Re: [fpc-pascal] FPC Advanced Records

2012-03-10 Thread Sven Barth 

Am 09.03.2012 23:26, schrieb Mark Morgan Lloyd:

Martin wrote:

On 09/03/2012 21:26, Mark Morgan Lloyd wrote:


but is there any way to define something like an
endianness-correcting type, i.e.:

Type TAWSHeader=Record
ThisSize: WordLE;
..

where by the time ThisSize is accessed any disparity has been corrected?



Not sure if this will be of any use.

but you can always define an assignment incompatible type
WordLE = record data: Word; end;

and define all required overloaded operators


I think that could be of a lot of use, but I'll need to tinker :-)

Noted your example, but apart from that is there any concise way to
define a type such as WordLE with the explicit restriction that it
requires an explicit definition of assignment operators (i.e. is never
subject to implicit casts or type conversions)? Can this be done such
that WordLE has a predefined size even if the assignment has to go via
code?



You could play around with a construct like the following:

type
  generic TMyLETypeT = record
Data: T;
class operator := (aRight: TMyLEType): TMyLEType;
class operator := (aRight: TMyLEType): T;
  end;

  TWordLE = specialize TMyLETypeWord;
  TLongWordLE = specialize TMyLETypeLongWord;
  // ...

class operator TMyLEType.:= (aRight: TMyLEType): TMyLEType;
begin
  Result.Data := aRight.Data
end;

class operator TMyLEType.:= (aRight: TMyLEType): T;
begin
  // convert the value to the correct endianess here
  // you might want to use SizeOf(T) for a determination of the
  // correct size of T, so you can swap correctly
end;

Regards,
Sven
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Re: [fpc-pascal] FPC Advanced Records

2012-03-10 Thread Mark Morgan Lloyd 

Sven Barth wrote:

Am 09.03.2012 23:26, schrieb Mark Morgan Lloyd:

Martin wrote:

On 09/03/2012 21:26, Mark Morgan Lloyd wrote:


but is there any way to define something like an
endianness-correcting type, i.e.:

Type TAWSHeader=Record
ThisSize: WordLE;
..

where by the time ThisSize is accessed any disparity has been 
corrected?




Not sure if this will be of any use.

but you can always define an assignment incompatible type
WordLE = record data: Word; end;

and define all required overloaded operators


I think that could be of a lot of use, but I'll need to tinker :-)

Noted your example, but apart from that is there any concise way to
define a type such as WordLE with the explicit restriction that it
requires an explicit definition of assignment operators (i.e. is never
subject to implicit casts or type conversions)? Can this be done such
that WordLE has a predefined size even if the assignment has to go via
code?



You could play around with a construct like the following:

type
  generic TMyLETypeT = record
Data: T;
class operator := (aRight: TMyLEType): TMyLEType;
class operator := (aRight: TMyLEType): T;
  end;

  TWordLE = specialize TMyLETypeWord;
  TLongWordLE = specialize TMyLETypeLongWord;
  // ...

class operator TMyLEType.:= (aRight: TMyLEType): TMyLEType;
begin
  Result.Data := aRight.Data
end;

class operator TMyLEType.:= (aRight: TMyLEType): T;
begin
  // convert the value to the correct endianess here
  // you might want to use SizeOf(T) for a determination of the
  // correct size of T, so you can swap correctly
end;


Thanks. Thinking about it, Martin's example probably fits my criteria of 
conciseness etc.


--
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

[Opinions above are the author's, not those of his employers or colleagues]
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RE : [fpc-pascal] FPC Advanced Records

2012-03-10 Thread Ludo Brands 
 
  where by the time ThisSize is accessed any disparity has been 
  corrected?
 
