Re: null Vs [] return arrays
bearophile Wrote: > Regan Heath: > > > conceptually it's nice to be able to express (exists but is empty) and > > (does not exist). > > You may want to express that, but for the implementation of the language > those two situations are the same, because in the [] literal the ptr is null. > So I think it's better for the programmer to not differentiate the two > situations, because they are not different. If the programmer tells them > apart, he/she is doing something bad in D, creating a false illusion. It's bad, when the language is driven by the implementation of a "reference" compiler by the copyright holder. This way compiler bugs and tricks become a language standard. See the story of VP7 codec.
Re: null Vs [] return arrays
On Tue, 05 Apr 2011 18:46:06 +0100, Steven Schveighoffer
wrote:
On Tue, 05 Apr 2011 13:24:49 -0400, Regan Heath
wrote:
On Fri, 01 Apr 2011 18:23:28 +0100, Steven Schveighoffer
wrote:
assert("" !is null); // works on D. Try it.
Yes, but that's because this is a string literal. It's not useful
where you're getting your input from somewhere else.. like in the other
2 use cases I mentioned.
But that isn't the same as []. Basically, if you have an existing
array, and you want to create a non-null empty array out of it, a slice
of [0..0] always works.
I know you mention it, but I want to draw attention to the original
problem, that [] returns a null array. Other cases where you are not
using [] or "" are a separate issue.
All the cases you have brought up involve strings, for which there is a
non-null array returned for "". I still have not yet seen a compelling
use case for making [] return non-null.
Ahh.. I see, I really should have renamed the thread title. I'm not, and
never was, arguing for [] (specifically) returning non-null. Sorry.
Quoting from your message previously (with added comment):
case 4:
return cast(char[])"".dup;
case 5:
return cast(char[])""[0..0]; // note lack of .dup
}
Drat, not sure what happened there. My source had the 'dup' when I went
back to it. Sorry.
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Re: null Vs [] return arrays
On Tue, 05 Apr 2011 13:24:49 -0400, Regan Heath
wrote:
On Fri, 01 Apr 2011 18:23:28 +0100, Steven Schveighoffer
wrote:
assert("" !is null); // works on D. Try it.
Yes, but that's because this is a string literal. It's not useful where
you're getting your input from somewhere else.. like in the other 2 use
cases I mentioned.
But that isn't the same as []. Basically, if you have an existing array,
and you want to create a non-null empty array out of it, a slice of [0..0]
always works.
I know you mention it, but I want to draw attention to the original
problem, that [] returns a null array. Other cases where you are not
using [] or "" are a separate issue.
All the cases you have brought up involve strings, for which there is a
non-null array returned for "". I still have not yet seen a compelling
use case for making [] return non-null.
The other use case may be a little more problematic depending on the
method used to read the input from the keyboard, IIRC one of the methods
returns null for a blank line of input, which I would have to detect and
'fix' using emptyArray if I wanted to pass it to something that cares
about the distinction.
That is up to the implementation of that function. D provides ways to
return an empty array that does not have a null pointer.
It's one thing to want an array with a non-null pointer, but it's
another thing entirely to want an array with a non-null pointer which
points to a valid heap address.
I don't specifically want either of those things. I just want _some
way_ to represent 'exists but is empty' and for it to be different to
'does not exist'. Currently D's arrays cannot do that, yet a plain old
pointer can.
Of course they can, you can check for null vs empty using "is null" or
".empty".
The issue you may have is that phobos does not always care about
preserving this distinction. One exmaple is dup. It is pointless to dup
an empty array (even if non-null) by creating a heap allocation, so it
just returns null.
In my opinion, [] means empty array. I don't care what the pointer is,
as long as the array is empty. The implementation can put whatever
value it wants for the pointer. If it wants to put null, that is
fine. null means I want a null pointer.
If I had it my way, all array literals would be immutable, and the
pointers would point to ROM (even empty ones). We should not be
constructing array literals at runtime. But my opinion is still that
you should not count on the pointer being anything because
it's not specified what it is.
