在什么情况下，我应该使用实例变量而不是其他变量类型？ [英] In what circumstances should I use instance variables instead of other variable types?

问题描述

我使用Ruby on Rails 3，我想知道在什么情况下，我应该使用实例变量而不是其他变量类型，如果有安全的原因。

I am using Ruby on Rails 3 and I would like to know in what circumstances should I use instance variables instead of other variable types and if there are security reason for those.

示例：

# Using an instance variable
@accounts = Account.find(...)

# Using a "local"\"normal" variable
account = Account.find(...)

推荐答案

一般来说，实例变量是局部的，并且持久存在于对象的实例中，而局部变量只是局部变量， / object / block范围。例如：

In general an instance variable is local and persisted inside an instance of an object, whereas a local variable is only local and persisted inside a function/object/block scope. For instance:

class User
  def name
    @name
  end

  def name= name
    @name = name
  end
end

def greet user
  name = user.name || 'John'
  p "Hi, my name is #{name}"
end

user = User.new
greet user
=> 'Hi, my name is John'
name
=> NameError: undefined local variable or method 'name' for main:Object
user.name = "Mike"
greet user
=> 'Hi, my name is Mike'
@name
=> nil

在greet函数 name 局部变量，只在该函数内定义。 name变量设置在函数的第一行， name = user.name || 'John'，但是它的值不会在函数之外持久化。当您尝试调用 name 时，您会得到一个 NameError ，因为名称只在greet函数中被定义为一个局部变量。

In the greet function name is a local variable that is only defined within that function. The name variable is set on the first line on the function, name = user.name || 'John', but its value is not persisted outside of the function. When you try calling name you get a NameError because name has only been defined as a local variable within the greet function.

@name 对于User类的用户实例是本地的。当你尝试调用它之外的上下文，你会得到 nil 。这是局部和实例变量之间的一个区别，实例变量返回nil，如果它们没有被定义，而本地非实例变量引发错误。

@name is local to the user instance of the User class. When you try calling it outside of that context you get nil. This is one difference between local and instance variables, instance variables return nil if they have not been defined, whereas local non-instance variables raise an Error.

注意，类型是特定上下文的本地。 @name 是在用户实例中定义的，所以当你调用 user.name 用户实例，其中定义了 @name 。 name 只定义在greet函数中，所以当你调用 pHi，我的名字是＃{name}您可以为 name 获取一个值，因为您在其定义的范围内。

Notice that both variable types are local to a specific context though. @name is defined within the user instance, so when you call user.name you are calling the name function in the user instance, in which @name is defined. name is only defined in the greet function, so when you call p "Hi, my name is #{name}" you are able to get a value for name because you are within the scope in which it is defined.

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