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digitalmars.D.bugs - [Issue 8486] New: Possibly wrong interaction of Variant and const arrays

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8486

           Summary: Possibly wrong interaction of Variant and const arrays
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody puremagic.com
        ReportedBy: cybevnm gmail.com



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 ?

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next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8486


klickverbot <code klickverbot.at> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |code klickverbot.at



---
Variant should generally ignore top-level const for »value types« (i.e. array
slices, pointers, integral types, …).

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prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8486




---
… the point being that this is, if anything, analogous to the constness of the
Variant instance itself, not the contained type.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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 … the point being that this is, if anything, analogous to the constness of
the
 Variant instance itself, not the contained type.


Unfortunately, Variant's handling of arrays is different to handling of class
references. So we have next situtation:

import std.variant;
class C { }
void main()
{
  {
    const C c;
    Variant v = Variant( c );
    assert( v.peek!( typeof(c) )() != null );
  }
  {
    const C[] cs;
    Variant v = Variant( cs );
    assert( v.peek!( typeof(cs) )() == null );
 }
}
But array top-level const cutting is understandable, it's the way it works in
all other places...

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Aug 01 2012
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8486




---
I meant that Variant should regard e.g. const(T[]) and const(T)[] or const(T*)
and const(T)* equivalent, i.e. allow implicit conversion between them.

The fact that const is automatically stripped from arrays is not
Variant-specific, template functions generally work like that in D.

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prev sibling parent d-bugmail puremagic.com writes:
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 I meant that Variant should regard e.g. const(T[]) and const(T)[] or const(T*)
 and const(T)* equivalent, i.e. allow implicit conversion between them.


Agree, it will resolve the problem.

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