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

[cy via Digitalmars-d-learn]

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...

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

[Marco Leise via Digitalmars-d-learn]

Am Sun, 07 Feb 2016 01:05:28 +
schrieb cy :

> 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

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

[Marco Leise via Digitalmars-d-learn]

Usually I want the keys to be declared "immutable" to signal
that their content must not change in order to provide stable
hashes. But when you remove items from the table you need to
call a const/immutable dtor that needs to be written for
everything that can be a hash table key.

What do you put into such a const/immutable dtor?
How do others deal with this?

-- 
Marco

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

[cy via Digitalmars-d-learn]

On Friday, 5 February 2016 at 22:18:50 UTC, Marco Leise wrote:
But when you remove items from the table you need to call a 
const/immutable dtor that needs to be written for everything 
that can be a hash table key.


You need to write destructors for hash keys? How would you use 
string literals as keys then? Could you provide an example 
maybe...?

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

[Marco Leise via Digitalmars-d-learn]

Am Sat, 06 Feb 2016 03:38:54 +
schrieb cy :

> On Friday, 5 February 2016 at 22:18:50 UTC, Marco Leise wrote:
> > But when you remove items from the table you need to call a 
> > const/immutable dtor that needs to be written for everything 
> > that can be a hash table key.
> 
> You need to write destructors for hash keys? How would you use 
> string literals as keys then? Could you provide an example 
> maybe...?

No, but they could have dtors because they contain malloc'd
data. E.g. string literals that don't live on the GC heap.

-- 
Marco