Inner struct accessing host member
I'd have thought that this would work:
struct A
{
int[] i;
B b;
struct B
{
void foo() { i ~= 1;}
}
}
void main()
{
A a;
a.b.foo();
}
But the compiler tells me 'need this for i of type int[]'.
Is there any way I can gain access on i inside B?
Re: Inner struct accessing host member
On Tuesday, 5 August 2014 at 20:32:08 UTC, Philippe Sigaud wrote:
I'd have thought that this would work:
struct A
{
int[] i;
B b;
struct B
{
void foo() { i ~= 1;}
}
}
void main()
{
A a;
a.b.foo();
}
But the compiler tells me 'need this for i of type int[]'.
Is there any way I can gain access on i inside B?
I know I've read this in TDPL, but don't recall enough.
Does this help :
http://www.digitalmars.com/d/archives/digitalmars/D/learn/Nested_struct_member_has_no_access_to_the_enclosing_class_data_38294.html
?
Re: Inner struct accessing host member
On Tue, Aug 5, 2014 at 11:37 PM, Martijn Pot via Digitalmars-d-learn wrote: > Does this help : > http://www.digitalmars.com/d/archives/digitalmars/D/learn/Nested_struct_member_has_no_access_to_the_enclosing_class_data_38294.html Yes, that helps: that explains why it does not wor :). I changed my code to use classes. It's a bit less handy, but it works.
Re: Inner struct accessing host member
> why it does not wor :). why it does not *work*, of course. Sigh.
Re: Inner struct accessing host member
On Tuesday, 5 August 2014 at 20:32:08 UTC, Philippe Sigaud wrote:
I'd have thought that this would work:
struct A
{
int[] i;
B b;
struct B
{
void foo() { i ~= 1;}
}
}
void main()
{
A a;
a.b.foo();
}
But the compiler tells me 'need this for i of type int[]'.
Is there any way I can gain access on i inside B?
programming Q, either youra newb or not, should rather be posted
to 'http://forum.dlang.org/group/digitalmars.D.learn'. Your post
appears on 'http://forum.dlang.org/group/digitalmars.D' which is
more related to the lang. design rather to programming Q. Take
care next time bro.
Re: Inner struct accessing host member
On Tuesday, 5 August 2014 at 20:32:08 UTC, Philippe Sigaud wrote:
But the compiler tells me 'need this for i of type int[]'.
Is there any way I can gain access on i inside B?
Been thinking about this a bit. I know some of my relies are in
the 2012 fourm posts regarding it, but access permissions seems
like the biggest reason, or rather lack of control of them.
So take your example:
struct A
{
int[] i;
B b;
}
Now let's make a couple instances of it; And assume it would
work...
A a;
immutable A i_a;
a.b.foo(); //fine
i_a.b.foo(); //won't run, due to not being const/immutable
So, a user decides let's copy the inner struct. If the struct
copies it's attached secondary pointer going to it's outer/host,
then:
A.B b = a.b;
A.B i_b = i_a.b;
A.B broken_b = cast(A.B) i_a.b;
b.foo(); //attached to a still, works...
i_b.foo(); //const or immutable, won't work.
broken_b.foo(); //i_a is accessible invisibly because overridden
or transformed assuming it would be converted or copied/moved as
appropriate.
return b; //if a is a local variable then b becomes invalid even
though it's a struct.
return i_b; //same as return b
return broken_b; //same as above two cases.
inner structs in a function where the struct is never passed
outside the function would probably work though...
void func() {
int[] i;
struct B {
void foo() { i ~= 1;}
}
B b;
b.foo(); //passed a reference to the current frame along with
it's local 'this', but since it never leaves the function it's
safe.
}
Now a current way to make it safe while still leaving it structs
could be passing a reference to either the outer struct or the
variable in question. For simplicity it would probably be the
struct.
struct A
{
int[] i;
B b;
struct B
{
void foo(ref A outer) { outer.i ~= 1;}
}
void bar() //call B foo
{
b.foo(this);
}
}
Or less safe is to use a pointer and assign it when b
instantiates to point back to A.. But if you pass B around
without A and A goes out of scope... same problem...
Maybe i'm over-thinking it.
Re: Inner struct accessing host member
On 08/05/14 22:32, Philippe Sigaud via Digitalmars-d-learn wrote:
> I'd have thought that this would work:
>
> struct A
> {
> int[] i;
> B b;
>
> struct B
> {
> void foo() { i ~= 1;}
> }
> }
>
> void main()
> {
> A a;
> a.b.foo();
> }
>
> But the compiler tells me 'need this for i of type int[]'.
