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<h1 id="-">实例变量、方法、类</h1>
<h2 id="-">对象的实例变量及方法</h2>
<p> <strong>实例变量（Instance Variables）</strong>是当你使用它们时，才会被建立的对象。因此，即使是同一个类的实例，也可以有不同的实例变量。</p>
<p> 从技术层面上来看，一个对象（实例）只是存储了它的实例变量和其所属类的引用。因此，一个对象的实例变量仅存在于对象中，方法（我们称之为<strong>实例方法（Instance Methods）</strong>）则存在于对象所属的类中。这也就是为什么同一个类的实例都共享类中的方法，却不能共享实例变量的原因了。</p>
<h2 id="-">类</h2>
<ul>
<li>类也是对象。</li>
<li>因为类也是一个对象，能应用于对象的皆可运用于类。类和任何对象一样，有它们自己的类，<code>Class</code>类即是<code>Class</code>类的实例。</li>
<li>与其它的对象一样，类也有方法。对象的方法即是其所属类的实例方法。亦即，任何一个类的方法就是<code>Class</code>类的实例方法。</li>
<li>所有的类有共同的祖先<code>Object</code>类（都是从<code>Object</code>类直接或间接继承而来），而<code>Object</code>类又继承自<code>BasicObject</code>类，Ruby类的根本。</li>
<li>类名是<strong>常量（Constant）</strong>。</li>
</ul>
<p> 下面的代码有助于你理解这些信息：</p>
<pre><code class="lang-ruby"><span class="hljs-comment"># 对象的方法即为其所属类的实例方法</span>
<span class="hljs-number">1</span>.methods == <span class="hljs-number">1</span>.<span class="hljs-keyword">class</span>.instance_methods
<span class="hljs-comment">#=> true</span>
<span class="hljs-comment"># 类的“溯源”</span>
<span class="hljs-constant">N</span> = <span class="hljs-constant">Class</span>.new
<span class="hljs-constant">N</span>.ancestors
<span class="hljs-comment">#=> [N, Object, Kernel, BasicObject]</span>
<span class="hljs-constant">N</span>.<span class="hljs-keyword">class</span>
<span class="hljs-comment">#=> Class</span>
<span class="hljs-constant">N</span>.superclass
<span class="hljs-comment">#=> Object</span>
<span class="hljs-constant">N</span>.superclass.superclass
<span class="hljs-comment">#=> BasicObject</span>
<span class="hljs-constant">N</span>.superclass.superclass.superclass
<span class="hljs-comment">#=> nil</span>
</code></pre>
<h2 id="-">类是开放的</h2>
<p> 在Ruby中，类始终都是开放的。你可以重定义一个类，甚至于像<code>String</code>或<code>Array</code>这样的标准库中的类。（译注：Ruby 2.0引入了<code>refine</code>来限定这种打开类的作用域。）</p>
<pre><code class="lang-ruby"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">String</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>writesize
<span class="hljs-keyword">self</span>.size
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
puts <span class="hljs-string">"Tell me my size!"</span>.writesize
</code></pre>
<blockquote>
<p>注意!</p>
<p> 当你打开一个类的时候，一定要万分小心！如果你随意向类中添加方法或数据，你可能会收到许多的BUG，比如，你定义了自己的<code>captalize()</code>方法并且漫不经心的覆盖了原<code>String</code>类中的<code>captalize()</code>方法，你很可能会收到风险。</p>
</blockquote>
<h2 id="-initialize-">多重initialize方法</h2>
<p> 下面将示范一个类的<strong>重载（Overloading）</strong>。我们编写了一个<code>Rectangule</code>类，该类用于将一个矩形呈现在网格上。当你在实例化一个<code>Rectangule</code>对象时，你可以使用两种方法：传递矩形的左上、右下的坐标，或者左上点坐标及矩形的宽度、高度。虽然Ruby中每个类只有一个<code>initialize</code>方法，但这种做法允许你的一个<code>initialize</code>就像两个不同的<code>initialize</code>一样。</p>
<pre><code class="lang-ruby"><span class="hljs-comment"># The Rectangle constructor accepts arguments in either</span>
<span class="hljs-comment"># of the following forms:</span>
<span class="hljs-comment"># Rectangle.new([x_top, y_left], length, width)</span>
<span class="hljs-comment"># Rectangle.new([x_top, y_left], [x_bottom, y_right])</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Rectangle</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>initialize(*args)
<span class="hljs-keyword">if</span> args.size < <span class="hljs-number">2</span> || args.size > <span class="hljs-number">3</span>
puts <span class="hljs-string">'Sorry. This method takes either 2 or 3 arguments.'</span>
<span class="hljs-keyword">else</span>
puts <span class="hljs-string">'Correct number of arguments.'</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-constant">Rectangle</span>.new([<span class="hljs-number">10</span>, <span class="hljs-number">23</span>], <span class="hljs-number">4</span>, <span class="hljs-number">10</span>)
<span class="hljs-constant">Rectangle</span>.new([<span class="hljs-number">10</span>, <span class="hljs-number">23</span>], [<span class="hljs-number">14</span>, <span class="hljs-number">13</span>])
