function ShowF1(i_sqlstr,i_db,i_key:string;Ds :TDataSource ); 請問是不是如果function 的傳入值是物件的話,代表的意思是 call by reference 不是 call by value 而i_sqlstr 這些string就是call by value 不知以上觀念是否有誤

[quote]function ShowF1(i_sqlstr,i_db,i_key:string;Ds :TDataSource );/quote] All parameters are passed by value. In fact, Ds will be passed as a pointer and a copy of it will be made for the function call.

引言: [quote]function ShowF1(i_sqlstr,i_db,i_key:string;Ds :TDataSource );/quote] All parameters are passed by value. In fact, Ds will be passed as a pointer and a copy of it will be made for the function call.

既然都是call by value傳入,為什麼在function修改會改變Ds的值?? call by value的定義不是就是在把原來的copy一份傳入,所以function內自己有一份i_sqlStr,改變這個值並不會影響外面的值,可是Ds只在function指定值就可以改變???

http://delphi.ktop.com.tw/topic.php?topic_id=24142 ccchen 版主指出此篇指出 function test(var ok:TObject):Boolean; 不需要用var了, Delphi之object都是call by reference, 不用var, 在函數中更改其值會反應至呼叫者    出知道到底是誰的對

Let us do an experiement then... Result is Label2 has not been changed. And I think you mis-understand ccchen...    1) function test(var ok:TObject):Boolean; 2) function test(ok:TObject):Boolean;    The above 2 functions will both pass a 4 bytes pointer, not the whole object and there is not way to pass a whole object...    

unit Unit1;    interface    uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;    type
  TForm1 = class(TForm)
    Label1: TLabel;
    Label2: TLabel;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
    procedure Test(ALabel: TLabel);
  public
    { Public declarations }
  end;    var
  Form1: TForm1;    implementation    {$R *.dfm}    procedure TForm1.Button1Click(Sender: TObject);
var
    ALabel : TLabel;
begin
    ALabel := Label1;
    Test(ALabel);
    ALabel.Caption := 'Button';
end;    procedure TForm1.Test(ALabel: TLabel);
begin
    ALabel.Caption := 'Test';
    ALabel := Label2;
end;    end.

引言: function ShowF1(i_sqlstr,i_db,i_key:string;Ds :TDataSource ); 請問是不是如果function 的傳入值是物件的話,代表的意思是 call by reference 不是 call by value 而i_sqlstr 這些string就是call by value 不知以上觀念是否有誤

以上的說法是正確的 補充一下: Delphi的object是很特殊的, 雖然實際上都是pointer, 由於call by address會使程式寫起來較麻煩, 老是要用@,^等operator. Delphi很體貼的讓我們可以直接使用, 而不用像C 老是須考慮用.還是-> 這就比較像是call by reference, 不完全是call by address 其實這只是名詞問題, 你只要知道object永遠是傳4 byte的pointer, 此pointer指向某物件之位址(如William兄所說)就沒錯了. 當然若你要寫C 時, call by value, call by address, call by reference就須弄清楚了

綜合以上兩位所講 ccchen前輩說這就比較像是call by reference, 不完全是call by address william前輩說是call by value 兩位都說是以pointer傳遞 我實作william的例子是不會改變 但是ccchen說 "都是call by reference, 不用var, 在函數中更改其值會反應至呼叫者" 我也實作一個範例 procedure TForm1.Button1Click(Sender: TObject); var i_sqlstr:string; ALabel : TLabel; begin DBGrid1.DataSource:=TDatasource.Create(nil ); i_sqlstr:='select * from ac'; ShowF1(i_sqlstr,'wdata','key1',DBGrid1.DataSource); end; procedure ShowF1(i_sqlstr,i_db,i_key:string;Ds :TDataSource ); var qry :TQuery; begin qry:=TQuery.Create(nil ); qry.SQL.Clear ; qry.SQL.Add(i_sqlstr); qry.DatabaseName :=i_db ; qry.Open ; Ds.DataSet:=qry; end; 真的是指定Ds後直接反應在呼叫者,所以哪我模糊了,不知可否解釋一下這兩個範例有何差別???