> Is there any way I can gain access on i inside B?
Not directly, but as you ask for /any/ way -- yes:
struct B
{
void foo() { outer.i ~= 1; }
ref A outer() inout @property { return
*cast(A*)(cast(void*)&this-A.b.offsetof); }
}
Note this will work only as long as you have just one B
instance in A and B is never created or copied outside of A.
artur
Re: Inner struct accessing host member
On Tuesday, 5 August 2014 at 22:14:23 UTC, abanstadya wrote: programming Q, either youra newb or not, should rather be posted to 'http://forum.dlang.org/group/digitalmars.D.learn'. Your post appears on 'http://forum.dlang.org/group/digitalmars.D' which is more related to the lang. design rather to programming Q. Take care next time bro. This *is* D.learn, bro.
Re: Inner struct accessing host member
On Tuesday, 5 August 2014 at 23:47:00 UTC, Artur Skawina via
Digitalmars-d-learn wrote:
Is there any way I can gain access on i inside B?
Not directly, but as you ask for /any/ way -- yes:
struct B
{
void foo() { outer.i ~= 1; }
ref A outer() inout @property { return
*cast(A*)(cast(void*)&this-A.b.offsetof); }
}
Note this will work only as long as you have just one B
instance in A and B is never created or copied outside of A.
OK. I have en entire graph, whose nodes are Bs inside A. So that
might not be totally appropriate for me. Thanks anyway, I always
forget about offsetof
Re: Inner struct accessing host member
Era: broken_b.foo(); //i_a is accessible invisibly because overridden or transformed assuming it would be converted or copied/moved as appropriate. return b; //if a is a local variable then b becomes invalid even though it's a struct. return i_b; //same as return b return broken_b; //same as above two cases. I see. I didn't know one could create an A.B 'outside'. I saw inner types as Voldemort types, but that is true only for inner structs in functions. Now a current way to make it safe while still leaving it structs could be passing a reference to either the outer struct or the variable in question. For simplicity it would probably be the struct. (...) Or less safe is to use a pointer and assign it when b instantiates to point back to A.. But if you pass B around without A and A goes out of scope... same problem... Maybe i'm over-thinking it. I already tried to propagate a ref through A's methods, but that made a mess: I have lots of methods, which have all to transmit this ref, only for *one* of them being able to update it. Thanks for you explanations :) I'm now using classes and inner classes. I'm not fond of classes, but that's working correctly.
Re: Inner struct accessing host member
On Wednesday, 6 August 2014 at 05:53:55 UTC, Philippe Sigaud wrote: I see. I didn't know one could create an A.B 'outside'. I saw inner types as Voldemort types, but that is true only for inner structs in functions. But we weren't creating them, we were copying them, no constructors were used. If the struct is private that may follow different rules as the struct can't leave the outer struct. hmmm static and private... other keywords to try, but offhand it's been a while i don't know if either would change the behavior. Could just be inner scope limitations. Might be other tags/modifiers... I feel helpless :( I already tried to propagate a ref through A's methods, but that made a mess: I have lots of methods, which have all to transmit this ref, only for *one* of them being able to update it. Thanks for you explanations :) I'm now using classes and inner classes. I'm not fond of classes, but that's working correctly. I'm not sure if it would help, but sometimes if you reverse the logic you might get what you want by putting the data in B instead of A. The example coming to mind is from the game 'Mark of the Ninja'. In order to speed up and get their code to work how they wanted, instead of having guards listen for sounds (which would be a constant pinging to look for sounds and other effects), the sound listened for the guards and notified them... So if the data is in B, A can access B's data; Although if you have another class C next to B, then communication between them becomes more of a pain probably... Just something to consider
Re: Inner struct accessing host member
> hmmm static and private... other keywords to > try, but offhand it's been a while i don't know if either would change the > behavior. Could just be inner scope limitations. Might be other > tags/modifiers... > > I feel helpless :( No need to ;-) Thanks for your help, don't sweat it too much. > I'm not sure if it would help, but sometimes if you reverse the logic you > might get what you want by putting the data in B instead of A. I have a lot of Bs (nodes in a graph). They compute some things and when they get a result, they update A's field. Each A holds the entry point to their inner graph of Bs and waits for the results. So I don't see how I could invert it, really. What *could* do it to have the graph of Bs in thread and sending results as messages to another thread, where A is waiting for them. It's just... I'm so used to being able to mix and compose 'concepts' in D: structs in functions, classes in classes in structs, functions returning functions returning structs, etc. I'm used to begin able to organise my code as I see the problem space. But here, with a struct-in-a-struct, I hit a wall. Not fun, but not problematic too...
