When is array-to-array cast legal, and what does actually happen?

[Daniel Keep]

> ...
>
> I see that neither the constructor nor the postblit is called.
> Apparently the bit representation is used. This has the risk of
> violating struct invariants.
> 
> Is it legal?
> 
> Thank you,
> Ali

cast is to value conversions what a tactical nuclear strike is to
peaceful negotiations.  cast is specifically *designed* to circumvent
the type system's protections [1].

If you want to do a value conversion, *do a value conversion*.  Allocate
a new array and convert each member.  cast doesn't call the constructor
or the postblit because it's doing a pointer conversion.

Your code is basically equivalent to this:

void main()
{
    auto tmp = "hello"d;
    auto mine = cast(MyChar*)(tmp.ptr)
        [0..(tmp.length*typeof(tmp[0]).sizeof)/MyChar.sizeof)];
}

That is, it's doing an unsafe, unchecked pointer cast, then re-slicing
the array.

To answer your question: yes, it's legal.  Not what you wanted, but legal.



[1] Except for int<->float.  Oh, and objects.  Really, this is one thing
I could just about strangle K&R for: conflating value-preserving,
non-value-preserving *AND* unsafe conversions all into a single
construct.  Walter, gets slapped with a fish for not putting a bullet in
cast's head when he had the chance.  Argh!