Let me try to explain...    

function test1(var ok:TObject):Boolean;
function test2(ok:TObject):Boolean;    AObject := TObject.Creae; { e.g. @AObject = $ABCDEF AObject^ = $123456 }
test1(AObject); { the actual pointer @ $ABCDEF is passed }
test2(AObject); { a copy of the pointer value $123456 is copied to a new location, e.g. $CCCCCC, and it is the poitner @CCCCCC is being passed }

我明白william的意思 可以請william前輩指點一下,Ds 是以call by value傳入 那為什麼在function指定Ds的值,呼叫方會有所改變 不是應該DS指定成什麼,都不會影響到DBGrid1.DataSource procedure TForm1.Button1Click(Sender: TObject); var i_sqlstr:string; begin DBGrid1.DataSource:=TDatasource.Create(nil ); i_sqlstr:='select * from ac'; ShowF1(i_sqlstr,'wdata','key1',DBGrid1.DataSource); end; procedure ShowF1(i_sqlstr,i_db,i_key:string;Ds :TDataSource ); var qry :TQuery; begin qry:=TQuery.Create(nil ); qry.SQL.Clear ; qry.SQL.Add(i_sqlstr); qry.DatabaseName :=i_db ; qry.Open ; Ds.DataSet:=qry; end; 發表人 - cyl 於 2003/01/14 17:41:26

Don't know if you could understand what I would like to express <>< face="Verdana, Arial, Helvetica">引言:我明白william的意思 可以請william前輩指點一下,Ds 是以call by value傳入 那為什麼在function指定值,回傳回呼叫方

procedure TForm1.Button1Click(Sender: TObject);
var i_sqlstr:string;
begin
DBGrid1.DataSource:=TDatasource.Create(nil); 
{e.g. DBGrid1.DataSource: address=$aaaa, value=$1111}
i_sqlstr:='select * from ac';
ShowF1(i_sqlstr,'wdata','key1',DBGrid1.DataSource);
end;    procedure ShowF1(i_sqlstr,i_db,i_key:string;Ds  :TDataSource );
{e.g. Ds: address=$bbbb, value=$1111 }
var qry :TQuery;
begin
   qry:=TQuery.Create(nil );
   qry.SQL.Clear ;
   qry.SQL.Add(i_sqlstr);
   qry.DatabaseName :=i_db ;
   qry.Open ;
   Ds.DataSet:=qry;
{@Ds.DataSet = @DBGrid1.DataSource here }
end;

procedure ShowF1(i_sqlstr,i_db,i_key:string;Ds  :TDataSource ); {e.g. Ds: address=$bbbb, value=$1111 } var qry :TQuery; begin qry:=TQuery.Create(nil ); qry.SQL.Clear ; qry.SQL.Add(i_sqlstr); qry.DatabaseName :=i_db ; qry.Open ; Ds.DataSet:=qry; {@Ds.DataSet = @DBGrid1.DataSource here } -->這句不懂 end; 分別指向不同的address.所以Ds改變內容,並不影響DBGrid1.DataSource?? 發表人 - cyl 於 2003/01/14 18:10:49

引言:{@Ds.DataSet = @DBGrid1.DataSource here } -->這句不懂 cyl 於 2003/01/14 18:10:49

typed wrongly... I means Ds.DataSet is the same as DBGrid.DataSource.DataSet. Assume: 1) DBGrid1.DataSource: address=$aaaa, value=$1111 2) Ds := DBGrid1.DataSource (i.e. @Ds=$bbbb Ds^=$1111) Then you may think Ds.DataSet as Ds^.DataSet and hence Ds.DataSet=DBGrid1.DataSource^.DataSet