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digitalmars.D.learn - Static attributes aren' immutable

reply bearophile <bearophileHUGS lycos.com> writes:
I'm playing some more with immutables in D2. This program compiles:

struct Foo {
    static int x;
}
void main() {
    immutable Foo f;
    Foo.x++;
    f.x++;
}

Is this code supposed to be correct? If I have an immutable struct I want all
of it to be immutable...

Bye and thank you,
bearophile
Mar 05 2010
next sibling parent reply div0 <div0 users.sourceforge.net> writes:
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bearophile wrote:

 I'm playing some more with immutables in D2. This program compiles:
 
 struct Foo {
     static int x;
 }
 void main() {
     immutable Foo f;
     Foo.x++;
     f.x++;
 }
 
 Is this code supposed to be correct? If I have an immutable struct I want all
of it to be immutable...
 
 Bye and thank you,
 bearophile


Yes it's correct.

x is really just a global variable, or a thread local variable if you
are using a newer compiler.

putting it in Foo simply puts it in a namespace.

You could argue that 'f.x++' should not be allowed
in order to emphasis that x is not part of an instance,
but that could make writing generic code more awkward.

IIRC, you can do:

immutable struct Foo {
    static int x;
}

Then you can't write to x, but then you can't ever
write to any instance members either.

- --
My enormous talent is exceeded only by my outrageous laziness.
http://www.ssTk.co.uk
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Mar 05 2010
parent reply bearophile <bearophileHUGS lycos.com> writes:
div0:

putting it in Foo simply puts it in a namespace.<


So my (wrong) idea of immutable applied to a struct was that every thing in
such namespace becomes immutable (I think this is a bit more intuitive).

What do you think of modifying D2 so in a situation like the one I've shown
even static arguments become const/immutable? Can this cause troubles to other
things?

Thank you to you and Lars T. Kyllingstad for the answers.

Bye,
bearophile
Mar 05 2010
parent reply =?ISO-8859-1?Q?Pelle_M=E5nsson?= <pelle.mansson gmail.com> writes:
On 03/05/2010 07:50 PM, bearophile wrote:

 div0:

 putting it in Foo simply puts it in a namespace.<


 So my (wrong) idea of immutable applied to a struct was that every thing in
such namespace becomes immutable (I think this is a bit more intuitive).

 What do you think of modifying D2 so in a situation like the one I've shown
even static arguments become const/immutable? Can this cause troubles to other
things?

 Thank you to you and Lars T. Kyllingstad for the answers.

 Bye,
 bearophile


Immutability (somewhat) guarantees the value will never ever change. The 
static attribute could be changed by a non-immutable instance, and can 
therefore not be immutable. It could be const for the immutable 
instance, but I don't see the gains from it.

I do, however, see the gains from not being able to access the static 
members through an instance.
Mar 05 2010
parent reply div0 <div0 users.sourceforge.net> writes:
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Pelle Månsson wrote:

 On 03/05/2010 07:50 PM, bearophile wrote:

 div0:

 putting it in Foo simply puts it in a namespace.<


 So my (wrong) idea of immutable applied to a struct was that every
 thing in such namespace becomes immutable (I think this is a bit more
 intuitive).

 What do you think of modifying D2 so in a situation like the one I've
 shown even static arguments become const/immutable? Can this cause
 troubles to other things?

 Thank you to you and Lars T. Kyllingstad for the answers.

 Bye,
 bearophile


 
 Immutability (somewhat) guarantees the value will never ever change. The
 static attribute could be changed by a non-immutable instance, and can
 therefore not be immutable. It could be const for the immutable
 instance, but I don't see the gains from it.
 
 I do, however, see the gains from not being able to access the static
 members through an instance.


Yeah, I *never* access static vars through an instance,
I always use the class name.

Somebody want to post in the main group?

Might be worth a bigger discussion; perhaps somebody can provide a good
use case for access through an instance we're not seeing.

- --
My enormous talent is exceeded only by my outrageous laziness.
http://www.ssTk.co.uk
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Mar 05 2010
next sibling parent bearophile <bearophileHUGS lycos.com> writes:
div0:

 Somebody want to post in the main group?


OK, I'll do it soon :-)

Bye,
bearophile
Mar 05 2010
prev sibling parent bearophile <bearophileHUGS lycos.com> writes:
div0:

 Yeah, I *never* access static vars through an instance,
 I always use the class name.
 
 Somebody want to post in the main group?


It seems there are few people that agree with you, but Walter has not answered
about this yet, so you can ask him his opinion. I like to use a tidy language.

Bye,
bearophile
Mar 06 2010
prev sibling parent "Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:
bearophile wrote:

 I'm playing some more with immutables in D2. This program compiles:
 
 struct Foo {
     static int x;
 }
 void main() {
     immutable Foo f;
     Foo.x++;
     f.x++;
 }
 
 Is this code supposed to be correct? If I have an immutable struct I want all
of it to be immutable...
 
 Bye and thank you,
 bearophile



It is correct, because x isn't a part of the struct, it's a global 
variable.  However, in my opinion that last line should cause a compiler 
error -- it shouldn't be possible to access static members through an 
instance of the struct.

-Lars
Mar 05 2010