 
 Not sure if this will be of any use.
 
 but you can always define an assignment incompatible type
 WordLE = record data: Word; end;
 
 and define all required overloaded operators 

Or an endian independant implementation:

type
  wordLE=packed record
L1,L2:byte;
  end;

operator := (w1:word) w2:wordLE;
  begin
w2.L2:=(w1 shr 8) and $FF;
w2.L1:=w1 and $FF;
  end;

operator := (w1:wordLE) w2:word;
  begin
w2:=((w1.L2 shl 8) and $FF00 ) or word(w1.L1);
  end;

Ludo

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Re: [fpc-pascal] FPC Advanced Records

2012-03-09 Thread Jonas Maebe 

On 09 Mar 2012, at 21:38, Mark Morgan Lloyd wrote:

 Can an Advanced Record, i.e. a record which as well as having fields has at 
 least one embedded function, be safely used as a cast or type transfer 
 overlaying unstructured bytes?

Yes.


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Re: [fpc-pascal] FPC Advanced Records

2012-03-09 Thread Jonas Maebe 

On 09 Mar 2012, at 22:26, Mark Morgan Lloyd wrote:

 Thanks Jonas, I don't think I need to do it but I thought it was a fair 
 question: better safe than sorry.
 
 OK, so hopefully I can get away with another. I'm looking at a program 
 written in Delphi, which reads a file containing the image of an IBM 
 mainframe tape. This has a record:
 
 Type TAWSHeader=Record
   ThisSize: Word;
 ..

If you want a record to have a predictable layout, always either
a) use {$packrecords c} if it's a translation of a C struct, or
b) declare it as packed

In other cases, you can (and will) have problems across different platforms or 
even compiler versions.

 but is there any way to define something like an endianness-correcting type, 
 i.e.:
 
 Type TAWSHeader=Record
   ThisSize: WordLE;
 ..
 
 where by the time ThisSize is accessed any disparity has been corrected?

No.


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Re: [fpc-pascal] FPC Advanced Records

2012-03-09 Thread Martin 

On 09/03/2012 21:26, Mark Morgan Lloyd wrote:


but is there any way to define something like an endianness-correcting 
type, i.e.:


Type TAWSHeader=Record
   ThisSize: WordLE;
..

where by the time ThisSize is accessed any disparity has been corrected?



Not sure if this will be of any use.

but you can always define an assignment incompatible type
   WordLE = record data: Word; end;

and define all required overloaded operators
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Re: [fpc-pascal] FPC Advanced Records

2012-03-09 Thread Mark Morgan Lloyd 

Jonas Maebe wrote:

On 09 Mar 2012, at 22:26, Mark Morgan Lloyd wrote:


Thanks Jonas, I don't think I need to do it but I thought it was a fair 
question: better safe than sorry.

OK, so hopefully I can get away with another. I'm looking at a program written 
in Delphi, which reads a file containing the image of an IBM mainframe tape. 
This has a record:

Type TAWSHeader=Record
  ThisSize: Word;
..


If you want a record to have a predictable layout, always either
a) use {$packrecords c} if it's a translation of a C struct, or
b) declare it as packed

In other cases, you can (and will) have problems across different platforms or 
even compiler versions.


Thanks Jonas, noted (although I invariably put lots of Assert(SizeOf()=) 
in initialization). It's actually pretty simple stuff, but derives from 
a card IBM designed which had a mainframe (big-endian) processor on it 
but plugged into an MCA (little-endian) system.


--
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

[Opinions above are the author's, not those of his employers or colleagues]
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Re: [fpc-pascal] FPC Advanced Records

2012-03-09 Thread Mark Morgan Lloyd 

Martin wrote:

On 09/03/2012 21:26, Mark Morgan Lloyd wrote:


but is there any way to define something like an endianness-correcting 
type, i.e.:


Type TAWSHeader=Record
   ThisSize: WordLE;
..

where by the time ThisSize is accessed any disparity has been corrected?



Not sure if this will be of any use.

but you can always define an assignment incompatible type
   WordLE = record data: Word; end;

and define all required overloaded operators


I think that could be of a lot of use, but I'll need to tinker :-)

Noted your example, but apart from that is there any concise way to 
define a type such as WordLE with the explicit restriction that it 
requires an explicit definition of assignment operators (i.e. is never 
subject to implicit casts or type conversions)? Can this be done such 
that WordLE has a predefined size even if the assignment has to go via code?


--
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

[Opinions above are the author's, not those of his employers or colleagues]
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