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
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