Re: Best way to reference an array in a child class...
On Friday, 7 March 2014 at 13:57:31 UTC, Steven Schveighoffer
wrote:
On Thu, 06 Mar 2014 17:44:09 -0500, captain_fid
wrote:
this() {items = [ {10, "first"}, {20, "second"}];}
strangely enough, when modeling this the first time (using
items as a class) and 'new item() syntax) there was no real
issue.
I thought using a static array of structs in the children
would be more efficient when instantiating the objects. Never
mind whether its true or not - Speed isn't a really a concern,
learning is.
I missed this the first time. That is a difference between my
code and your code. Mine creates a new instance of an array on
*object* initialization, yours creates ONE instance of an
array, that all objects share.
This is not necessarily a good thing. Because you've created a
mutable version of the array. I believe it is initialized on
startup from the heap.
One really bad thing is, the same array is used if you
initialize from multiple threads. And it's mutable, making it
implicitly shared even though it shouldn't be. You should make
the array immutable and static, or else initialize it in the
constructor. I don't know your use case, so it's hard to say
what you should do.
-Steve
I don't know your use case, so it's hard to say what you
should do.
Steve, I don't think I know my use case -- So it's not you. I'm
attempting to model hardware, where 'items' in this case are an
static array of registers. Single threaded (for now). Mainly,
this a learning opportunity to get a better understanding of D
for future comparison (vs. C++).
This is not necessarily a good thing. Because you've created a
mutable version of the array. I believe it is initialized on
startup from the heap.
Lot to learn. I understand initialized from the heap, but do you
mean at program startup, or object instantiation? If program, I
would have expected that with __gshared (globals) only, not with
this.
One really bad thing is, the same array is used if you
initialize from multiple threads. And it's mutable, making it
implicitly shared even though it shouldn't be. You should make
the array immutable and static, or else initialize it in the
constructor.
I appreciate your suggestions and the patience.
Re: Best way to reference an array in a child class...
On Thu, 06 Mar 2014 17:44:09 -0500, captain_fid wrote:
this() {items = [ {10, "first"}, {20, "second"}];}
strangely enough, when modeling this the first time (using items as a
class) and 'new item() syntax) there was no real issue.
I thought using a static array of structs in the children would be more
efficient when instantiating the objects. Never mind whether its true or
not - Speed isn't a really a concern, learning is.
I missed this the first time. That is a difference between my code and
your code. Mine creates a new instance of an array on *object*
initialization, yours creates ONE instance of an array, that all objects
share.
This is not necessarily a good thing. Because you've created a mutable
version of the array. I believe it is initialized on startup from the heap.
One really bad thing is, the same array is used if you initialize from
multiple threads. And it's mutable, making it implicitly shared even
though it shouldn't be. You should make the array immutable and static, or
else initialize it in the constructor. I don't know your use case, so it's
hard to say what you should do.
-Steve
Re: Best way to reference an array in a child class...
On Friday, 7 March 2014 at 00:10:20 UTC, bearophile wrote:
captain_fid:
struct S
{
int a;
string b;
}
class A
{
S[] items;
abstract void doit();
}
class B: A
{
this() {items = [ {10, "first"}, {20, "second"}];}//
line 21
override void doit() { }
}
(21): Error: found '}' when expecting ';' following statement
(21): Error: found ',' instead of statement
For reasons I don't know that {} syntax doesn't always work in
D. So try:
this() {
this.items = [S(10, "first"), S(20, "second")];
}
Bye,
bearophile
Going to have to buy the whole bar a round. Thanks bearophile for
the quick fix!
Re: Best way to reference an array in a child class...
captain_fid:
struct S
{
int a;
string b;
}
class A
{
S[] items;
abstract void doit();
}
class B: A
{
this() {items = [ {10, "first"}, {20, "second"}];}//
line 21
override void doit() { }
}
(21): Error: found '}' when expecting ';' following statement
(21): Error: found ',' instead of statement
For reasons I don't know that {} syntax doesn't always work in D.
So try:
this() {
this.items = [S(10, "first"), S(20, "second")];
}
Bye,
bearophile
Re: Best way to reference an array in a child class...
Well, actually... take it back.
