如何在TVArrec中设置字符串（或ansistring）常量？

Question

我想将格式化参数生成Args传递到 format 功能。 我找到了一些例子，但我找不到如何在中分配字符串常量tvarrec 会员。以下代码失败了使用E2089 Invalid typecast的编译。

procedure TForm1.Button1Click(Sender: TObject);
var Arguments: array of TVarRec;
begin
  SetLength(Arguments, 2);

  Arguments[0].VInteger := 111;
  Arguments[1].VAnsiString :=  PAnsiString('Text'); // I want to set this member

  ShowMessage(Format('Decimal: %d, String: %s', Arguments));
end;

.

任何人都可以建议我如何将字符串（或ansistring）设置为tvarrec成员？我正在使用Delphi 2009。

谢谢很多

Answer 1

This seems to work in XE:

var
  Args: array[0..1] of TVarRec;
  S: AnsiString;
  U: UnicodeString;
begin
  S := 'Hello';
  U := 'world';
  Args[0].VType := vtAnsiString;
  Args[0].VAnsiString := Pointer(S);
  Args[1].VType := vtUnicodeString;
  Args[1].VUnicodeString := Pointer(U);

  Writeln(Format('%s, %s!', Args));
end;

Answer 2

Just my two cents. TOndrej's answer is correct, but I just wanted to emphasize some points. And comments are not a good place to do it.

Please be aware that the AnsiString must be referenced during all time TVarRec will be used.

For instance, if you write a function setting an array of TVarRec, you should ensure that you made a temporary copy of the AnsiString created within the function to a variable which will remain until all time the TVarRec array is used. Otherwise you may have some random access violation (not every time, but only when the MM reassign the ansistring memory).

For instance, the following code is incorrect:

type
  TVarRec2 = array[0..1] of TVarRec;

procedure SetVarRec(a,b: integer; var Result: TVarRec2);
begin
  Result[0].VType := vtAnsiString;
  Result[0].VString := pointer(AnsiString(IntToStr(a)));
  Result[1].VType := vtUnicodeString;
  Result[1].VString := pointer(UnicodeString(IntToStr(b)));
end;

Because the AnsiString and UnicodeString temporary variables will be freed when the procedure ends, and Results[].VString will still point to those freed memory...

Using a class or a record with some temporary private string may do the trick:

type
  TMyVar2 = record
  private
    tmpA: AnsiString;
    tmpB: UnicodeString; 
  public
    VarRec: TVarRec2;
    procedure SetVarRec(a,b: integer);
  end;

procedure TMyVar2.SetVarRec(a,b: integer);
begin
  VarRec[0].VType := vtAnsiString;
  tmpA := AnsiString(IntToStr(a));
  VarRec[0].VString := pointer(tmpA);
  VarRec[1].VType := vtUnicodeString;
  tmpB := UnicodeString(IntToStr(b));
  VarRec[1].VString := pointer(tmpB);
end;

Of course, in your program you may have an already existing class. In this case, you would better use a method, and some private temporary strings. For the method to be multi-thread safe (i.e. re-entrant) you should provide the temporary strings as parameters...

I use this trick to have valid a dynamic array of TVarData containing some AnsiString content in a class. In fact, TVarData and TVarRec both uses such non referenced pointer to strings, which can be confusing.

Be aware that issues involving pointer(S) like statements can be difficult to track.