I would like to know if data containers of type packed record are zeroed when the item is declared?

type
  TMyRecord = packed record
    AField1: byte;
    AField2: word;
    AField3: single;
    AField4: boolean;
    AField5: array[0..15] of Cardinal;
  end;
procedure SomeProc;
var
  myRec: TMyRecord;
begin
 ...

Will random data fill myRec until I assign each field or will the entire record be zeroed at this point?

No, as with all local unmanaged variables it contains random data. The best you can do is add this as the first line:

begin
   MyRec:=Default(TMyRecord);

It will in effect zero out the record.

Michael.

On Thu, 8 Oct 2020 10:47:52 +0200 (CEST), Michael Van Canneyt via fpc-pascal

OK,
is it possible to define the TMyRecord with default values so that when I do Default(TMyRecord) it will be non-zero defaults written instead of zeros?

No. You will always need code for that.

Michael.

OK, is it possible to define the TMyRecord with default values so that when I do Default(TMyRecord) it will be non-zero defaults written instead of zeros?

You can define typed constant and assign it to variable.

type
   TMyRecord = record
     X, Y: Integer;
     S: String;
   end;
const
   MYDEFREC: TMyRecord = (X: 1; Y: 2; S: 'ABC');
var
   R: TMyRecord;
begin
   R := MYDEFREC;
end.

On Thu, 8 Oct 2020 13:04:43 +0200 (CEST), Michael Van Canneyt via fpc-pascal

OK,
I have now added code that sets those fields that should have non-zero values to the default on startup. All others are handled by the Default() call.
Works fine.
Thank you for your clarification!

I never understood why we can't initialize fields at init time for records. Pascal seems to be the only language that doesn't support this in 2020. Thinking of C++, C#, PHP, Swift and many more all let you do this. Not even Delphi supports this AKAIK.

type
 TMyRecord = record
   X, Y: Integer = 0;
   S: String = '';
 end;

Regards,
Ryan Joseph

Of course you can:

var
   x : record a,b : integer; end = (a:1;b:3);
begin
end.

This is identical to how C++ does it. From cppreference.com:

typedef struct { int k; int l; int a[2]; } T;
T x = {.l = 43, .k = 42, .a[1] = 19, .a[0] = 18 };

Michael.

On Thu, 8 Oct 2020 23:00:32 +0200 (CEST), Michael Van Canneyt via fpc-pascal

If you do that can you be selective and initialize only some fields or must you list all or none?

Like:

type
  TmyRec = record
    a, b, c: integer;
    d, e: double
  end = (a:2; b:3; e:3.1415);
var
  charlie: TmyRec;
begin
  Default(charlie);

What will charlie.c and charlie.d contain now?
And are a, b and e left alone at the default values?

Or are all zero?

You can do selected fields.
Fields you didn't specify are undefined.

But the code you posted is wrong.

Michael.

You need to make a method for tha. You can't even use constructors because they must have parameters for some reason but you can use a static class function (this requires advanced records mode switch of course). There is also an initialize operator that is in 3.2.0 I believe.

Sven did say one time that he MAY consider allowing a record initialization syntax inside code blocks (see below). That would be a nice option but they require all fields to be initialized. Having default values like C++ would be best though.

type
 TmyRec = record
   a, b, c: integer;
   d, e: double;
   class function Create: TMyRec; static;  // this syntax works.
   class operator Initialize(var self: TMyRec); // init operator gets called when the record is declared
 end;
var
 charlie: TmyRec = (a:2; b:3; e:3.1415);
begin
 charlie := TmyRec(a:2; b:3; e:3.1415); // this would be a nice addition to avoid constructors

Regards,
Ryan Joseph