</code></pre>
<p> 上述代码还不足以编写一个完整的<code>Rectangule</code>程序，但却足以演示方法重载是如何实现的。对<code>initialize</code>方法的重载可使得其具有处理可变参数的能力。这种技巧不但适用于<code>initialize</code>方法，也适用于其他方法！</p>
<h2 id="-">匿名类</h2>
<p> 一个<strong>匿名类（Anonymous Class）</strong>也被称作<strong>单例类（Singleton Class）</strong>，<strong>特征类（Eigenclass）</strong>，<strong>鬼魂类（Ghost Class）</strong>，<strong>元类（Metaclass）</strong>或者<strong>uniclass</strong>。</p>
<blockquote>
<p> 关于eigenclass这个名字的由来：大多数人叫它singlton classes，另一部分人叫它metaclasses，意思是the class of a class。但是这些名字都不足以描述它。Ruby之父Matz至今还没有给出一个官方的名字，但是他似乎喜欢叫它eigenclass，eigen这个词来自于德语，意思是one’s own。所以，eigenclass就被翻译为“an object’s own class”。而eigenclass的方法名字，则取了一个比较科学严肃的名字叫Singlton Methods。
——参考自<a href="[blackanger对《Metaprogramming Ruby》的笔记(5">blackanger对Metaprogramming Ruby的笔记(5)</a>](<a href="http://book.douban.com/people/blackanger/annotation/4086938/" target="_blank">http://book.douban.com/people/blackanger/annotation/4086938/</a>))</p>
<p> “特征类”是一个很好的命名。这个“特征”和线性代数中“特征值”、“特征向量”的“特征”是一个意思。“特征值”、“特征向量”的名称是由德国数学家 David Hilbert （大卫·希尔伯特）在 1904 年使用并得以流传的，德语单词“Eigen”本意为“自己的”。Eigenclass，也就意味着“自己的类”，用于 Ruby 的“单例类”概念之上十分贴切，因为“单例类”实际上就是一个对象独有的类。
——参考自紫苏的<a href="http://szsu.wordpress.com/2011/01/17/%E7%89%B9%E5%BE%81%E7%B1%BB/" target="_blank">特征类</a>一文</p>
</blockquote>
<p> Ruby中每个对象都有其自己的匿名类，一个类能拥有方法，但是只能对该对象本身其作用：当我们对一个具体的对象添加方法时，Ruby会插入一个新的匿名类于父类之间，来容纳这个新建立的方法。值得注意的是，匿名类通常是不可见（Hidden）的。它没有名字因此不能像其他类一样，通过一个常量来访问。你不能为这个匿名类实例化一个新的对象。</p>
<p> 下面展示了建立匿名类的一些方法：</p>
<pre><code class="lang-ruby"><span class="hljs-comment"># 1</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Rubyist</span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span><span class="hljs-keyword">self</span>.who
<span class="hljs-string">"Geek"</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-comment"># 2</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Rubyist</span></span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-inheritance"><</span><span class="hljs-inheritance">< <span class="hljs-parent">self</span></span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>who
<span class="hljs-string">"Geek"</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
<span class="hljs-comment"># 3</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Rubyist</span></span>
<span class="hljs-keyword">end</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span><span class="hljs-constant">Rubyist</span>.who
<span class="hljs-string">"Geek"</span>
<span class="hljs-keyword">end</span>
<span class="hljs-comment">#4</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Rubyist</span></span>
<span class="hljs-keyword">end</span>
<span class="hljs-constant">Rubyist</span>.instance_eval <span class="hljs-keyword">do</span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>who
<span class="hljs-string">"Geek"</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
puts <span class="hljs-constant">Rubyist</span>.who <span class="hljs-comment"># => Geek</span>
<span class="hljs-comment"># 5</span>
<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-inheritance"><</span><span class="hljs-inheritance">< <span class="hljs-parent">Rubyist</span></span></span>
<span class="hljs-function"><span class="hljs-keyword">def</span> </span>who
<span class="hljs-string">"Geek"</span>
<span class="hljs-keyword">end</span>
<span class="hljs-keyword">end</span>
</code></pre>
<p> 上述5段代码，分别定义了<code>Rubylist.who</code>方法，该方法返回<code>"Geek"</code>。</p>
<p> 任何时候，一旦你看到如上述标号为#5的代码，<code>class</code>关键字后面紧接着<code><<</code>，你就应该确信这里为<code><<</code>右边的对象打开了一个匿名类。</p>
<p> 你可以参看<a href="http://banisterfiend.wordpress.com/2008/11/25/a-complete-ruby-class-diagram/" target="_blank">Complete Ruby Class Diagram</a> 一文，文中展示了Ruby 1.8.6中的用户定义类及他们的超类的关系。而下面这幅由Artem S贡献的Ruby层次关系（Ruby 2.0.0）也非常有用：</p>
<p><img src="http://rubylearning.com/images/Ruby_2.0.0_Hierarchy.png" alt="Screen"></p>
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