Inherited records
lazarus / 64-FPC development

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SonnyBoyXXl · 2018-01-13 21:22

Hello together,
so my last silly question for today

Is it possible to derive records in FPC (maybe trunk version) like in
C++?

For example to translate this
Quotestruct CD3DX12_PACKED_MIP_INFO : public D3D12_PACKED_MIP_INFO{};
to something like this

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  type   
TD3DX12_PACKED_MIP_INFO = record(TD3D12_PACKED_MIP_INFO)    end; 
Thx

SonnyBoy

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taazz · 2018-01-13 21:25

the old object construct can be used instead of records for the same
purpose.

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SonnyBoyXXl · 2018-01-13 21:42

When you have your own record definition (with no external
requirements) : yes.

But I'm using header definitions from Microsoft. And struct definied
in the C++ .h headers are records in Pascal.
When I'm using external functions in a DLL which have an struct as
input parameter this should be a "record", not a "class".
Is a class definied in FPC the "same"  as a struct in C++?

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marcov · 2018-01-13 21:43

Afaik object is fairly struct like till you use virtual methods.

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Zoran · 2018-01-13 21:48

Yes, unlike classes (whose instances are always created on the heap,
meaning they are implicit pointers), objects can be created on the
stack, so they are very similar to C++ structs (classes). See:
https://www.freepascal.org/docs-html/current/ref/refch5.html

Yes, you can simply use an object instead of record and derive from it
-- see this example of inheritance, but add just fields, not methods,
and you will get what you asked for.

Edit: important note -- just to clarify -- the word object has two
unrelated meanings in Pascal -- The word object is often use with
meaning "instance of a class", but this is not what we are talking
about here. The objects and classes are two distinct concepts of OOP
which Object Pascal has -- see here:
http://wiki.freepascal.org/Programming_Using_Objects

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marcov · 2018-01-13 22:23

Only virtual methods, as demonstrated by :

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Type 
 DrawingObject = Object      x, y : single;      height, width :
single;      procedure Draw;   end;    DrawingObject2 = Object      x,
y : single;      height, width : single;      procedure Draw;
virtual;   end; procedure DrawingObject.Draw; begin end;procedure
DrawingObject2.Draw; begin end;  begin    
writeln(sizeof(drawingobject));                // 16, 4 singles; 
writeln(sizeof(drawingobject2));               // 20 in 32-bit.
4*single + 1 x vmt pointer due to virtualend.

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Thaddy · 2018-01-14 06:43

@SonnyBoy:
Your example shows just an empty record.
If it is just a case of naming the C++ record try this:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i type   
TD3DX12_PACKED_MIP_INFO =  TD3D12_PACKED_MIP_INFO;// or strongly
typed:type    TD3DX12_PACKED_MIP_INFO = type
TD3D12_PACKED_MIP_INFO; Usually such C(++) records are opaque to
Pascal code. Therefor in recent trunk a special record and pointer
type were added to the system unit: TOpaqueData and OpaquePointer that
are for use in just such cases.
You can declare these in the same manner as I described above and the
compiler will provide type checking.
 Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i type   
PD3D12_PACKED_MIP_INFO = OpaquePointer;  // pointer to TOpaqueData   
TD3D12_PACKED_MIP_INFO  = TOpaqueData;  // declared as an empty
"opaque" record    PD3DX12_PACKED_MIP_INFO = ^TD3DX12_PACKED_MIP_INFO 
  TD3DX12_PACKED_MIP_INFO =  TD3D12_PACKED_MIP_INFO;// or strongly
typed:type    TD3DX12_PACKED_MIP_INFO = type
TD3D12_PACKED_MIP_INFO; This works because we usually need the
pointers and not the structure itself and these are now  typed
pointers in the same fashion as C++ expects.
You need a very recent trunk for this. 3.1.1-r37942

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SonnyBoyXXl · 2018-01-14 09:10

Hello Thaddy,

you are right, this was a very simple example.
@marcov and Zoran: objects can't handle unions and class operators (?)

So let me give a more complex example:

Quote//---------------------------------------------------------------
---------------
// 2D Vector; 32 bit floating point components
struct XMFLOAT2
{
    float x;
    float y;

    XMFLOAT2() XM_CTOR_DEFAULT
    XM_CONSTEXPR XMFLOAT2(float _x, float _y) : x(_x), y(_y) {}
    explicit XMFLOAT2(_In_reads_(2) const float *pArray) :
x(pArray[0]), y(pArray[1]) {}

    XMFLOAT2& operator= (const XMFLOAT2& Float2) { x = Float2.x; y =
Float2.y; return *this; }
};

// 2D Vector; 32 bit floating point components aligned on a 16 byte
boundary
__declspec(align(16)) struct XMFLOAT2A : public XMFLOAT2
{
    XMFLOAT2A() XM_CTOR_DEFAULT
    XM_CONSTEXPR XMFLOAT2A(float _x, float _y) : XMFLOAT2(_x, _y) {}
    explicit XMFLOAT2A(_In_reads_(2) const float *pArray) :
XMFLOAT2(pArray) {}

    XMFLOAT2A& operator= (const XMFLOAT2A& Float2) { x = Float2.x; y =
Float2.y; return *this; }
};

