为什么这个类/实例变量没有被初始化? [英] Why this class/instance variable is not being intialized?

问题描述

我正在尝试使用 gnu-smalltalk.在带有变量的简单类的以下代码中，我发现它没有被初始化为给定的值:

I am trying to use gnu-smalltalk. In following code of a simple class with a variable, I find it is not getting initialized to the given value:

Object subclass: Myclass[
    |mainval|
    mainval := 555.
    getmainval [^mainval]
]

gc := Myclass new.
gc getmainval printNl.

对象创建时没有任何错误.但是，输出是:

The object gets created without any error. But, the output is:

nil

虽然我原以为是 555.

while I had expected it to be 555.

如果我添加一个方法来为它赋值并在创建类的实例后调用它，它就可以工作.

If I add a method to assign a value to it and call it after creating an instance of the class, it works.

问题出在哪里，如何解决?谢谢.

Where is the problem and how can it be corrected? Thanks.

推荐答案

我认为您已经很接近了，但是您尝试了一个不起作用的快捷方式，因为您期望将值设置为类定义中的实例变量(在方法和本地有效性之外)将返回值.实际上，您想在方法中使用实例变量，例如init 表示对象的内部状态.

I think you were close but you tried a shortcut which does not work due to the fact that you are expecting that setting value to an instance variable within class definition (outside a method and local validity) will return value. In reality you want to use an instance variable within a method e.g. init to express the inner state of an object.

正如我在之前的一些答案中所指出的，我不是 GNU Smalltalk 专家.我正在使用 Smalltalk/X-jv 分支进行编程.

As I have noted in some previous answers, I'm not a GNU Smalltalk expert. I'm using Smalltalk/X-jv branch for programming.

我冒昧地将您的代码重写为更多类似 GNU Smalltalk 的代码:

I have taken the liberty to rewrite your code to more GNU Smalltalk like code:

Object subclass: MyClass [
    | mainVal |
    <comment:
        'testing main value'>
    MyClass class >> new [
        <category: 'instance creation'>
        | myClass |
        myClass := super new.
        myClass init.
        ^ myClass
    ]
    init [
        <category: 'initialization'>
        mainVal := 555.
    ]
    mainVal [
        ^ mainVal
    ]
]

说明:

我正在定义 mainVal 实例变量.然后我重新定义了一个 new 类方法，它不需要，但为了说明目的，我这样做了.(new 消息继承自 Object 类)

I'm defining the mainVal instance variable. Then I'm redefining a class method new which is not needed but for illustration purposes I'm doing so. (The new message is inherited from Object class)

我在类方法new中所做的.我将 new 消息发送到创建 anObject 实例的 superclass，然后使用 myClass 初始化 myClasscode>init 然后返回它.

What I'm doing in the class method new. I'm sending the new message to the superclass which creates instance of anObject, then initializing the myClass with init and then returing it.

之后您可以看到 init 类，它将您的实例变量初始化为您希望的 555 值.

Afterwards you can see the init class which initialize your instance variable to your wished value of 555.

然后你有一个 getter(在 Smalltalk 中通常没有 get 前缀，但这只是"风格问题)mainVal.

Then you have a getter (in Smalltalk it is usual to have it without the get prefix, but that is "only" matter of style) the mainVal.

然后是调用对象并获取值的代码.

Then your code to call the object and get value.

gc := MyClass new.
gc mainVal

没有new消息重新定义的最小示例:

Object subclass: MyClass [
    | mainVal |
    <comment:
        'testing main value'>
    init [
        <category: 'initialization'>
        mainVal := 555.
    ]
    mainVal [
        ^ mainVal
    ]
]

您不能忘记的是，在 Smalltalk 中通常没有构造函数的默认调用，因此您必须手动发送 init 消息(另一种选择是重新定义 new> 消息，就像我上面所做的那样).

What you must not forget that in Smalltalk there is usually no default calling of a constructor so you have to send the init message manually (the other option is to redefine new message like I did above).

然后使用它看起来像这样:

Then using it would look like this:

gc := MyClass new.
a MyClass
st> gc init
a MyClass
st> gc mainVal
555

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