CODEWITHSUNDEEP
linuxwindowsdelphidelphi-10.2-tokyo
JP Hendriks
JP Hendriks

Reputation: 43

Segmentation fault in easy Delphi program in Linux - Windows is fine

I have this simple program, written in Delphi 10.2, and running nicely on Windows, but crashing on Linux.

The essence is that the class used has code to be executed in its destructor.

type
  Kwak = class
  public
    index: integer;
    constructor Create(index:integer);
    destructor Free;
  end;

constructor Kwak.Create(index:integer);
begin
  self.index := index;
  writeln('Welcome Kwak '+inttostr(index));
end;

destructor Kwak.Free;
begin
  writeln('Bye Kwak '+inttostr(index));
end;

If I use this in a calling procedure, like this one:

procedure myProc1;
var 
  myKwak:Kwak;
begin
  myKwak := Kwak.Create(15);
  myKwak.Free;
end;

This runs fine on Windows, but causes a segmentation error on Linux the moment myKwak leaves scope (the end is encountered in myProc1).

I guess this all has to do with Automatic Reference Counting on Linux compiler.

If I use FreeAndNil(), the program doesn't crash, but doesn't call the destructor either.

What is an elegant solution?

Please tell me your suggestions.

Upvotes: 3

Views: 323

Answers (1)

Remy Lebeau
Remy Lebeau

Reputation: 596397

destructor Free; // <-- WRONG!

This is wrong. The correct destructor is called Destroy() instead, and it is virtual in TObject so you need to override it in derived classes:

type 
  Kwak = class
  public
    index: integer;
    constructor Create(index:integer);
    destructor Destroy; override;
  end;

constructor Kwak.Create(index:integer);
begin
  inherited Create;
  self.index := index;
  writeln('Welcome Kwak '+inttostr(index));
end;

destructor Kwak.Destroy;
begin
  writeln('Bye Kwak '+inttostr(index));
  inherited;
end;

On non-ARC systems, TObject.Free() is a non-virtual instance method that calls the Destroy() destructor if Self is not nil.

On ARC systems, the compiler silently replaces all calls to Free() with a nil assignment, decrementing an object's reference count. This is so the same code can be used with similar semantics on both ARC and non-ARC systems. An object's Destroy() destructor is called when its reference count falls to 0.

Upvotes: 7

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