D Language Gotchas

[Jonathan M Davis]

On Wednesday, November 06, 2013 07:26:57 qznc wrote:
> On Wednesday, 6 November 2013 at 04:57:57 UTC, Jonathan M Davis
> 
> wrote:
> > On Tuesday, November 05, 2013 13:27:54 Jacob Carlborg wrote:
> >> On 2013-11-05 12:16, Meta wrote:
> >> > One gotcha relates to enums. Writing `enum a = [0, 1, 2]` is
> >> > a really
> >> > bad idea, because everywhere you use a, it constructs a new
> >> > array at
> >> > runtime. The [0, 1, 2] is "pasted in", and you'll have a
> >> > bunch of
> >> > allocations you didn't expect. This doesn't just happen with
> >> > arrays, but
> >> > that's the most common case. What *is* okay is using string
> >> > enums, as
> >> > strings are a bit special due to being immutable.
> >> 
> >> Isn't the problem rather that [0, 1, 2] allocates in the first
> >> place,
> >> regardless if an enum is used or not.
> > 
> > In most cases, array literals need to allocate, regardless of
> > whether they're
> > an enum or not. The cases where they don't need to allocate
> > (e.g. when
> > initializing a static array) shouldn't allocate, but an array
> > literal by
> > itself is a dynamic array, and pretty much has to allocate in
> > most cases in
> > order to be safe. Having an enum which is an array literal
> > rather than making
> > it an immutable constant then results in allocations that you
> > might not want,
> > but that's not really the literal's fault.
> 
> Can you work around that? Maybe something like:
> 
>    enum a = immutable([0, 1, 2]);

What's to work around? It's very much on purpose that enums work that way. If 
you don't want that behavior, then simply create a variable instead. e.g.

immutable a = [0, 1, 2];

The fact that enums are treated as manifest constants isn't a problem so long 
as you understand that that's what they're supposed to do. If you don't want a 
manifest constant, then don't use an enum. The only thing that

enum a = [0, 1, 2];

gains you over

immutable a = [0, 1, 2];

is making it a manifest constant instead of a variable. Which you choose 
depends on what behavior you want. Neither is wrong, and which is desirable 
depends entirely on what you're doing.

- Jonathan M Davis