www.digitalmars.com         C & C++   DMDScript  

digitalmars.D - (Manifest) constants inside structs

reply =?ISO-8859-15?Q?S=F6nke_Ludwig?= writes:
I'm currently trying to upgrade from DMD 2.008 to 2.010. There's one problem I 
could not find a work-around. There are a bunch of structs which have constants 
defined inside:

   struct S {
     static const S some_const1 = {0};
     static const S some_const2 = {1};

     int member;
   }

This works fine, until there is a second struct, which uses on of these 
constants in an equivalent initializer:

   struct T {
     static const T dependent_constant = {S.some_const1};
     // <- error: cannot convert const(S) to S

     S member;
   }

I've tried to use an enum instead, but that causes a segfault in the compiler:

   struct S {
     enum S some_const1 = {0}; // compiler segfault without error message
   }

A possible solution would be to put the constants outside of the struct. 
However, the actual structs look more like:

   template(S, int N) Vector {
     static const S zero = {...};
     static const S one = {...};
   }
   alias Vector!(int, 2) Vec2i;

   // in code used as Vec2i.one

Since the types used for S are not known, putting the constants outside of the 
struct is not possible.

I think the semantics should be changed to allow assignment of const values to 
non-const variables, as long as the specific struct does not contain a
reference 
  /pointer of some form (directly or indirectly).

Also manifest constants inside structs using enum would be nice.

Does anybody have another idea how to sidestep this issue for now?
I think I'm running out of ideas after also having tried a bunch of other
things.
Jan 22 2008
next sibling parent reply =?ISO-8859-15?Q?S=F6nke_Ludwig?= writes:
Finally found a dirty hack that works:

T ct_strip_const_helper(T)( const(T)[] t ){ return (cast(T[])t)[0]; }
T ct_strip_const(T)( const(T) t ){ return ct_strip_const_helper([t]); }

struct S {
   static const S ess_const = {1};

   int member;
}


struct T {
   static const T tee_const = {ct_strip_const(S.ess_const)};

   S member;
}
Jan 22 2008
parent Robert Fraser <fraserofthenight gmail.com> writes:
Sönke Ludwig wrote:
 Finally found a dirty hack that works:
 
 T ct_strip_const_helper(T)( const(T)[] t ){ return (cast(T[])t)[0]; }
 T ct_strip_const(T)( const(T) t ){ return ct_strip_const_helper([t]); }
 
 struct S {
   static const S ess_const = {1};
 
   int member;
 }
 
 
 struct T {
   static const T tee_const = {ct_strip_const(S.ess_const)};
 
   S member;
 }

Eww! Here's to hoping the enum-in-structs thing gets fixed.
Jan 22 2008
prev sibling parent reply Walter Bright <newshound1 digitalmars.com> writes:
Both of these should go into bugzilla: http://d.puremagic.com/issues/
Jan 22 2008
parent reply =?ISO-8859-15?Q?S=F6nke_Ludwig?= writes:
Walter Bright wrote:
 Both of these should go into bugzilla: http://d.puremagic.com/issues/

Done. Also I just noticed that the crash only applies to the specific case where the same struct is used as the enum type, in which it is declared: struct S { enum S s_const = ...; }
Jan 23 2008
parent Walter Bright <newshound1 digitalmars.com> writes:
Sönke Ludwig wrote:
 Done.

Thanks.
Jan 23 2008