为什么在这里访问冲突？ [英] Why Access Violation here?

问题描述

我正在尝试在某些Delphi项目中构建自己的类。代码是这样的：

I'm trying to construct my own class in some Delphi Project. The code is like this:

type
 TMyClass = class(TObject)
private
 hwnMain, hwnChild: HWND;
 Buffer, URL: string;
 Timer: TTimer;
public
 procedure ScanForClass;
end;

var
Form1: TForm1;
TimerAccess: TMyClass;

implementation

procedure TForm1.FormCreate(Sender: TObject);
begin
 TimerAccess.ScanForClass;
end;

procedure TMyClass.ScanForClass;
begin
 Timer:= TTimer.Create(Application); **here I get Access Violation!!**
 Timer.Interval:= 5000;
 Timer.Enabled:= true;

为什么会发生访问冲突？

why getting that access violation?

推荐答案

您的代码在使用该类之前不会创建该类的实例。

Your code does not create an instance of the class before using it.

因此它将在此代码中引发访问冲突异常：

So it will raise a access violation Exception in this code:

procedure TForm1.FormCreate(Sender: TObject);
begin
  TimerAccess.ScanForClass;
end;

因为TimerAccess仍未初始化（未定义）。

because TimerAccess is still uninitialized (undefined).

在FormCreate中，调用构造函数并将实例分配给变量

in FormCreate, call the constructor and assign the instance to the variable

procedure TForm1.FormCreate(Sender: TObject);
begin
  TimerAccess := TMyClass.Create; 
  TimerAccess.ScanForClass;
end;

在FormDestroy中，调用析构函数进行清理：

in FormDestroy, call the destructor to cleanup:

procedure TForm1.FormDestroy(Sender: TObject);
begin
  TimerAccess.Free;
end;

注意：如果有很多TForm1实例，该代码将不起作用，因为变量TimerAccess是全局变量，并且每个Form实例都会在FormCreate中分配一个新的TMyClass实例，从而导致内存泄漏。一种解决方案是使TimerAccess成为Form类的属性（或字段）。

Note: the code will not work if there are many instances of TForm1, because the variable TimerAccess is global, and every Form instance will assign a new instance of TMyClass in FormCreate, thus causing memory leaks. One solution would be to make TimerAccess a property (or field) of the Form class.

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