Undo struct slicing by type-punning

[Ali Çehreli via Digitalmars-d-learn]

On 07/14/2014 02:34 PM, ponce wrote:

 > On Monday, 14 July 2014 at 18:43:36 UTC, Ali Çehreli wrote:
 >> On 07/14/2014 10:35 AM, ponce wrote:
 >>
 >> > Ok, solved it, I just use pointer casts and it seems to work
 >> when the
 >> > struct is sliced.
 >>
 >> I think there is a terminology issue here. Slicing cannot be undone;
 >> once the object is sliced, the non-A parts are gone.
 >>
 >
 > Well they are not really gone if the struct is passed by-ref,

Oh, it's not slicing in that case... but wait! There is a problem here. :)

In C++, both structs and classes are value types and they both support 
inheritance. To observe slicing in C++, one needs pass-by-value and 
inheritance, which is available by structs and classes.

In D, structs don't support inheritance and classes don't support 
pass-by-value. However... Enter 'alias this' and we have D's version of 
slicing.

 > you can recover the "gone" parts: http://dpaste.dzfl.pl/d64863fd4c6d

Aggreed but in D's case the non-A parts are not gone; as long as A 
lives, we know that B is alive. This is different from C++ where due to 
the necessary pass-by-value, they are truly gone (the bits may still be 
there but a C++ code should not do as D does).

Ali