动态常量赋值 [英] Dynamic constant assignment

问题描述

class MyClass
  def mymethod
    MYCONSTANT = "blah"
  end
end

给我错误:

SyntaxError:动态常量赋值错误

SyntaxError: dynamic constant assignment error

为什么这被认为是一个动态常数?我只是给它分配一个字符串.

Why is this considered a dynamic constant? I'm just assigning a string to it.

推荐答案

您的问题是每次运行该方法时，您都会为常量分配一个新值.这是不允许的，因为它使常量非常量；即使字符串的内容是相同的(无论如何)，每次调用该方法时，实际的字符串object本身都是不同的.例如:

Your problem is that each time you run the method you are assigning a new value to the constant. This is not allowed, as it makes the constant non-constant; even though the contents of the string are the same (for the moment, anyhow), the actual string object itself is different each time the method is called. For example:

def foo
  p "bar".object_id
end

foo #=> 15779172
foo #=> 15779112

也许如果您解释了您的用例——为什么要更改方法中常量的值——我们可以帮助您更好地实现.

Perhaps if you explained your use case—why you want to change the value of a constant in a method—we could help you with a better implementation.

也许你更愿意在类上有一个实例变量?

Perhaps you'd rather have an instance variable on the class?

class MyClass
  class << self
    attr_accessor :my_constant
  end
  def my_method
    self.class.my_constant = "blah"
  end
end

p MyClass.my_constant #=> nil
MyClass.new.my_method

p MyClass.my_constant #=> "blah"

如果您真的想更改方法中常量的值，并且您的常量是字符串或数组，则可以作弊"并使用 #replace 方法使对象采用新值而不实际更改对象:

If you really want to change the value of a constant in a method, and your constant is a String or an Array, you can 'cheat' and use the #replace method to cause the object to take on a new value without actually changing the object:

class MyClass
  BAR = "blah"

  def cheat(new_bar)
    BAR.replace new_bar
  end
end

p MyClass::BAR           #=> "blah"
MyClass.new.cheat "whee"
p MyClass::BAR           #=> "whee"

这篇关于动态常量赋值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！