When I did try this syntax, I receive(d) the following error. I
then went and created this test which I thought couldn't go wrong.
with both dmd and gdc ...
struct S
{
int a;
string b;
}
class A
{
S[] items;
abstract void doit();
}
class B: A
{
this() {items = [ {10, "first"}, {20, "second"}];}// line
21
override void doit() { }
}
(21): Error: found '}' when expecting ';' following statement
(21): Error: found ',' instead of statement
Re: Best way to reference an array in a child class...
this() {items = [ {10, "first"}, {20, "second"}];}
strangely enough, when modeling this the first time (using items
as a class) and 'new item() syntax) there was no real issue.
I thought using a static array of structs in the children would
be more efficient when instantiating the objects. Never mind
whether its true or not - Speed isn't a really a concern,
learning is.
Re: Best way to reference an array in a child class...
On Thursday, 6 March 2014 at 22:16:50 UTC, Steven Schveighoffer
wrote:
On Thu, 06 Mar 2014 17:05:12 -0500, captain_fid
wrote:
On Thursday, 6 March 2014 at 21:26:11 UTC, Ali Çehreli wrote:
On 03/06/2014 12:02 PM, Steven Schveighoffer wrote:
> The best way
> to reference an array in a child class, especially one of a
static type,
> is to not have another copy in the child class :)
Agreed. Alternatively, a member function in the child class
could return a slice.
Ali
Steve, thanks for the link and the nicely written article.
Also for the assessment on pointer syntax, I'd love to avoid
if possible (especially w/ limitations you noted).
I had been spending time over at http://dlang.org/arrays.html
and had forgotten (or never understood) dynamic arrays were
passed by slices. That link only talks about passing static
arrays (IIRC). Keeping track of 'what being passed how' is
tough for this programmer.
Your suggestion Ali (of not accessing the base member in the
child was great) and it works properly.
Believe if I understand what you are suggesting above is never
to have the array in base? simply retrieve slice through the
child member function?
I think what Ali means is:
class A
{
abstract S[] items();
}
class B : A
{
S[] _items = [ {10, "first"}, {20, "second"}];
override S[] items() { return _items;}
}
What I was saying is, if you know the items are going to be
stored in the object, just store them in the base:
class A
{
S[] items;
}
class B : A
{
this() {items = [ {10, "first"}, {20, "second"}];}
}
This way, both the derived and the base will always see the
same items, and you only store it in the object once. Obviously
if you store the items elsewhere in some derivatives, Ali's
idea is preferable.
-Steve
Yes Steve!
Even if the title of the thread is off, the original intent was
what you've shown. For a reason (probably duplicate definitions
in base and child) I would end up with items in B and not visible
in A.
I didn't understand the syntax for performing the above.
I owe you guys a beer (or something). Definitely a learning
lesson.
Re: Best way to reference an array in a child class...
On 03/06/2014 02:05 PM, captain_fid wrote:
> Your suggestion Ali (of not accessing the base member in the child was
> great) and it works properly.
>
> Believe if I understand what you are suggesting above is never to have
> the array in base? simply retrieve slice through the child member
function?
Yes, but it also depends on the need. I assumed that the base class
needed a slice of elements from the base class.
Will the slice of elements ever change? If no, then the base could take
them at construction time and use them in the future:
import std.stdio;
class B
{
const(int[]) items;
this(const(int[]) items)// <-- Requires elements
// during construction
{
this.items = items;
}
final void do_work()
{
writeln(items);
}
}
class D : B
{
this()
{
super([ 1, 2, 3]);// Items are determined here
}
}
void main()
{
auto d = new D();
d.do_work();
}
Alternatively, and especially if the elements will change at run time,
the derived class can hold on to the elements and can provide them as
the base class needs:
import std.stdio;
class B
{
abstract const(int)[] items();
final void do_work()
{
writeln(items());
}
}
class D : B
{
int[] myItems;
this()
{
myItems = [ 1, 2, 3];
}
void add(int[] items)
{
myItems ~= items;
}
override const(int)[] items()
{
return myItems;
}
}
void main()
{
auto d = new D();
d.do_work(); // 1, 2, 3
// Add more later on
d.add([ 4, 5, 6 ]);
d.do_work(); // 1, 2, 3, 4, 5, 6
}
Ali
Re: Best way to reference an array in a child class...
