关于 Record & TList 的一点吐槽

来源:转载

需求是把record 保存在TList中,并可以随时改变TList中Items的各项值。

 

 代码很简单:

 MyRec = record

MyStr: string;

constructor Create(AMyStr: string);

end;

TForm4 = class(TForm)

Button1: TButton;

procedure Button1Click(Sender: TObject);

public

MyList: TList<MyRec>;

end;

constructor MyRec.Create(AMyStr: string);

begin

MyStr := AMyStr;

end;

procedure TForm4.Button1Click(Sender: TObject);

begin

MyList := TList<MyRec>.Create;

MyList.Add(MyRec.Create('str'));

MyList.Items[0].MyStr := 'www'; //[Error] Unit4.pas(46): E2064 Left side cannot be assigned to

ShowMessage(MyList.Items[0].MyStr);

end;

 

 

无法赋值?想了一下,大概是因为左侧取出的是变量值本身,而不是变量,所以无法赋值。那么我取值地址再赋值总可以吧?于是改动如下:

 

procedure TForm4.Button1Click(Sender: TObject);

var

ps: ^string;

begin

MyList := TList<MyRec>.Create;

MyList.Add(MyRec.Create('str'));

ps := @MyList.Items[0].MyStr; //[DCC Error] Unit4.pas(45): E2036 Variable required

ps^ := 'www';

ShowMessage(MyList.Items[0].MyStr);

end;

 

 

必须为变量?又想了一下,大概是因为左侧取出的是变量值本身,而不是变量,所以无法取地址。那没办法,只能改一下TList,了,于是改动如下

Tpye

PMyRec = ^MyRec;

procedure TForm4.Button1Click(Sender: TObject);

begin

MyList := TList<PMyRec>.Create;

MyList.Add(MyRec.Create('str')); //[DCC Error] Unit4.pas(42): E2010 Incompatible types: 'PMyRec' and 'MyRec'

MyList.Items[0].MyStr := 'www';

ShowMessage(MyList.Items[0].MyStr);

end;

 

 

要是Create能返回指针就好了。不多说,继续改:

procedure TForm4.Button1Click(Sender: TObject);

begin

MyList := TList<PMyRec>.Create;

MyList.Add(@MyRec.Create('str')); //[DCC Error] Unit4.pas(42): E2036 Variable required

MyList.Items[0].MyStr := 'www';

ShowMessage(MyList.Items[0].MyStr);

end;

 

 

……。继续改:

procedure TForm4.Button1Click(Sender: TObject);

var

r: MyRec;

begin

MyList := TList<PMyRec>.Create;

r := MyRec.Create('str');

MyList.Add(@r);

MyList.Items[0].MyStr := 'www';

ShowMessage(MyList.Items[0].MyStr);

end;

 

 

看起来似乎是可以了,但是:

procedure TForm4.Button2Click(Sender: TObject);

begin

ShowMessage(MyList.Items[0].MyStr); // Access Violation!

end;

 

 

想了一下,大概是r的内存被释放了。于是改动如下:

procedure TForm4.Button1Click(Sender: TObject);

var

p: PMyRec;

begin

MyList := TList<PMyRec>.Create;

New(p);

p.Create('str');

MyList.Add(p);

MyList.Items[0].MyStr := 'www';

ShowMessage(MyList.Items[0].MyStr);

end;

 

 

OK了,不过真是麻烦,不用的时候还要dispose。感觉没有C++灵活。


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