[fpc-pascal] type definitions etc

[Vinzent Hoefler]

On Thursday 26 May 2005 12:06, Hans Maartensson wrote:

> In a program I had a recursive definition like:
>
> type sometype = record
>    n: longint;
>    p: ^sometype
>    end;

This was never allowed in Pascal, AFAIK. The standard solution is to 
define the pointer type first:

|type
|   ptr_sometype = ^sometype;
|type
|   some_type =
|   record
|      p : ptr_sometype;
|      ...
|   end;

> Also the following call to a win32 API function is not allowed any
> more:
>
> invalidaterect(windows, 0, true);
>
> The error is that the 2nd parameter should be of type 'rect'.

Have you actually tried "InvalidateRect (windows, NIL, True);"?

> The 2nd parameter in this example must be of 'var' type, which means
> that it is the pointer to the parameter that is transferred to the
> function. Transferring a nil pointer is in pascal language the same
> as leaving the parameter undefined. But that is not possible in
> FreePascal, is it?

It is, because of function overloading:

function InvalidateRect(hWnd:HWND; var lpRect:RECT; 
bErase:WINBOOL):WINBOOL; external 'user32' name 'InvalidateRect';
function InvalidateRect(hWnd:HWND;lpRect:LPRECT; 
bErase:WINBOOL):WINBOOL; external 'user32' name 'InvalidateRect';

So the second form takes a pointer, thus passing NIL should be perfectly 
allowed, AFAICS.

> Finally I would like to ask about the following.
> The compiler does not allow a procedure definition like:
>
> procedure someproc(p: ^double);
>
> whereas the following is perfectly possible:
>
> type Tpdouble = ^double;
> procedure f(p: Tpdouble);
>
> Is there a good reason for this? Why isn't  ^double a valid type
> everywhere?

Because

type1 = ^double;
type2 = ^double;

should be considered different types. So if you define the type in the 
procedure declaration, you couldn't call it, just because you can't 
declare a variable of the same type for the argument. The only allowed 
argument would be NIL, which I think is a little bit pointless.

BTW, why don't you use "procedure f(const/var/out p : double);" instead 
to get rid of as much pointers as possible and use the right parameter 
modes which also have the side effect of being more self documenting?


Vinzent.