Sure, I agree with all that, but I still want some way of representing
both states and detecting both states and the problem is that if the
language cannot do it at a fundamental level, and requires some weird
hack or reliance on string literals then when I use any 3rd party
library, or phobos itself it will tell me null and I will have to guess
which state it actually means and 'fix' it manually.
The array has the ability to store whether it's null and empty or just
empty, you are just expecting every function to care about that
distinction, which most don't.
That seems to work, but it's hideous syntax for something that is not
that uncommon IMO.
My opinion is that it is uncommon, but it can be abstracted:
template emptyArray(T)
{
enum emptyArray = (cast(T*)0)[1..1];
}
rename as desired.
Not useful if you're getting your input from somewhere else, vs trying
to create a new empty array.
Again, not relevant. Getting an empty-but-not-null array from a
non-null-non-empty array is trivial. This whole thread is about [].
That said, if I were to want this I'd use the literal instead as it
seems safer, eg.
template emptyArray(T)
{
enum emptyArray = cast(T[])""[0..0];
}
Either way should be safe. Nothing should use data outside the array
bounds.
This code seems to disagree with your results for case 5 (dmd 2.052):
auto x = cast(char[])""[0..0];
assert(x.ptr != null); // no failure
Nope, my case 5 had a 'dup' which you're missing. If I add a new case
returning a literal as you have there I get the same result as you. I
was intentionally avoiding the literal because I knew it would be
non-null (I believe D null terminates literals) and because I want to be
able to detect these states on more than just empty string literals.
Quoting from your message previously (with added comment):
case 4:
return cast(char[])"".dup;
case 5:
return cast(char[])""[0..0]; // note lack of .dup
}
-Steve
Re: null Vs [] return arrays
On Fri, 01 Apr 2011 18:23:28 +0100, Steven Schveighoffer
wrote:
On Fri, 01 Apr 2011 11:52:47 -0400, Regan Heath
wrote:
But, in D it seems I cannot do this. In D I would have to pass an
additional boolean parameter, or add another level of indirection i.e.
pass a string[]*. The same problem exists in C if I want to pass an
'int' or any primitive type, I have to pass it as int*, use a boolean,
or invent a 'special' value which means essentially
NULL/not-set/ignored.
assert("" !is null); // works on D. Try it.
Yes, but that's because this is a string literal. It's not useful where
you're getting your input from somewhere else.. like in the other 2 use
cases I mentioned.
However.. I've realised that in those cases, where you have an input array
you can slice, eg.
auto y = "a=123&b=&c=456"
auto z = y[8..8];
I do get an array 'z', with a non-null ptr. So, provided D doesn't lose
this information when I pass it around I am fine. The other use case may
be a little more problematic depending on the method used to read the
input from the keyboard, IIRC one of the methods returns null for a blank
line of input, which I would have to detect and 'fix' using emptyArray if
I wanted to pass it to something that cares about the distinction.
I'm not too bothered what syntax gets used, provided it was something
that you don't accidently use when you do not want it, and wasn't too
horrible to use as I don't see this as being a very uncommon occurance
(which would warrant/allow ugliness of syntax). "[]" >> seems logical,
as does "new T[]", both are not "null" so the programmer was obviously
trying to do something other than pass null.
It's one thing to want an array with a non-null pointer, but it's
another thing entirely to want an array with a non-null pointer which
points to a valid heap address.
I don't specifically want either of those things. I just want _some way_
to represent 'exists but is empty' and for it to be different to 'does not
exist'. Currently D's arrays cannot do that, yet a plain old pointer can.
In my opinion, [] means empty array. I don't care what the pointer is,
as long as the array is empty. The implementation can put whatever
value it wants for the pointer. If it wants to put null, that is fine.
null means I want a null pointer.
If I had it my way, all array literals would be immutable, and the
pointers would point to ROM (even empty ones). We should not be
constructing array literals at runtime. But my opinion is still that
you should not count on the pointer being anything because
it's not specified what it is.
Sure, I agree with all that, but I still want some way of representing
both states and detecting both states and the problem is that if the
language cannot do it at a fundamental level, and requires some weird hack
or reliance on string literals then when I use any 3rd party library, or
phobos itself it will tell me null and I will have to guess which state it
actually means and 'fix' it manually.
