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

[Tomas Hajny]

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.

Tomas