On Thu, 06 Mar 2014 17:05:12 -0500, captain_fid wrote:
On Thursday, 6 March 2014 at 21:26:11 UTC, Ali Çehreli wrote:
On 03/06/2014 12:02 PM, Steven Schveighoffer wrote:
> The best way
> to reference an array in a child class, especially one of a
static type,
> is to not have another copy in the child class :)
Agreed. Alternatively, a member function in the child class could
return a slice.
Ali
Steve, thanks for the link and the nicely written article. Also for the
assessment on pointer syntax, I'd love to avoid if possible (especially
w/ limitations you noted).
I had been spending time over at http://dlang.org/arrays.html and had
forgotten (or never understood) dynamic arrays were passed by slices.
That link only talks about passing static arrays (IIRC). Keeping track
of 'what being passed how' is tough for this programmer.
Your suggestion Ali (of not accessing the base member in the child was
great) and it works properly.
Believe if I understand what you are suggesting above is never to have
the array in base? simply retrieve slice through the child member
function?
I think what Ali means is:
class A
{
abstract S[] items();
}
class B : A
{
S[] _items = [ {10, "first"}, {20, "second"}];
override S[] items() { return _items;}
}
What I was saying is, if you know the items are going to be stored in the
object, just store them in the base:
class A
{
S[] items;
}
class B : A
{
this() {items = [ {10, "first"}, {20, "second"}];}
}
This way, both the derived and the base will always see the same items,
and you only store it in the object once. Obviously if you store the items
elsewhere in some derivatives, Ali's idea is preferable.
-Steve
Re: Best way to reference an array in a child class...
On Thursday, 6 March 2014 at 21:26:11 UTC, Ali Çehreli wrote: On 03/06/2014 12:02 PM, Steven Schveighoffer wrote: > The best way > to reference an array in a child class, especially one of a static type, > is to not have another copy in the child class :) Agreed. Alternatively, a member function in the child class could return a slice. Ali Steve, thanks for the link and the nicely written article. Also for the assessment on pointer syntax, I'd love to avoid if possible (especially w/ limitations you noted). I had been spending time over at http://dlang.org/arrays.html and had forgotten (or never understood) dynamic arrays were passed by slices. That link only talks about passing static arrays (IIRC). Keeping track of 'what being passed how' is tough for this programmer. Your suggestion Ali (of not accessing the base member in the child was great) and it works properly. Believe if I understand what you are suggesting above is never to have the array in base? simply retrieve slice through the child member function?
Re: Best way to reference an array in a child class...
On 03/06/2014 12:02 PM, Steven Schveighoffer wrote: > The best way > to reference an array in a child class, especially one of a static type, > is to not have another copy in the child class :) Agreed. Alternatively, a member function in the child class could return a slice. Ali
Re: Best way to reference an array in a child class...
Steve and Ali, Thanks for the quick helpful suggestions. In this case I probably don't need the expansion (but certainly look forward to understanding either way). I'll certainly hit that reference (Special thanks Ali for your book. I've enjoyed it over the past few months)
Re: Best way to reference an array in a child class...
On Thu, 06 Mar 2014 15:02:14 -0500, Steven Schveighoffer wrote: Second, it's very difficult to get an array, just by itself, in the heap. And you don't want to store a reference to a stack-frame slice. Sorry, I meant an array *reference*, not an array. Clearly allocating an array in the heap is easy, but allocating a pointer to an array is not :) -Steve
Re: Best way to reference an array in a child class...
On Thu, 06 Mar 2014 14:47:49 -0500, Ali Çehreli wrote:
On 03/06/2014 11:40 AM, Steven Schveighoffer wrote:
>> class A
>> {
>>S[]* pointer_to_list;
>>abstract...
>> }
> I would highly suggest to just use S[] and not S[]*. A slice is
already
> a reference (coupled with a length).
But what if there are elements added to B.items later on? I assumed the
OP wanted a true reference.
First, it's very cumbersome to work with a pointer to an array. All array
syntax sugar does not peek through the reference, you have to * everything.
Second, it's very difficult to get an array, just by itself, in the heap.
And you don't want to store a reference to a stack-frame slice.
I think there is a good reason you seldom see code that has pointers to
array in D, the above code snippet seems to me like a programmer who
doesn't understand what a D slice is.