That seems to work, but it's hideous syntax for something that is not
that uncommon IMO.
My opinion is that it is uncommon, but it can be abstracted:
template emptyArray(T)
{
enum emptyArray = (cast(T*)0)[1..1];
}
rename as desired.
Not useful if you're getting your input from somewhere else, vs trying to
create a new empty array.
That said, if I were to want this I'd use the literal instead as it seems
safer, eg.
template emptyArray(T)
{
enum emptyArray = cast(T[])""[0..0];
}
This code seems to disagree with your results for case 5 (dmd 2.052):
auto x = cast(char[])""[0..0];
assert(x.ptr != null); // no failure
Nope, my case 5 had a 'dup' which you're missing. If I add a new case
returning a literal as you have there I get the same result as you. I was
intentionally avoiding the literal because I knew it would be non-null (I
believe D null terminates literals) and because I want to be able to
detect these states on more than just empty string literals.
R
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Re: null Vs [] return arrays
On Fri, 01 Apr 2011 11:52:47 -0400, Regan Heath
wrote:
On Fri, 01 Apr 2011 13:38:45 +0100, Steven Schveighoffer
wrote:
On Fri, 01 Apr 2011 06:38:56 -0400, Regan Heath
wrote:
On Mon, 28 Mar 2011 17:54:29 +0100, bearophile
wrote:
Steven Schveighoffer:
So essentially, you are getting the same thing, but using [] is
slower.
It seems I was right then, thank you and Kagamin for the answers.
This may be slightly OT but I just wanted to raise the point that
conceptually it's nice to be able to express (exists but is empty) and
(does not exist). Pointers/references have null as a (does not exist)
"value" and this is incredibly useful. Try doing the same thing with
'int' .. it requires you either use int* or pass an additional boolean
to indicate existence.. yuck.
I'd suggest if someone types '[]' they mean (exists but is empty) and
if they type 'null' they mean (does not exist) and they may be relying
on the .ptr value to differentiate these cases, which is useful. If
you're not interested in the difference, and you need performance, you
simply use 'null'. Everybody is happy. :)
The distinction is useful if you have something to reference (e.g. an
empty slice that points at the end of a pre-existing non-empty array).
But [] is a new array, no point in allocating memory just so the
pointer can be non-null. Can you come up with a use case to show why
you'd want such a thing?
Ok. Recently I wrote (in C) a function proxy interface. I had to
execute a set of functions from one thread, and wanted to 'call' them
from potentially many. So, I set up the thread, added events, and a
queue, etc and I wrote a proxy function for 'calling' them from the many
threads which looks like...
void proxy(int func, ...) {}
So, it accepts a variable list of args, places them in a structure,
places that in the queue, and waits on an event for the proxy thread to
execute the command and return the result. Lets say the function I am
executing is a database lookup, lets say I have a database field which
is a string, lets say it can be NULL (database definition allows
NULLS). Now, lets say I want to do these lookups:
1. lookup all objects where the field is NULL
2. lookup all objects where the field is "reganwashere"
3. lookup all objects where the field is "" (empty/non-null)
#1 and #2 are simple enough. I call proxy like..
proxy(LOOKUP, NULL);
proxy(LOOKUP, "reganwashere");
and in the actual lookup function, invoked by proxy, I call:
pFieldValue = va_arg(pArgs, char*);
and I get NULL, and "reganwashere".
In C, case #2 would also be easy, I would call proxy(LOOKUP, "") and in
the actual lookup function pFieldValue would be "" (not NULL).
But, in D it seems I cannot do this. In D I would have to pass an
additional boolean parameter, or add another level of indirection i.e.
pass a string[]*. The same problem exists in C if I want to pass an
'int' or any primitive type, I have to pass it as int*, use a boolean,
or invent a 'special' value which means essentially NULL/not-set/ignored.
assert("" !is null); // works on D. Try it.
Your plan would mean that [] is a memory allocation. I'd rather not
have the runtime do the lower performing thing unless there is a good
reason.
