resolving circular reference
lazarus / 62-General

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440bx · 2022-03-07 07:35

Hello,

I've run into circular references in the translation of C functions
and structures.  The sample below shows the problem using an
abbreviated version of the structures and functions involved: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 {$APPTYPE
CONSOLE} program _CircularPointers; type  { _DDHAL_DESTROYDRIVERDATA 
                                               }  
PDDHAL_DESTROYDRIVERDATA = ^TDDHAL_DESTROYDRIVERDATA; 
TDDHAL_DESTROYDRIVERDATA = record    lpDD                       :
PDDRAWI_DIRECTDRAW_GBL;  { defined below!    }     { more fields     
                                                      }  end;   {
_DDHAL_DESTROYDRIVER                                                 
   }   PDDHAL_DESTROYDRIVER = function                           (   
                       { _in_ } InDriver : PDDHAL_DESTROYDRIVERDATA  
                       )                         : DWORD; stdcall;   {
_DDHAL_DDCALLBACKS                                                   
   }   PDDHAL_DDCALLBACKS = ^TDDHAL_DDCALLBACKS;  TDDHAL_DDCALLBACKS =
record    dwSize               : DWORD;    dwFlags              :
DWORD;    DestroyDriver        : PDDHAL_DESTROYDRIVER;  end;   {
_DDHAL_CALLBACKS                                                     
   }   PDDHAL_CALLBACKS = ^TDDHAL_CALLBACKS;  TDDHAL_CALLBACKS =
record    cbDDCallbacks              : TDDHAL_DDCALLBACKS;  end;   {
_DDRAWI_DIRECTDRAW_GBL                                               
   }   PDDRAWI_DIRECTDRAW_GBL = ^TDDRAWI_DIRECTDRAW_GBL; 
TDDRAWI_DIRECTDRAW_GBL = record    lpDDCBtmp                :
PDDHAL_CALLBACKS;     { many more fields                             
                         }  end; beginend. 
The problem is there is a circular reference to
PDDRAWI_DIRECTDRAW_GBL.  As a diagram ( "->" means "references")

  PDDRAWI_DIRECTDRAW_GBL                           <- circular
reference
    -> PDDHAL_CALLBACKS                             |
         -> TDDHAL_DDCALLBACKS                      |
              -> PDDHAL_DESTROYDRIVER               |
                   -> PDDHAL_DESTROYDRIVERDATA      |
                        -> PDDRAWI_DIRECTDRAW_GBL  -^ circular
reference


The only solution I can think of so far is to change the parameter of
the PDDHAL_DESTROYDRIVER function to be a plain "pointer" and move the
definition of P/TDDHAL_DESTROYDRIVERDATA to be below the definition of
PDDRAWI_DIRECTDRAW_GBL. 

That solution is undesirable because the parameter of
PDDHAL_DESTROYDRIVER becomes a plain pointer instead of a pointer to a
specific structure.

My question is: is there a way in Pascal to declare these structures
without losing any information (such as the parameter type of
PDDHAL_DESTROYDRIVER) ?

Thank you for your help.

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Zvoni · 2022-03-07 08:47

WorksCode: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i program
Project1; type  PFirstRecord = ^TFirstRecord;   PSecondRecord =
^TSecondRecord;  TSecondRecord = Record    FirstField  : Integer;   
SecondField : String;    ThirdField  : PFirstRecord;  end;  
PThirdRecord = ^TThirdRecord;  TThirdRecord = Record    Field1     :
Integer;    Field2     : PSecondRecord;  end;   PFourthRecord =
^TFourthRecord;  TFourthRecord = Record    Field100    :
PThirdRecord;  end;   TFirstRecord = Record    Field1000  :
PSecondRecord;  end;  beginend.

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marcov · 2022-03-07 08:47

Only reference types can be forwardly declared in Pascal (and only
within one single TYPE block), but in this case that is enough. Move
all pointer definitions to the top and it will compile.

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440bx · 2022-03-07 15:09

@Zvoni and Marcov,

Thank you both for your help.

@marcov, you are right.  Declaring all the pointers first solved the
problem.  I should have thought of that but, obviously I didn't.