Constant of class can be changed?

[d-_-b]

So I'm going to assume I'm miss-using the const keyword in the class, but it feels wrong that I can change the const value at runtime, why?

ideone code snippet with stdout.

Code: Pascal  [Select][+][-]

1. program const_in_class;
2. {$MODE objfpc}
3. type
4.   TKlazz = class
5.   public
6.     const
7.       c: char = 'w';
8.   end;
9. begin
10.         write(TKlazz.c);
11.         TKlazz.c := 't';
12.         write(TKlazz.c);
13.         TKlazz.c := 'f';
14.         write(TKlazz.c);
15. end.
16.  

 

[440bx]

So I'm going to assume I'm miss-using the const keyword in the class, but it feels wrong that I can change the const value at runtime, why?

ideone code snippet with stdout.

Code: Pascal  [Select][+][-]

1. program const_in_class;
2. {$MODE objfpc}
3. type
4.   TKlazz = class
5.   public
6.     const
7.       c: char = 'w';
8.   end;
9. begin
10.         write(TKlazz.c);
11.         TKlazz.c := 't';
12.         write(TKlazz.c);
13.         TKlazz.c := 'f';
14.         write(TKlazz.c);
15. end.
16.  

 

using the keyword "const" the way it is used in the code you presented is a _writeable_ "constant" declaration.  What it really is, is a regular variable (allocated in the data segment - not on the stack), not a constant.

read what the reference manual says about "writeable constants".

HTH.

[Zoran]

It has nothing to do with classes.
If you read the manual (https://www.freepascal.org/docs-html/current/ref/refch2.html#x20-190002)
you will understand the difference between ordinary (true) constant and (unfortunately named) typed constant.

[howardpc]

Although const declarations within a class declaration do seem to be treated differently from global const declarations.
The directive {$WriteableConst off} raises a compiler error if you try to assign a new value to a global typed constant, but has no effect on assigning a new value to a typed const inside a class, which can freely be made.
Would this be a bug, or is it intended?

[Thaddy]

Would this be a bug, or is it intended?

I think that is a bug, because code like this should not work and throw a compile time error in J- state:

Code: Pascal  [Select][+][-]

1. {$mode delphi}{$H+}{$J-}
2. type
3.   TTestme=class
4.   const
5.     c_one:integer = 1;
6.     c_two:integer = 10;
7.     c_three = 100;  // untyped works of course
8.     c_four = 'W';   //
9.   public
10.     constructor create(a,b:integer);
11.   end;
12.  
13.   constructor TTestme.create(a,b:integer);
14.   begin
15.     inherited create;
16.     c_one := a;
17.     c_two := b;
18.   end;
19. var
20.   a:TTestme;  
21.   b:TTestme;
22. begin
23.   a:=TTestme.Create(1000,10000);
24.   a.free;
25.   writeln(b.c_one,b.c_two);
26. end.

Also demo's that they really work as class const : the values set in a, propagate to b.
I can see one reason why it can't work: {$J+/-} is a local switch, not a global switch.....
( I also tested with {$push}{$J-}....{$pop}, same effect: writable)

A way around it is to have the consts in a strict private section and access them through read-only properties.

Code: Pascal  [Select][+][-]

1. type
2.   TTestme=class
3.   strict private
4.   const
5.     c_one:integer = 1;
6.     c_two:integer = 10;
7.   public
8.   class property one:integer read c_one;
9.   class property two:integer read c_two;
10.   end;

But to my horror even strict private class const fields can be written to, even over unit boundaries!!
So apart from being always writable, the visibility specifier also has no meaning for class constants. (there is some logic to the latter, but not the former)

[Thaddy]

reported as issue 0036114 on Mantis.

[Thaddy]

reported as issue 0036114 on Mantis.

And fixed in trunk. (Tnx Sven)