获取给定名称的实例变量的值

问题描述

通常，如何获得对我在字符串中具有其名称的对象的引用?

In general, how can I get a reference to an object whose name I have in a string?

更具体地说，我有一个参数名称列表(成员变量-动态生成的，因此我不能直接引用它们).

More specifically, I have a list of the parameter names (the member variables - built dynamically so I can't refer to them directly).

每个参数都是一个具有from_s方法的对象.

Each parameter is an object that also has an from_s method.

我想做类似以下的事情(这当然是行不通的...):

I want to do something like the following (which of course doesn't work...):

define_method(:from_s) do | arg |
    @ordered_parameter_names.each do | param |
        instance_eval "field_ref = @#{param}"
        field_ref.from_s(param)
    end
end

推荐答案

最惯用的方法是:

some_object.instance_variable_get("@#{name}")

无需使用+或intern； Ruby会很好地处理这个问题.但是，如果您发现自己进入另一个对象并拉出其ivar，则很有可能破坏了封装.

There is no need to use + or intern; Ruby will handle this just fine. However, if you find yourself reaching into another object and pulling out its ivar, there's a reasonably good chance that you have broken encapsulation.

如果您明确想要访问ivar，则正确的做法是使其成为访问器.请考虑以下内容:

If you explicitly want to access an ivar, the right thing to do is to make it an accessor. Consider the following:

class Computer
  def new(cpus)
    @cpus = cpus
  end
end

在这种情况下，如果您执行Computer.new，则将被迫使用instance_variable_get到达@cpus.但是，如果您这样做，则可能意味着@cpus是公开的.您应该做的是:

In this case, if you did Computer.new, you would be forced to use instance_variable_get to get at @cpus. But if you're doing this, you probably mean for @cpus to be public. What you should do is:

class Computer
  attr_reader :cpus
end

现在您可以执行Computer.new(4).cpus.

请注意，您可以重新打开任何现有类，并将私有的ivar变成阅读器.由于访问器只是一种方法，因此您可以Computer.new(4).send(var_that_evaluates_to_cpus)

Note that you can reopen any existing class and make a private ivar into a reader. Since an accessor is just a method, you can do Computer.new(4).send(var_that_evaluates_to_cpus)

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