什么是 Shallow copy？

引用 Wikipedia 中的定义如下：

One method of copying an object is the shallow copy. In the process of shallow copying A, B will copy all of A’s field values. If the field value is a memory address it copies the memory address, and if the field value is a primitive type it copies the value of the primitive type.

The disadvantage is if you modify the memory address that one of B’s fields point to, you are also modifying what A’s fields point to.

什么是 Deep copy？

引用 Wikipedia 中的定义如下：

An alternative is a deep copy. Here the data is actually copied over. The result is different from the result a shallow copy gives. The advantage is that A and B do not depend on each other but at the cost of a slower and more expensive copy.

两者最主要区别就是如何处理引用对象，Shallow copy 只复制了对象地址，而 Deep copy 则复制了被引用对象本身。

Ruby 中的 Shallow copy

Ruby 提供的两个复制对象的方法 Object#dup 和 Object#clone 都是 Shallow copy。

Ruby 中的 Deep copy

Ruby 默认没有提供 Deep copy，通常有三种方法解决这个问题：

方法一，使用序列化和反序列化：

class Object
  def deep_clone
    Marshal::load Marshal.dump(self)
  end
end

不足之处是 Marshel 不能 dump 下面这些对象：

  1. anonymous Class/Module.
  2. objects which related to its system (ex: Dir, File::Stat, IO, File, Socket and so on)
  3. an instance of MatchData, Data, Method, UnboundMethod, Proc, Thread, ThreadGroup, Continuation
  4. objects which defines singleton methods

方法二：

class Object
  def dclone
    case self
      when Fixnum,Bignum,Float,NilClass,FalseClass,
           TrueClass,Symbol
        klone = self
      when Hash
        klone = self.clone
        self.each{|k,v| klone[k] = v.dclone}
      when Array
        klone = self.clone
        klone.clear
        self.each{|v| klone << v.dclone}
      else
        klone = self.clone
    end
    klone.instance_variables.each {|v|
      klone.instance_variable_set(v,
        klone.instance_variable_get(v).dclone)
    }
    klone
  end
end

方法三：

class Object
  def deep_clone
    _deep_clone({})
  end

  protected
  def _deep_clone(cloning_map)
    return cloning_map[self] if cloning_map.key? self
    cloning_obj = clone
    cloning_map[self] = cloning_obj
    cloning_obj.instance_variables.each do |var|
      val = cloning_obj.instance_variable_get(var)
      begin
        val = val._deep_clone(cloning_map)
      rescue TypeError
        next
      end
      cloning_obj.instance_variable_set(var, val)
    end
    cloning_map.delete(self)
  end
end

class Array
  protected
  def _deep_clone(cloning_map)
    return cloning_map[self] if cloning_map.key? self
    cloning_obj = super
    cloning_map[self] = cloning_obj
    cloning_obj.map! do |val|
      begin
        val = val._deep_clone(cloning_map)
      rescue TypeError
        #
      end
      val
    end
    cloning_map.delete(self)
  end
end

class Hash
  protected
  def _deep_clone(cloning_map)
    return cloning_map[self] if cloning_map.key? self
    cloning_obj = super
    cloning_map[self] = cloning_obj
    pairs = cloning_obj.to_a
    cloning_obj.clear
    pairs.each do |pair|
      pair.map! do |val|
        begin
          val = val._deep_clone(cloning_map)
        rescue TypeError
          #
        end
        val
      end
      cloning_obj[pair[0]] = pair[1]
    end
    cloning_map.delete(self)
  end
end

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