Actually, given the subject of this post, I take it back. The best way to
reference an array in a child class, especially one of a static type, is
to not have another copy in the child class :)
> You can read more about D arrays and slices here:
>
> http://dlang.org/d-array-article.html
My I suggest you do the same. :o)
TL;DR ;)
-Steve
Re: Best way to reference an array in a child class...
On 03/06/2014 11:40 AM, Steven Schveighoffer wrote:
>> class A
>> {
>>S[]* pointer_to_list;
>>abstract...
>> }
> I would highly suggest to just use S[] and not S[]*. A slice is already
> a reference (coupled with a length).
But what if there are elements added to B.items later on? I assumed the
OP wanted a true reference.
> You can read more about D arrays and slices here:
>
> http://dlang.org/d-array-article.html
My I suggest you do the same. :o)
> -Steve
Ali
Re: Best way to reference an array in a child class...
On Thu, 06 Mar 2014 14:31:52 -0500, captain_fid wrote:
On Thursday, 6 March 2014 at 19:19:29 UTC, captain_fid wrote:
Sorry for the very basic question. Much still alludes me with this
language. I appreciate the forum.
struct S
{
Wow sorry for that. I'm a moron... don't press ...
struct S
{
int a;
string b;
}
class A
{
S[]* pointer_to_list;
abstract...
}
class B: A
{
S[] items = [ {10, "first"}, {20, "second"}];
this() {
pointer_to_list = &items;
}
}
My problem is in de-referencing later (seg fault). Is this even the best
way?
I Really need to access the array in a derived class. 'S' (I believe)
really is best as a Structure.
Any suggestions. Thanks in advance (and sorry for the rough start).
I would highly suggest to just use S[] and not S[]*. A slice is already a
reference (coupled with a length).
You can read more about D arrays and slices here:
http://dlang.org/d-array-article.html
-Steve
Re: Best way to reference an array in a child class...
On 03/06/2014 11:31 AM, captain_fid wrote:
On Thursday, 6 March 2014 at 19:19:29 UTC, captain_fid wrote:
Sorry for the very basic question. Much still alludes me with this
language. I appreciate the forum.
struct S
{
Wow sorry for that. I'm a moron... don't press ...
struct S
{
int a;
string b;
}
class A
{
S[]* pointer_to_list;
abstract...
}
class B: A
{
S[] items = [ {10, "first"}, {20, "second"}];
this() {
pointer_to_list = &items;
}
}
My problem is in de-referencing later (seg fault). Is this even the best
way?
I Really need to access the array in a derived class. 'S' (I believe)
really is best as a Structure.
Any suggestions. Thanks in advance (and sorry for the rough start).
You need to dereference the pointer by * and you would be using a slice:
import std.stdio;
import std.conv;
struct S
{
int a;
string b;
}
class A
{
S[]* pointer_to_list;
void access()
{
foreach (e; *pointer_to_list) {// <-- NOTE *
writeln(e);
}
}
}
class B: A
{
S[] items = [ {10, "first"}, {20, "second"}];
this() {
pointer_to_list = &items;
}
}
void main()
{
auto b = new B();
foreach (i; 30 .. 40) {
b.items ~= S(i, i.to!string);
}
b.access();
}
As a side note, I would make A take the slice as a reference parameter,
rather that B accessing A's member directly. It is better for
maintainability:
class A
{
S[]* pointer_to_list;
this(ref S[] list)// <-- BETTER
{
this.pointer_to_list = &list;
}
// ...
}
class B: A
{
// ...
this() {
super(items);// <-- BETTER
}
}
Ali
Re: Best way to reference an array in a child class...
On Thursday, 6 March 2014 at 19:19:29 UTC, captain_fid wrote:
Sorry for the very basic question. Much still alludes me with
this language. I appreciate the forum.
struct S
{
Wow sorry for that. I'm a moron... don't press ...
struct S
{
int a;
string b;
}
class A
{
S[]* pointer_to_list;
abstract...
}
class B: A
{
S[] items = [ {10, "first"}, {20, "second"}];
this() {
pointer_to_list = &items;
}
}
My problem is in de-referencing later (seg fault). Is this even
the best way?
I Really need to access the array in a derived class. 'S' (I
believe) really is best as a Structure.
Any suggestions. Thanks in advance (and sorry for the rough
start).
Best way to reference an array in a child class...
Sorry for the very basic question. Much still alludes me with
this language. I appreciate the forum.
struct S
{