I'm not too bothered what syntax gets used, provided it was something
that you don't accidently use when you do not want it, and wasn't too
horrible to use as I don't see this as being a very uncommon occurance
(which would warrant/allow ugliness of syntax). "[]" seems logical, as
does "new T[]", both are not "null" so the programmer was obviously
trying to do something other than pass null.
It's one thing to want an array with a non-null pointer, but it's another
thing entirely to want an array with a non-null pointer which points to a
valid heap address.
In my opinion, [] means empty array. I don't care what the pointer is, as
long as the array is empty. The implementation can put whatever value it
wants for the pointer. If it wants to put null, that is fine. null means
I want a null pointer.
If I had it my way, all array literals would be immutable, and the
pointers would point to ROM (even empty ones). We should not be
constructing array literals at runtime. But my opinion is still that you
should not count on the pointer being anything because it's not specified
what it is.
As an alternative, you could use (cast(T *)null)[1..1] if you really
needed it (this also would be higher performing, BTW since the runtime
array literal function would not be called).
That seems to work, but it's hideous syntax for something that is not
that uncommon IMO.
My opinion is that it is uncommon, but it can be abstracted:
template emptyArray(T)
{
enum emptyArray = (cast(T*)0)[1..1];
}
rename as desired.
To remind myself what D does, and try and find another way to achive the
same thing I wrote a test case:
Re: null Vs [] return arrays
On Fri, 01 Apr 2011 13:38:45 +0100, Steven Schveighoffer
wrote:
On Fri, 01 Apr 2011 06:38:56 -0400, Regan Heath
wrote:
On Mon, 28 Mar 2011 17:54:29 +0100, bearophile
wrote:
Steven Schveighoffer:
So essentially, you are getting the same thing, but using [] is
slower.
It seems I was right then, thank you and Kagamin for the answers.
This may be slightly OT but I just wanted to raise the point that
conceptually it's nice to be able to express (exists but is empty) and
(does not exist). Pointers/references have null as a (does not exist)
"value" and this is incredibly useful. Try doing the same thing with
'int' .. it requires you either use int* or pass an additional boolean
to indicate existence.. yuck.
I'd suggest if someone types '[]' they mean (exists but is empty) and
if they type 'null' they mean (does not exist) and they may be relying
on the .ptr value to differentiate these cases, which is useful. If
you're not interested in the difference, and you need performance, you
simply use 'null'. Everybody is happy. :)
The distinction is useful if you have something to reference (e.g. an
empty slice that points at the end of a pre-existing non-empty array).
But [] is a new array, no point in allocating memory just so the pointer
can be non-null. Can you come up with a use case to show why you'd want
such a thing?
Ok. Recently I wrote (in C) a function proxy interface. I had to execute
a set of functions from one thread, and wanted to 'call' them from
potentially many. So, I set up the thread, added events, and a queue, etc
and I wrote a proxy function for 'calling' them from the many threads
which looks like...
void proxy(int func, ...) {}
So, it accepts a variable list of args, places them in a structure, places
that in the queue, and waits on an event for the proxy thread to execute
the command and return the result. Lets say the function I am executing
is a database lookup, lets say I have a database field which is a string,
lets say it can be NULL (database definition allows NULLS). Now, lets say
I want to do these lookups:
1. lookup all objects where the field is NULL
2. lookup all objects where the field is "reganwashere"
3. lookup all objects where the field is "" (empty/non-null)
#1 and #2 are simple enough. I call proxy like..
proxy(LOOKUP, NULL);
proxy(LOOKUP, "reganwashere");
and in the actual lookup function, invoked by proxy, I call:
pFieldValue = va_arg(pArgs, char*);
and I get NULL, and "reganwashere".
In C, case #2 would also be easy, I would call proxy(LOOKUP, "") and in
the actual lookup function pFieldValue would be "" (not NULL).
But, in D it seems I cannot do this. In D I would have to pass an
additional boolean parameter, or add another level of indirection i.e.
pass a string[]*. The same problem exists in C if I want to pass an 'int'
or any primitive type, I have to pass it as int*, use a boolean, or invent
a 'special' value which means essentially NULL/not-set/ignored.
