Ruby变量中的实例变量

清疚 2022-12-06 15:27 210阅读 0赞

Instance variables begin with an at sign (@) and can be referenced only within class methods. They differ from local variables in that they don't exist within any particular [scope][]. Instead, a similar variable table is stored for each instance of a class. Instance variables live within a class instance, so as long as that instance stays alive, so will the instance variables.

实例变量以at符号(@)开头，并且只能在类方法中引用。 它们与局部变量的区别在于它们不在任何特定[范围内][scope] 。 而是为类的每个实例存储类似的变量表。 实例变量位于类实例中，因此只要该实例保持活动状态，实例变量也将保持活动状态。

Instance variables can be referenced in any method of that class. All methods of a class use the same instance [variable table][], as opposed to local variables where each method will have a different variable table. It is possible to access instance variables without first defining them, however. This will not raise an exception, but the variable's value will be *nil* and a warning will be issued if you've run Ruby with the *-w* switch.

实例变量可以在该类的任何方法中引用。 类的所有方法都使用相同的实例[变量表][variable table] ，而不是局部变量，在局部变量中，每个方法将具有不同的变量表。 但是，可以在不首先定义实例变量的情况下访问实例变量。 这不会引发异常，但是如果您使用*-w*开关运行Ruby，则变量的值将为*nil，*并且将发出警告。

This example demonstrates the use of instance variables. Note that the [shebang][] contains the *-w* switch, which will print warnings should they occur. Also, note the incorrect usage outside of a method in the class scope. This is incorrect and discussed below.

本示例演示了实例变量的用法。 请注意， [shebang][]包含*-w*开关，如果出现警告，它将打印警告。 另外，请注意类范围内方法外部的不正确用法。 这是不正确的，下面将进行讨论。

Why is the *@test* variable incorrect? This has to do with scope and how Ruby implements things. Within a method, the instance variable scope refers to the particular instance of that class. However, in the class scope (inside the class, but outside of any methods), the scope is the *class instance* scope. Ruby implements the class hierarchy by instantiating *Class* objects, so there is a *second instance* at play here. The first [instance][] is an instance of the *Class* class, and this is where *@test* will go. The second instance is the instantiation of *TestClass*, and this is where *@value* will go. This gets a bit confusing, but just remember to never use *@instance\_variables* outside of methods. If you need class-wide storage, use *@@class\_variables*, which can be used anywhere in the class scope (inside or outside of methods) and will behave the same.

为什么*@test*变量不正确？ 这与范围和Ruby如何实现有关。 在方法内，实例变量范围是指该类的特定实例。 但是，在类范围(在类内部，但在任何方法之外)，范围是*类实例*范围。 Ruby通过实例化*Class*对象来实现类层次结构，因此这里有*第二个实例*在起作用。 第一个[实例][instance]是*Class*类的一个实例，这就是*@test的*去向。 第二个实例是*TestClass*的实例化，这是*@value的*去向。 这有点令人困惑，但要记住不要在方法之外使用*@instance\_variables* 。 如果您需要类范围的存储，请使用*@@ class\_variables* ，它可以在类范围内的任何位置(方法内部或外部)使用，并且行为相同。

## [存取器][Link 1] **(** [Accessors][Link 1] **)** ##

You normally cannot access instance variables from outside of an object. For instance, in the above example, you cannot simply call *t.value* or *t.@value* to access the instance variable *@value*. This would break the rules of *encapsulation*. This also applies to instances of child classes, they cannot access instance variables belonging to the parent class even though they're technically the same type. So, in order to provide access to instance variables, *accessor* methods must be declared.

通常，您无法从对象外部访问实例变量。 例如，在上面的示例中，您不能简单地调用*t.value*或*t。@ value*来访问实例变量*@value* 。 这将破坏*封装*规则。 这也适用于子类的实例，即使它们在技术上是相同的类型，它们也无法访问属于父类的实例变量。 因此，为了提供对实例变量的访问，必须声明访问*器*方法。

The following example demonstrates how accessor methods can be written. However, note that [Ruby][] provides a shortcut and that this example only exists to show you how the accessor methods work. It's generally not common to see accessor methods written in this way unless some sort of additional logic is needed for the accessor.

下面的示例演示如何编写访问器方法。 但是，请注意， [Ruby][]提供了一个快捷方式，并且该示例仅用于向您展示访问器方法的工作方式。 除非访问器需要某种其他逻辑，否则通常不常见以这种方式编写访问器方法。

The shortcuts make things a bit easier and more compact. There are three of these helper methods. They must be run in the class scope (inside the class but outside of any methods), and will dynamically define methods much like the methods defined in the above example. There's no magic going on here, and they look like language keywords, but they really are just dynamically defining methods. Also, these accessors typically go at the top of the class. That gives the reader an instant overview of which member variables will be available outside the class or to child classes.

快捷方式使事情变得更轻松，更紧凑。 这些辅助方法有三种。 它们必须在类范围内运行(在类内部但在任何方法之外)，并且将动态定义方法，就像上面示例中定义的方法一样。 这里没有魔术，它们看起来像语言关键字，但实际上它们只是动态定义方法。 同样，这些访问器通常位于类的顶部。 这使读者可以快速了解哪些成员变量将在类外部或子类中可用。

There are three of these accessor methods. They each take a list of symbols describing the instance variables to be accessed.

这些访问器方法共有三种。 它们每个都带有描述要访问的实例变量的符号列表。

*  **attr\_reader** \- Define "reader" methods, such as the *name* method in the above example.
    
    **attr\_reader-**定义“ reader”方法，例如上例中的*name*方法。
 *  **attr\_writer** \- Define "writer" methods such as the *age=* method in the above example.
    
    **attr\_writer-**定义“ writer”方法，例如上述示例中的*age =*方法。
 *  **attr\_accessor** \- Define both "reader" and "writer" methods.
    
    **attr\_accessor-**定义“读取器”和“写入器”方法。

## 何时使用实例变量 **(** When to use Instance Variables **)** ##

Now that you know what instance variables are, when do you use them? Instance variables should be used when they represent the state of the object. A student's name and age, their grades, etc. They shouldn't be used for temporary storage, that's what local variables are for. However, they could possibly be used for temporary storage between method calls for multi-stage computations. However if you're doing this, you may want to rethink your method composition and make these variables into method parameters instead.

现在您知道什么是实例变量，什么时候使用它们？ 当实例变量代表对象状态时，应使用实例变量。 学生的姓名，年龄，成绩等。不应将其用于临时存储，这是局部变量的用途。 但是，它们可能会用于方法调用之间的临时存储，以进行多阶段计算。 但是，如果执行此操作，则可能需要重新考虑方法的组成并将这些变量设置为方法参数。

> 翻译自: [https://www.thoughtco.com/instance-variables-2908385][https_www.thoughtco.com_instance-variables-2908385]

[scope]: https://www.thoughtco.com/scope-2034287
[variable table]: https://www.thoughtco.com/nameerror-undefined-local-variable-2907927
[shebang]: https://www.thoughtco.com/commenting-ruby-code-2908193
[instance]: https://www.thoughtco.com/vbnet-what-happened-to-control-arrays-4079042
[Link 1]: https://www.thoughtco.com/accessors-and-mutators-2034335
[Ruby]: https://www.thoughtco.com/what-is-ruby-2907828
[https_www.thoughtco.com_instance-variables-2908385]: https://www.thoughtco.com/instance-variables-2908385