[fpc-pascal] Initialization of constant record member of pointer type

[Tomas Hajny]

On 2020-12-02 16:01, LacaK via fpc-pascal wrote:
> Dňa 2.12.2020 o 13:55 Tomas Hajny via fpc-pascal napísal(a):
>> On 2020-12-01 11:39, Ladislav Karrach via fpc-pascal wrote:
>>>> 
>>>> Because MyConst1 is not an *untyped* constant. Only untyped 
>>>> constants can be used in constant expressions (a pointer to 
>>>> something can be considered an untyped constant).
>>>> 
>>>> The following might work though I did not test it:
>>>> 
>>>> === code begin ===
>>>> 
>>>> const
>>>>    MyStr = 'abc'
>>>>    MyConst1: AnsiString = MyStr;
>>>>    MyConst2: TMyRec = (l: Length(MyStr); a: @MyConst1[1]);
>>>> 
>>>> 
>>>> === code end ===
>>>> 
>>> Yes it works, but I must define 2 constants (MyStr and MyConst1),
>>> which is not so nice ;-)
>>> 
>>> It would be nice to have support for true constants:
>>> 
>>> const
>>>    MyConst1 = 'abc' ;
>>>    MyConst2: TMyRec = (l: Length(MyConst1); a: @MyConst1[1]);
>>> 
>>> But may be that there are technical reasons why it is problematic 
>>> (may
>>> be that true constants are stored in another memory locations, which
>>> can not be easy addressed)
>> 
>> Yes, that's one of possible reasons.
>> 
>> Regarding Length in constant value assignment - remember that Length 
>> is a function. In case of real constants, the call may be replaced by 
>> the compiler with the constant value. However typed constants may not 
>> be constant, so generic replacement throughout the source code at 
>> compile time is not possible (if it appears in the main body or some 
>> function, the length may already be changed to something else) and 
>> selective replacement may result in a behaviour not expected by the 
>> user. The only questionable case is IMHO the case of {$WRITEABLECONST 
>> OFF} - the compiler _might_ be able to perform the compile-time 
>> substition in that case (and thus allow using Length of such a 
>> constant in constant value assignment), but it doesn't do that in that 
>> case either apparently.
> 
> Yes I have no problem with {$WRITEABLECONST OFF} if it will allow 
> Length().
> 
> Somewhere I read (may be in Delphi documentation) that some intristic
> functions like Low(), High(), Pred() and also Length() etc. can be
> used in constant expressions because compiler can evaluate them if
> theirs arguments are also constants.
> Which is the case in my example.
> For now {$WRITEABLECONST OFF} does not help ;-)

Yes, that's what I mentioned as well, but it might be changed for this 
particular case. You might create a ticket for that if Delphi allows to 
use Length that way; obviously, you might create such a ticket even if 
Delphi doesn't allow to do that. ;-)

Tomas