Custom hash table key is const, how to call dtors?

[Marco Leise via Digitalmars-d-learn]

Am Sun, 07 Feb 2016 01:05:28 +0000
schrieb cy <dlang at verge.info.tm>:

> On Saturday, 6 February 2016 at 03:57:16 UTC, Marco Leise wrote:
> 
> > No, but they could have dtors because they contain malloc'd 
> > data. E.g. string literals that don't live on the GC heap.
> 
> Character arrays allocated with glibc malloc are immutable? News 
> to me...

err... well... you got a point there, but then new string(100)
is probably allocated with malloc, too deep down in druntime.

immutable really means that there is no mutable reference to
the data. At any point in your code you can cast something to
immutable when you that no mutable references will exist
thereafter. We do this all the time during construction of
immutable stuff, because when something is newly created,
there is only one unique reference that is turned immutable
after construction and you are set.

You can go the same route with other MM schemes such as
malloc, just that without a GC you are responsible for not
freeing the immutable data as long as there are references to
it. For example (and this applies to Ds GC'd AA, too) you must
not delete entries while you iterate over the keys. There is
no way to say "Hey I just borrowed the list of keys, please
disallow any writes to it."

For now, issues related to dtor-constness need to be fixed.
Then working with immutable data structures is a lot less of a
mine field.

-- 
Marco