Question about constant pointer typecasts.

[Dany]

Hi,

why does the expression

Code: Select all

Ptr := ^const TStateTransition(Machine.TransitionTable);

give an error "syntac error: expected "pointer to record (?T57)" but "pointer to array()" found,
while the expression

Code: Select all

Ptr := ^const TStateTransition(^const byte(Machine.TransitionTable));

compiles without error?

Apparently 2 subsequent type castings are needed here?

The types used are:

Code: Select all

type
     TGetEventProc = function: byte;
     
     TActionProc = procedure(Id, From, Towards, Event: byte);
     
     TStateTransition =
     record
       Fromstate, ToState, Event: byte;
       Action: ^TActionProc;
     end;
     
     TStateMachineTable = array[1] of TStateTransition; // dummy size
     
     TStateMachine =
     record
       Ident           : byte;
       TransitionTable : ^const TStateMachineTable;
       NrTransitions   : byte;
       GetEventProc    : ^TGetEventProc;
       Running         : boolean;  // running or stopped
       CurrentState    : byte;     // the current state of the state machine
       CurrentEvent    : byte;     // the current event to react upon
       NextState       : byte;     // the future state of the state machine
     end;

The usage of the above types in the expression:

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procedure StateMachine_Step(var Machine: TStateMachine);
var Index      : byte;
    Routine    : ^TActionProc;
    Ptr         : ^const TStateTransition;
    Found      : boolean;
begin
  ....      
      Ptr := ^const TStateTransition(^const byte(Machine.TransitionTable)); // point to first entry in the State Transition table  
  ...
end;

Thanks in advance!

[JimKueneman]

I want to know for the dsPIC compiler as well. What appears two pointer types are compatible. I am doing this:

TBase = record
State: byte;
Count: byte;
end;
PBase = ^TBase;

TData = record
State: byte;
Count: byte;
Data: array[0..50] of char;
end;
PData = ^TData;

..... More records expanding TBase

Now I code what type it is in the state field;

Base: PBase;
Data: PData;

if Base^.State and STATE_IS_TDATA <> 0 then
Data := PData( Base);

gives the same type of error. I have to cast it to a non pointer type then back:

if Base^.State and STATE_IS_TDATA <> 0 then
Data := PData( Word( Base));

Then all is well, sort of because now the code is assuming a 16 bit pointer for the dsPIC and is not portable between micros. I have fixed this by creating a pointer type:

{ifdef P30}Pointer = Word;{endif}
{ifdef P32}Pointer = DWord;{endif}
etc...

so now I do this:

if Base^.State and STATE_IS_TDATA <> 0 then
Data := PData( Pointer( Base));

so it "looks" like I am casting it to a generic pointer but not really.

That said I would like to suggest a generic pointer type so it is automatically based microcontroller you are compiling for and the size of the pointer.

Jim

[srdjan]

Hi,

why does the expression

Code: Select all

Ptr := ^const TStateTransition(Machine.TransitionTable);

give an error "syntac error: expected "pointer to record (?T57)" but "pointer to array()" found,

Should be an error here, these types are not compatible and the error message is correct.

while the expression

Code: Select all

Ptr := ^const TStateTransition(^const byte(Machine.TransitionTable));

compiles without error?

Byte pointer acts as an generic pointer, it can be passed to any other pointer type and vice verse with the exception of function pointers.
Therefore, by forcing this 'two step' casting, compiler makes sure that this is something you
wanted to do, not something you have done by mistake.

[srdjan]

so it "looks" like I am casting it to a generic pointer but not really.

That said I would like to suggest a generic pointer type so it is automatically based microcontroller you are compiling for and the size of the pointer.

We already have an pascal version with generic type pointers, just needs to be released
Unfortunately, it will take some time to properly test this one
For now, you can use byte pointers as generic pointers like Danny did.
This will take care of your pointer size issue mentioned here.

[Dany]

We already have an pascal version with generic type pointers, just needs to be released
Unfortunately, it will take some time to properly test this one
For now, you can use byte pointers as generic pointers like Danny did.
This will take care of your pointer size issue mentioned here.

Thanks!