There are plenty of other use cases, essentially anywhere where you have
something that can exist in one of 3 states:
1. NULL (not set)
2. "" (set, to blank)
3. "anything" (set, to anything)
Like.. parsing input from a web page, where a field can:
1. not be present on the page (NULL)
2. be present, but left blank ("")
3. be present, contains "anything" ("anything")
This one came up a lot when I worked with web software, we had to be able
to detect whether the user was trying to set something to a blank string,
and in some cases we wanted that to remove the setting entirely (null & ""
being identical ok) or actually set it to a blank string (null & "" being
identical, not ok).
Or... saving settings to a file from user input, where the user selects a
setting from a menu, then enters the value and could:
1. not select setting A, therefore save no value(NULL)
2. select the setting A, enter blank string ("")
3. select the setting A, enter the value "anything" ("anything")
Granted (and this was the response 2 years back when this topic came up) I
can "work around" the deficiency by using a map/hash/dictionary where I
insert key/value pairs, then I can ask it if the key exists. But, this is
essentially another level of indirection like an int* or string[]* and is
more heavy weight than I might want/need.
Ultimately, and people may disagree here, I don't have a problem with
pointers, and this is a really 'nice' feature of using pointers, and it
seems D's arrays don't share it, which bothers me.
Your plan would mean that [] is a memory allocation. I'd rather not
have the runtime do the lower performing thing unless there is a good
reason.
I'm not too bothered what syntax gets used, provided it was something that
you don't accidently use when you do not want it, and wasn't too horrible
to use as I don't see this as being
Re: null Vs [] return arrays
On Fri, 01 Apr 2011 06:38:56 -0400, Regan Heath wrote: On Mon, 28 Mar 2011 17:54:29 +0100, bearophile wrote: Steven Schveighoffer: So essentially, you are getting the same thing, but using [] is slower. It seems I was right then, thank you and Kagamin for the answers. This may be slightly OT but I just wanted to raise the point that conceptually it's nice to be able to express (exists but is empty) and (does not exist). Pointers/references have null as a (does not exist) "value" and this is incredibly useful. Try doing the same thing with 'int' .. it requires you either use int* or pass an additional boolean to indicate existence.. yuck. I'd suggest if someone types '[]' they mean (exists but is empty) and if they type 'null' they mean (does not exist) and they may be relying on the .ptr value to differentiate these cases, which is useful. If you're not interested in the difference, and you need performance, you simply use 'null'. Everybody is happy. :) The distinction is useful if you have something to reference (e.g. an empty slice that points at the end of a pre-existing non-empty array). But [] is a new array, no point in allocating memory just so the pointer can be non-null. Can you come up with a use case to show why you'd want such a thing? Your plan would mean that [] is a memory allocation. I'd rather not have the runtime do the lower performing thing unless there is a good reason. As an alternative, you could use (cast(T *)null)[1..1] if you really needed it (this also would be higher performing, BTW since the runtime array literal function would not be called). -Steve
Re: null Vs [] return arrays
On 04/01/2011 12:38 PM, Regan Heath wrote: On Mon, 28 Mar 2011 17:54:29 +0100, bearophile wrote: Steven Schveighoffer: So essentially, you are getting the same thing, but using [] is slower. It seems I was right then, thank you and Kagamin for the answers. This may be slightly OT but I just wanted to raise the point that conceptually it's nice to be able to express (exists but is empty) and (does not exist). Pointers/references have null as a (does not exist) "value" and this is incredibly useful. Try doing the same thing with 'int' .. it requires you either use int* or pass an additional boolean to indicate existence.. yuck. I'd suggest if someone types '[]' they mean (exists but is empty) and if they type 'null' they mean (does not exist) and they may be relying on the .ptr value to differentiate these cases, which is useful. If you're not interested in the difference, and you need performance, you simply use 'null'. Everybody is happy. :) That's the way I understand this distinction. Unfortunately, D does not really allow this, by semantically treating both indifferently (eg one can put a new element into an null array). Denis -- _ vita es estrany spir.wikidot.com
