Function return parameter code generation misery

[cryborg2]

How to tell the compiler to generate a code that treat Pointer type function return parameter as a "var" parameter just like it does with string type function return parameters?


function Test1{EAX}: string; assembler;
asm
// from here it is like we have a header like this: procedure Test1(var S{EAX}: string); assembler;
end;

function Test2{EAX}: Pointer; assembler;
asm
// from here it is like we have a header like this: procedure Test2(out P{EAX}: Pointer); assembler;
end;


procedure Test;
  var
    s: string;
    P: Pointer;
begin
  GetMem(P, MemBytes);

  s:=Test1; // In this case we pass the string to Test1 as a "var" parameter! Then Test1 can access the string as it was a real "var" parameter.
  {
    assembler:
    LEA     EAX,DWORD PTR [EBP-12]      (lea -0xc(%ebp),%eax)
    CALL    <TEST1>
  }

  P:=Test2; // In this case we not pass the pointer's address, we just get an address defined by Test2.
  {
    assembler:
    CALL    <TEST2>
    MOV     DWORD PTR [EBP-16],EAX      (mov %eax,-0x10(%ebp))
  }

  FreeMem(P);
end;

 

[cryborg2]

This is important to me because I want to use my custom string data type with the same fashion as the built-in strings (except for concatenation of course).

[PascalDragon]

How types are passed is fixed by the ABI and is not up to a user. So a Pointer will always be passed by value in the result register.

[alpine]

This is important to me because I want to use my custom string data type with the same fashion as the built-in strings (except for concatenation of course).

IMO It is due to the fact that String have a special treatment in FPC.
With the help of advanced records and operator/function overloading you can define whatever custom data type you want.
What do you mean with "the same fashion as"?

[cryborg2]

This is important to me because I want to use my custom string data type with the same fashion as the built-in strings (except for concatenation of course).

IMO It is due to the fact that String have a special treatment in FPC.
With the help of advanced records and operator/function overloading you can define whatever custom data type you want.
What do you mean with "the same fashion as"?

I want to use the ":=" operator to get my virtual string type return value, and to do so, the called function must have acces to my VirtualString object -which have it's own memory management- to transfer data through function return mechanism.

[PascalDragon]

Then you need to use advanced records with operator overloads. Anything else is just hacks.

[ASerge]

I want to use the ":=" operator to get my virtual string type return value, and to do so, the called function must have acces to my VirtualString object -which have it's own memory management- to transfer data through function return mechanism.

You can, but not Pointer. Example:

Code: Pascal  [Select][+][-]

1. {$MODE OBJFPC}
2. {$LONGSTRINGS ON}
3. {$MODESWITCH ADVANCEDRECORDS}
4. {$APPTYPE CONSOLE}
5. type
6.   TVirtualString = record
7.   strict private
8.     FLength: SizeInt;
9.     FData: PAnsiChar;
10.     procedure CopyFrom(const Src; ALength: SizeInt);
11.     class operator Copy(constref Src: TVirtualString; var Dst: TVirtualString);
12.     class operator Finalize(var Self: TVirtualString);
13.     class operator Initialize(var Self: TVirtualString);
14.   public
15.     class operator := (const S: string): TVirtualString;
16.     function AsString: string;
17.   end;
18.  
19. class operator TVirtualString. := (const S: string): TVirtualString;
20. begin
21.   Result.CopyFrom(Pointer(S)^, Length(S));
22. end;
23.  
24. function TVirtualString.AsString: string;
25. begin
26.   SetString(Result, FData, FLength);
27. end;
28.  
29. class operator TVirtualString.Copy(constref Src: TVirtualString; var Dst: TVirtualString);
30. begin
31.   Dst.CopyFrom(Src.FData^, Src.FLength);
32. end;
33.  
34. class operator TVirtualString.Finalize(var Self: TVirtualString);
35. begin
36.   ReAllocMem(Self.FData, 0);
37.   Self.FLength := 0;
38. end;
39.  
40. class operator TVirtualString.Initialize(var Self: TVirtualString);
41. begin
42.   Self.FLength := 0;
43.   Self.FData := nil;
44. end;
45.  
46. procedure TVirtualString.CopyFrom(const Src; ALength: SizeInt);
47. begin
48.   FLength := ALength;
49.   ReAllocMem(FData, FLength);
50.   if FLength > 0 then
51.     Move(Src, FData^, FLength)
52. end;
53.  
54. // End TVirtualString
55.  
56. function SomeFunction: TVirtualString;
57. begin
58.   Result := '123';
59. end;
60.  
61. var
62.   V: TVirtualString;
63. begin
64.   V := '456';
65.   V := SomeFunction; // As you want
66.   Writeln(V.AsString);
67.   Readln;
68. end.