Static array as immutable

[Jonathan M Davis]

On Tuesday, December 12, 2017 10:35:15 Ivan Trombley via Digitalmars-d-learn 
wrote:
> On Tuesday, 12 December 2017 at 09:48:09 UTC, Jonathan M Davis
>
> wrote:
> > On Tuesday, December 12, 2017 07:33:47 Ivan Trombley via
> >
> > Digitalmars-d-learn wrote:
> >> Is there some way that I can make this array immutable?
> >>
> >>    static float[256] ga = void;
> >>    static foreach (i; 0 .. 256)
> >>
> >>        ga[i] = (i / 255.0f) ^^ (1 / 2.2f);
> >
> > If you want anything to be immutable, you either have to
> > initialize it directly or give it a value in a static
> > constructor (and the static constructor solution won't work for
> > local variables). So, you'd need to do something like
> >
> > static immutable float[256] ga = someFuncThatGeneratesGA();
> >
> > If the function is pure, and there's no way that the return
> > value was passed to the function, then its return value can be
> > assigned to something of any mutability, since the compiler
> > knows that there are no other references to it, and it can
> > implicitly cast it, or if the type is a value type (as in this
> > case), then you just get a copy, and mutability isn't an issue.
> > Alternatively to using a pure function, you can use
> > std.exception.assumeUnique to cast to immutable, but that
> > relies on you being sure that there are no other references to
> > the data, and it may not work at compile-time, since casting is
> > a lot more restrictive during CTFE. So, in general, using a
> > pure function is preferable to assumeUnique.
> >
> > - Jonathan M Davis
>
> Ah, it doesn't work. I get this error using the ^^ operator:
>
> /usr/include/dmd/phobos/std/math.d(5724,27): Error: cannot
> convert &real to ubyte* at compile time
> /usr/include/dmd/phobos/std/math.d(6629,24):        called from
> here: signbit(x)
> /usr/include/dmd/phobos/std/math.d(6756,16):        called from
> here: impl(cast(real)x, cast(real)y)
>
> :(

Well, if the code you need to initialize a variable can't be run at compile
time, then that variable can't be a variable that needs to be initialized at
compile time and be immutable.

- Jonathan M Davis