Come impostare la costante di String (o Ansistring) in Tvarrec?
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15-11-2019 - |
Domanda
Voglio passare gli argomenti di formattazione Args
in Formato funzione.
Ho trovato alcuni esempi di ciò, ma non riesco a scoprire come assegnare costante di stringa in Tvarrec membro.Il codice seguente non riesce sulla compilazione con E2089 Invalid typecast
.
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;
.
Qualcuno può suggerirmi come impostare la costante della stringa (o ansistrating) per il membro di Tvarrec?Sto usando Delphi 2009.
Grazie mille
Soluzione
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;
Altri suggerimenti
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.