
// c code
#include <stdio.h>
struct card {
char *name;
char *addr;
};
int main(){
struct card c;
c.name = "xxx";
printf("card: %s\n", c.name);
return 0;
};
有人会问，我用类就可以实现同样的功能了嘛，
# ruby code
class Card
attr_accessor :name, :addr
end
c = Card.new
c.name = 'yafei lee'
puts c.name
但是，我们却忽略了很多结构体的功能：对比，快速声明，序列化。这正是 Struct 与 OpenStruct 所带给大家的.
同样的问题，我分别的用 Struct 和 OpenStruct 看下：
# ruby code
# struct
Card = Struct.new(:name, :addr)
c = Card.new
c.name = 'yafei lee'
puts c.name
c2 = Card.new
c2.name = 'yafei lee'
puts c2 == c # true
OpenStruct:
# ruby code
require 'ostruct'
Card = OpenStruct
c = Card.new
c.name = 'yafei lee'
puts c.name
c2 = Card.new
c2.name = 'yafei lee'
puts c2.name
puts c2 == c # true
可以看得出，Struct 与 OpenStruct 让声明结构体更加简单纯粹，而不需要声明一个 "冗余" 的类，加一堆属性.
除此之外，它俩都可以帮助将 ==, eql? 这些事情做好。最后还实现了 marshal_dump 与 marshal_load, 满足持久化的功能：什么是 Marshal 持久化？
最后，讲一下它俩之间的区别：
-
Struct需要开头明确声明字段; 而OpenStruct人如其名，随时可以添加属性 -
Struct性能优秀; 而OpenStruct差不少，约有几十倍，具体的性能差距可看这里：http://stackoverflow.com/questions/1177594/ruby-struct-vs-openstruct -
Struct是 Ruby 解释器内置，用 C 实现;OpenStruct是 Ruby 标准库，Ruby 实现 - API 有些不同：Struct API 与 OpenStruct
来自 WinDy Blog
其实 struct 或者 openstruct 非常适合做 money，address 之类值对象，增加代码可读性。
http://www.sitepoint.com/value-objects-explained-with-ruby/ http://blog.codeclimate.com/blog/2012/10/17/7-ways-to-decompose-fat-activerecord-models/
Struct , OpenStruct, Hash 各自适用场合 Use Struct if 1.you need a data container and fields are fixed and known beforehand (i.e. at time of writing of the code), 2.you need a structured Hash key, 3.you want to quickly define a class with a few fields, 4.you need to detect errors caused by misspelled field names.
Use OpenStruct if 1.the number of fields is fixed at runtime but varies frequently during development time (this is often the case for objects that should hold the result of command line option parsing), 2.you need a mock or a want to quickly have objects at hand which can be filled via usual attribute setters and getters. You might want to replaced with a proper class (or Struct) later — with explicit attribute declarations via attr_accessor and relatives.
Use Hash if 1.the number of fields is unknown at coding time, 2.there is a potentially unlimited number of fields (e.g. when reading key values from a file as is often the case for script based text processing). 原文http://blog.rubybestpractices.com/posts/rklemme/017-Struct.html
笔记一则：
class Gear
# 车轮的抽象数据结构
Wheel = Struct.new(:rim, :tire)
# 整理输入数据
def wheelify(data)
data.collect do |per|
Wheel.new(per[0], per[1])
end
end
attr_reader :wheels
def initialize(data)
@wheels = wheelify data
end
def diameters
wheels.collect { |wheel| diameter wheel }
end
# 计算车轮直径
def diameter(wheel)
wheel.rim + (wheel.tire * 2)
end
end
input = [
[622, 20],
[622, 23],
[559, 30],
[559, 40],
]
p Gear.new(input).diameters
=begin
此时的 Wheel 还没有必要单独抽出来, 用 Struct 是一个方便当下又面向未来的方法.
依赖行为而不依赖数据: 使用存取器而不是直接操作实例变量.
隐藏具体的数据结构: 将输入数据的格式处理单独封装, 将无语义的数据索引转化为有语义的属性值.
=end
