如何在构造体主体中初始化不可用的成员？

发布于 2025-01-31 22:30:24 字数 527 浏览 2 评论 0原文

我以这种方式创建了我的班级，但是我必须初始化不可用的实例字段“文本”。尝试在此构造函数中添加初始化器表达式，或在此构造函数中添加字段初始化器，或将其标记为“迟到”。我想知道是否有一种方法可以以“ python'样式进行此类creation”可能，谢谢。

class Lexer {
  String _text;
  int _pos;
  String _current_char;

  Lexer(String text) {
    this._text = text;
    this._pos = -1;
    this._current_char = '';
    this.advance();
  }

  void advance() {
    this._pos++;
    this._current_char = this._pos < this._text.length ? this._text[this._pos] : '';
  }
}

原文

I've created my class in Dart this way, but I'm getting the Non-nullable instance field 'text' must be initialized. Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'. I would like to know if there's a way to do it in a 'Python' style where this kind of class creation is possible, thank you in advance.

class Lexer {
  String _text;
  int _pos;
  String _current_char;

  Lexer(String text) {
    this._text = text;
    this._pos = -1;
    this._current_char = '';
    this.advance();
  }

  void advance() {
    this._pos++;
    this._current_char = this._pos < this._text.length ? this._text[this._pos] : '';
  }
}

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花辞树 2025-02-07 22:30:24

class Lexer {
  String _text;
  int _pos;
  String _current_char;

这声明了几个具有类型字符串的成员。由于它们被声明为String而不是String？，因此这些成员是不可用的;它们不允许永远是null。（这是DART 2.12的新NULL-SAFETY功能的一部分。）

DART在两个阶段中初始化对象。当构造函数的 body 运行时，DART期望所有成员变量已经初始化。因为您的成员不可拆卸，并且尚未将其初始化为非零值，所以这是一个错误。错误消息解释了您可以做什么：

必须初始化不可删除的实例字段“文本”。尝试添加初始化器表达式，或在此构造函数中添加字段初始化器，或将其标记为“迟到”。

使用初始化器表达式。这意味着使用 initializer list ：< /p>

  lexer（字符串文本）
  ：_text = text，
    _pos = -1，
    _current_char =''{
  进步（）;
}

请注意，如果您使用同名的构造参数初始化成员，则可以使用速记：
lexer（this._text）：_pos = -1， _current_char =''{ 进步（）; }

添加字段初始化器。这意味着在类声明中初始化成员 inline 。

  class lexer {
  字符串_text ='';
  int _pos = -1，
  字符串_current_char ='';

将您的成员标记为late 。这意味着您保证将在任何尝试使用它们之前初始化变量。
```
  class lexer {
  晚字符串_text;
  晚int _pos，
  晚字符串_current_char;
 
```
使您的成员无效，默认情况下可以隐式为null：
```
  class lexer {
  细绳？ _文本;
  int？ _pos，
  细绳？ _current_char;
 
```
但是，这将要求所有访问明确检查成员在使用之前是否没有null。

您还可能要阅读： DART立即或在构造函数中分配给变量？

class Lexer {
  String _text;
  int _pos;
  String _current_char;

This declares several members with type String. Since they are declared as String and not as String?, these members are non-nullable; they are not allowed to ever be null. (This is part of the new null-safety feature from Dart 2.12.)

Dart initializes objects in two phases. When the constructor's body runs, Dart expects all member variables to already be initialized. Because your members are non-nullable and haven't been initialized to non-null values yet, this is an error. The error message explains what you can do:

Non-nullable instance field 'text' must be initialized. Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'.

Use initializer expressions. This means using an initializer list:
```
Lexer(String text)
  : _text = text,
    _pos = -1,
    _current_char = '' {
  advance();
}
```
Note that if you're initializing members with a construction parameter of the same name, you can use shorthand:
```
Lexer(this._text)
  : _pos = -1,
    _current_char = '' {
  advance();
}
```
Adding field initializers. This means initializing members inline in the class declaration.
```
class Lexer {
  String _text = '';
  int _pos = -1,
  String _current_char = '';
```
Marking your members as late. This means that you promise that the variables will be initialized before anything attempts to use them.
```
class Lexer {
  late String _text;
  late int _pos,
  late String _current_char;
```
Making your members nullable, which allows them to be implicitly null by default:
```
class Lexer {
  String? _text;
  int? _pos,
  String? _current_char;
```
However, that will require that all accesses explicitly check that the members aren't null before using them.