Don wrote:
Ary Borenszweig wrote:
Michiel Helvensteijn escribió:
> Walter Bright wrote:
>
>> immutable - data that cannot change or be changed (imagine it is
stored
>> in ROM)
>>
>> const - read only view of data, you cannot change it but others can
>>
>> enum - compile time constant, has no storage
>>
>> The only place these overlap is in the declaration of symbolic
>> constants. C++ has all three, but in a way that is context dependent
>> that very few people are aware of.
>
> Aren't immutable and enum the same thing? At least to the programmer?
Yesterday I was thiking the same thing. I think the only difference to
the programmer between those is that you can take the address of
something immutable, but can't take the address of a compile time
constant.
Now I wonder why would you like to take the address of something
that's immutable. The times I remember passing something's address is
to change it, something like:
int x = 10;
foo(&x);
// now x might have changed
But immutable variables cannot change, so no need to pass it by
reference.
The other usage is performance. If it's an immutable big struct you
might want to pass it by reference instead of copying the whole thing.
If you can't pass a reference to something immutable you couldn't do
with this optimization. But in this case, the compiler could choose to
rewrite mentions to big immutable variables as references to those in
a transparent way to the programmer. Of course this is harder for the
compiler writer, but it's much easier for the user.
(as a starting point the compiler could do without this optimization
and see if the users are having problems without it)
I can't come up with other cases where you'd want to pass a reference
to something immutable.
So let's suppose we don't need to take an immutable varaible's
address. We get rid of "enum" then and unify "immutable" and "enum"
into the single "immutable" keyword. The compiler still has to
determine what to put in ROM and what not. But I think this one is
easy: primitive types can go in ROM, also maybe small immutable
structs, maybe primitive arrays, but not classes or references to
classes.
Is there something fundamentally wrong with this reasoning?
AFAIK, the only reason 'enum' manifest constants exist in D2 is because
of linking limitations. OPTLINK isn't smart enough to be able to discard
immutable references. There's no need to use them, unless you have
something like the Windows headers, where they're an enormous bloat if
they aren't discarded. Most people should forget they exist.
Since objconv works with OMF now, how hard would it be to get a workflow
going to use a different linker on Windows? Every other post these days
seems to be "oh, it's an optlink issue"
