Little bit confusing PChar operation

[Thaddy]

Classes are also pointers, as opposed to objects (which behave like records)

[egsuh]

Classes are also pointers, as opposed to objects (which behave like records)

I know. The classes there may be accessed even after TaqValue has been released.

TaqValue was not designed as self-data-containing record. I'm using this as an intermediary storage --- calculated data actually. For an example, an expression like "2 * 3" will be stored in TExpression as input string. The calculated value (i.e. integer 6) is stored in TaqValue.

[ASerge]

I can mix fixed fields, methods, and variant fields in record.

Only when {$LONGSTRINGS OFF}. Error: Data types which require initialization/finalization cannot be used in variant records.

[egsuh]

Only when {$LONGSTRINGS OFF}. Error: Data types which require initialization/finalization cannot be used in variant records.

In my example, string is not defined within variant part of the record. Do I still need {$LONGSTRINGS OFF}?

What I need in the end is an expression parser and evaluator.

https://wiki.freepascal.org/How_To_Use_TFPExpressionParser

I found this page now, and there are similar approach to my idea (If I had read this in the past, I don't have to think over now).

Code: Pascal  [Select][+][-]

1. type
2.   TResultType = (rtBoolean, rtInteger, rtFloat, tDateTime, rtString);
3.  
4.   TFPExpressionResult = record
5.     ResString   : String;
6.     Case ResultType : TResultType of
7.       rtBoolean  : (ResBoolean  : Boolean);
8.       rtInteger  : (ResInteger  : Int64);
9.       rtFloat    : (ResFloat    : TExprFloat);
10.       rtDateTime : (ResDateTime : TDatetime);
11.       rtString   : ();
12.   end;
13.  

I need one more type --- set or array. And there could be variables, whose values should be given from outside.

[ASerge]

In my example, string is not defined within variant part of the record. Do I still need {$LONGSTRINGS OFF}?

The message I replied to was:

Code: Pascal  [Select][+][-]

1. type
2.     RRec= record
3.           ThingToDo: string;
4.  
5.           procedure SetRecValue(rtype:integer; AValue:string);
6.  
7.           case rectype: integer of
8.                  1:  (Month: string);  // <- THIS!!!
9.                  2:  (Date: integer);
10.     end;

[egsuh]

In my example, string is not defined within variant part of the record. Do I still need {$LONGSTRINGS OFF}?

The message I replied to was:

Code: Pascal  [Select][+][-]

1. type
2.     RRec= record
3.           ThingToDo: string;
4.  
5.           procedure SetRecValue(rtype:integer; AValue:string);
6.  
7.           case rectype: integer of
8.                  1:  (Month: string);  // <- THIS!!!
9.                  2:  (Date: integer);
10.     end;

OMG. In real test program, it was String[10]. Sorry for the error. I ran test program on another PC and wrote it here based on memory. My point here was ThingTodo. But this was already documented.

[alpine]

@egsuh
Here is how to solve your issue from the original post. The additional property will help managing memory through it's getter/setter:

Code: Pascal  [Select][+][-]

1. {$MODESWITCH ADVANCEDRECORDS}
2. type
3.   TResultType = (rtBoolean, rtInteger, rtFloat, rtDateTime, rtString);
4.  
5.   TExprFloat = Double;
6.  
7.   { TFPExpressionResult }
8.  
9.   TFPExpressionResult = record
10.   private
11.     function GetResString: AnsiString;
12.     procedure SetResString(AValue: AnsiString);
13.     class operator Finalize(var ASelf: TFPExpressionResult); inline;
14.   public
15.     property ResString: AnsiString read GetResString write SetResString;
16.     //ResString   : String;
17.     Case ResultType : TResultType of
18.       rtBoolean  : (ResBoolean  : Boolean);
19.       rtInteger  : (ResInteger  : Int64);
20.       rtFloat    : (ResFloat    : TExprFloat);
21.       rtDateTime : (ResDateTime : TDatetime);
22.       rtString   : (ResString_: PChar);
23.   end;
24.  
25. { TFPExpressionResult }
26.  
27. function TFPExpressionResult.GetResString: AnsiString;
28. begin
29.   Result := ResString_;
30. end;
31.  
32. procedure TFPExpressionResult.SetResString(AValue: AnsiString);
33. var
34.   L: SizeInt;
35. begin
36.   if Assigned(ResString_) then
37.   begin
38.     FreeMem(ResString_);
39.     ResString_ := Nil;
40.   end;
41.   L := Length(AValue);
42.   if L > 0 then
43.   begin
44.     GetMem(ResString_, L + 1);
45.     Move(AValue[1], ResString_^, L + 1);
46.   end;
47. end;
48.  
49. class operator TFPExpressionResult.Finalize(var ASelf: TFPExpressionResult);
50. begin
51.   if Assigned(ASelf.ResString_) then
52.     FreeMem(ASelf.ResString_);
53. end;
54.  
55. var
56.   R: TFPExpressionResult;
57.  
58. begin
59.   R.ResString := 'AnsiString';
60.   WriteLn('''', R.ResString, '''');
61.   R.ResString := 'Another AnsiString';
62.   WriteLn('''', R.ResString, '''');
63.   R.ResString := '';
64.   WriteLn('''', R.ResString, '''');
65. end.  

[egsuh]

@alpine

Really appreciate your comment. Actually I'm thinking of defining function like:

Code: Pascal  [Select][+][-]

1. function TFPExpressionResult.GetResString: AnsiString;
2. begin
3.     case ResultType of
4.         rtBoolean: Result := BoolToStr(ResBoolean, true);
5.         rtInteger: Result := IntToStr(ResInteger);
6.         // ......
7.         rtString:  Result := ResString_;
8.    end;
9. end;

for external access. In most cases results are converted to string in the end (like 35 in "You answered your age is 35.", etc.).