//------
So this is my example (maybe not perfect) to convert this to pascal
when inherited records are possible:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i type   
TTwoSingleArray = array [0..1] of single;     // 2D Vector; 32 bit
floating point components     { TXMFLOAT2 }     TXMFLOAT2 = record   
   class operator Initialize(var AFloat2:TXMFLOAT2);       
constructor Create(_x, _y: single);        class operator Explicit(a:
TTwoSingleArray): TXMFLOAT2;        case integer of            0: (x:
single;                y: single);            1: (f:
TTwoSingleArray);    end;     // 2D Vector; 32 bit floating point
components aligned on a 16 byte boundary{$PACKRECORD 16}     {
TXMFLOAT2A }     TXMFLOAT2A = record (TXMFLOAT2)        class operator
Initialize (var AFloat2A:TXMFLOAT2A);        constructor Create(_x,
_y: single);        class operator Explicit(a: TTwoSingleArray):
TXMFLOAT2A;    end;  implementation { TXMFLOAT2A } class operator
TXMFLOAT2A.Initialize(var AFloat2A: TXMFLOAT2A);begin   
ZeroMemory(AFloat2A,SizeOf(TXMFLOAT2A));end; constructor
TXMFLOAT2A.Create(_x, _y: single);begin    inherited;end; class
operator TXMFLOAT2A.Explicit(a: TTwoSingleArray): TXMFLOAT2A;begin   
inherited;end;  { TXMFLOAT2 } class operator TXMFLOAT2.Initialize(var
AFloat2: TXMFLOAT2);begin   
ZeroMemory(AFloat2,SizeOf(TXMFLOAT2));end; constructor
TXMFLOAT2.Create(_x, _y: single);begin    x:=_x;    y:=_y;end; class
operator TXMFLOAT2.Explicit(a: TTwoSingleArray): TXMFLOAT2;begin   
Result.f:=a;end;


struct - public sequences are rare in the MS API as I checked
yesterday. But if you have a definition like this

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
 TDWRITE_FONT_METRICS = record        designUnitsPerEm: UINT16;      
ascent: UINT16;        descent: UINT16;        lineGap: INT16;       
capHeight: UINT16;        xHeight: UINT16;        underlinePosition:
INT16;        underlineThickness: UINT16;       
strikethroughPosition: INT16;        strikethroughThickness: UINT16; 
 end; TDWRITE_FONT_METRICS1 = record        {TDWRITE_FONT_METRICS}   
   designUnitsPerEm: UINT16;        ascent: UINT16;        descent:
UINT16;        lineGap: INT16;        capHeight: UINT16;       
xHeight: UINT16;        underlinePosition: INT16;       
underlineThickness: UINT16;        strikethroughPosition: INT16;     
 strikethroughThickness: UINT16;      { New }        glyphBoxLeft:
INT16;         glyphBoxTop: INT16;         glyphBoxRight: INT16;     
 glyphBoxBottom: INT16;        subscriptPositionX: INT16;       
subscriptPositionY: INT16;        subscriptSizeX: INT16;       
subscriptSizeY: INT16;        superscriptPositionX: INT16;       
superscriptPositionY: INT16;        superscriptSizeX: INT16;       
superscriptSizeY: INT16;        hasTypographicMetrics: longbool;   
end; 
inherited records can save code

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
TDWRITE_FONT_METRICS1 = record (TDWRITE_FONT_METRICS)      { New }   
   glyphBoxLeft: INT16;         glyphBoxTop: INT16;        
glyphBoxRight: INT16;        glyphBoxBottom: INT16;       
subscriptPositionX: INT16;        subscriptPositionY: INT16;       
subscriptSizeX: INT16;        subscriptSizeY: INT16;       
superscriptPositionX: INT16;        superscriptPositionY: INT16;     
 superscriptSizeX: INT16;        superscriptSizeY: INT16;       
hasTypographicMetrics: longbool;    end;

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dicepd · 2018-01-15 19:12

I can think of another case where inheritance in adv records would be
really useful.

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
TGLZVector4f =  record  {lots of methods here}   class operator
-(constref A: TGLZVector4f): TGLZVector4f; overload;    case Byte of 
   0: (V: TGLZVector4fType);      1: (X, Y, Z, W: Single);      2:
(Red, Green, Blue, Alpha: Single);      3: (AsVector3f :
TGLZVector3f);         4: (ST,UV : TGLZVector2f);      5: (Left, Top,
Right, Bottom: Single);      6: (TopLeft,BottomRight : TGLZVector2f);
       end; 
For most of the 'types' in the accessors negate works fine, the
negative of each of the members. However when it comes to using as a
homogenous vector it all falls down and needs some way of overriding
negate to only negate X,Y,Z.
Currently means whole new record for homogenous or ugly HmgNegate
method.

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dicepd · 2018-01-15 20:38

The other way around the above would be to allow operators in record
helpers.

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
TGLZHmgVector4f = TGLZVector4f; TGLZVectorHelper = record helper for
TGLZVector4f  public   class operator -(constref A: TGLZHmgVector4f):
TGLZVector4f; overload;end;

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Thaddy · 2018-01-16 06:45

Note you should always add the methods *after* the fields, so fields
first, then the methods, albeit only to reflect it is still a record.

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dicepd · 2018-01-16 08:23

Quote from: Thaddy on January 16, 2018, 07:45:39 amNote you should
always add the methods *after* the fields, so fields first, then the
methods, albeit only to reflect it is still a record.

The compiler does not give a hoot about the order, just your
convention?

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Thaddy · 2018-01-16 08:27

Depends. e.g.  field re-ordering. You can not rely on that. Now it is
OK, but that is implementation detail. Better to keep it in record
order as intended. See all official examples.