During initializing Variant, D discards top level const of array, which leads to little unintuitive behaviour. Consider code:

import std.stdio;
import std.variant;
void main()
{
  const int[] arr;
  Variant v = Variant( arr );
  writeln( v.peek!( typeof( arr ) )() );
  writeln( v.peek!( const(int)[] )() );
  writeln( v.type() );
}

...and output:
%dmd main.d && ./main.d
null
7FFF358AE298
const(int)[]

As you can see peek works successfully not for original array type, but for type without top level const. Is Variant supposed to work in that way ?

On Monday, July 30, 2012 23:44:56 cybevnm wrote:
> During initializing Variant, D discards top level const of array, which leads to little unintuitive behaviour. Consider code:
> 
> import std.stdio;
> import std.variant;
> void main()
> {
> const int[] arr;
> Variant v = Variant( arr );
> writeln( v.peek!( typeof( arr ) )() );
> writeln( v.peek!( const(int)[] )() );
> writeln( v.type() );
> }
> 
> ...and output:
> %dmd main.d && ./main.d
> null
> 7FFF358AE298
> const(int)[]
> 
> As you can see peek works successfully not for original array type, but for type without top level const. Is Variant supposed to work in that way ?

Probably not. When arrays are passed to templated functions, they're passed as tail-const (so the constness on the array itself - but not its elements - is stripped), which in general is _way_ more useful than passing them as fully const. However, Variant predates that behavior by quite a while, and it's well-passed due for having extensive work done on its implementation (it's API should be fine, but it was implemented when D was much younger, and we can do a much better job of it now). There's a discussion on that in the main newsgroup at the moment actually.

In any case, please create a bug report for this:

http://d.puremagic.com/issues

- Jonathan M Davis

On Monday, 30 July 2012 at 20:56:30 UTC, Jonathan M Davis wrote:
> On Monday, July 30, 2012 23:44:56 cybevnm wrote:
>> During initializing Variant, D discards top level const of array, which
>> leads to little unintuitive behaviour. Consider code:
>> 
>> import std.stdio;
>> import std.variant;
>> void main()
>> {
>> const int[] arr;
>> Variant v = Variant( arr );
>> writeln( v.peek!( typeof( arr ) )() );
>> writeln( v.peek!( const(int)[] )() );
>> writeln( v.type() );
>> }
>> 
>> ...and output:
>> %dmd main.d && ./main.d
>> null
>> 7FFF358AE298
>> const(int)[]
>> 
>> As you can see peek works successfully not for original array type, but
>> for type without top level const. Is Variant supposed to work in that way ?
>
> Probably not. When arrays are passed to templated functions, they're passed as
> tail-const (so the constness on the array itself - but not its elements - is
> stripped), which in general is _way_ more useful than passing them as fully
> const. However, Variant predates that behavior by quite a while, and it's
> well-passed due for having extensive work done on its implementation (it's API
> should be fine, but it was implemented when D was much younger, and we can do a
> much better job of it now). There's a discussion on that in the main newsgroup
> at the moment actually.
>
> In any case, please create a bug report for this:
>
> http://d.puremagic.com/issues
>
> - Jonathan M Davis

http://d.puremagic.com/issues/show_bug.cgi?id=8486