What does “Packed Now Forces Byte Alignment of Records” mean?

Question

The What\'s New for Delphi XE2 contains the following.

Packed Now Forces Byte Alignment of Records

If you have legacy code that uses the

Answer 1

As I am not the Delphi compiler guy I can also mostly guess as others did: The record alignment might not be meant for aligning the members inside the record, but the record itself instead.

If you declare a record variable it is aligned to some address in memory, that is most likely aligned on a 4-byte boundary. This has been the case (as tested in D2007) for packed and unpacked records.

Now in XE2 a packed record is placed in a 1-byte boundary, while unpacked records are placed on some even boundary, which can be controlled by the align keyword. Like this:

type
  TRecAligned = record
    b1: byte;
    u1: uint64;
  end align 16;

  TRecPackedAligned = packed record
    b1: byte;
    u1: uint64;
  end align 16;

The packed record is still aligned on a 1-byte boundary while the unpacked record is aligned to a 16-byte boundary.

As I said, it is only a guess. The wording of the Embarcadero quote isn't that much clear on the suject.