Re: null Vs [] return arrays
2011/4/1 Regan Heath : > On Mon, 28 Mar 2011 17:54:29 +0100, bearophile > wrote: >> >> Steven Schveighoffer: >> >>> So essentially, you are getting the same thing, but using [] is slower. >> >> It seems I was right then, thank you and Kagamin for the answers. > > This may be slightly OT but I just wanted to raise the point that > conceptually it's nice to be able to express (exists but is empty) and (does > not exist). Pointers/references have null as a (does not exist) "value" and > this is incredibly useful. Try doing the same thing with 'int' .. it > requires you either use int* or pass an additional boolean to indicate > existence.. yuck. > > I'd suggest if someone types '[]' they mean (exists but is empty) and if > they type 'null' they mean (does not exist) and they may be relying on the > .ptr value to differentiate these cases, which is useful. If you're not > interested in the difference, and you need performance, you simply use > 'null'. Everybody is happy. :) > > R > For associative arrays it certainly would be nice to be able to do something like string[string] options = [:]; so that functions can manipulate an empty aa without using ref. Torarin
Re: null Vs [] return arrays
Regan Heath: > conceptually it's nice to be able to express (exists but is empty) and > (does not exist). You may want to express that, but for the implementation of the language those two situations are the same, because in the [] literal the ptr is null. So I think it's better for the programmer to not differentiate the two situations, because they are not different. If the programmer tells them apart, he/she is doing something bad in D, creating a false illusion. Bye, bearophile
Re: null Vs [] return arrays
On Mon, 28 Mar 2011 17:54:29 +0100, bearophile wrote: Steven Schveighoffer: So essentially, you are getting the same thing, but using [] is slower. It seems I was right then, thank you and Kagamin for the answers. This may be slightly OT but I just wanted to raise the point that conceptually it's nice to be able to express (exists but is empty) and (does not exist). Pointers/references have null as a (does not exist) "value" and this is incredibly useful. Try doing the same thing with 'int' .. it requires you either use int* or pass an additional boolean to indicate existence.. yuck. I'd suggest if someone types '[]' they mean (exists but is empty) and if they type 'null' they mean (does not exist) and they may be relying on the .ptr value to differentiate these cases, which is useful. If you're not interested in the difference, and you need performance, you simply use 'null'. Everybody is happy. :) R
Re: null Vs [] return arrays
Steven Schveighoffer: > So essentially, you are getting the same thing, but using [] is slower. It seems I was right then, thank you and Kagamin for the answers. Bye, bearophile
Re: null Vs [] return arrays
On Sun, 27 Mar 2011 09:37:47 -0400, bearophile
wrote:
I have compiled this little D2 program:
int[] foo() {
return [];
}
int[] bar() {
return null;
}
void main() {}
Using DMD 2.052, dmd -O -release -inline test2.d
This is the asm of the two functions:
_D5test23fooFZAicomdat
L0: pushEAX
mov EAX,offset FLAT:_D11TypeInfo_Ai6__initZ
push0
pushEAX
callnear ptr __d_arrayliteralT
mov EDX,EAX
add ESP,8
pop ECX
xor EAX,EAX
ret
_D5test23barFZAicomdat
xor EAX,EAX
xor EDX,EDX
ret
Is this expected and desired? Isn't it better to compile the foo() as
bar()?
Probably. The runtime that allocates an array looks like this (irrelevant
parts collapsed):
extern (C) void* _d_arrayliteralT(TypeInfo ti, size_t length, ...)
{
auto sizeelem = ti.next.tsize();// array element size
void* result;
...
if (length == 0 || sizeelem == 0)
result = null;
else
{
...
}
return result;
}
So essentially, you are getting the same thing, but using [] is slower.
-Steve
Re: null Vs [] return arrays
bearophile Wrote: > Kagamin: > > > [] is not null, it's an array of 0 elements, what is done exactly. > > edx points to the allocated array. > > I don't understand what you say. I think the caller of foo() and bar() > receive the same thing, two empty registers. I think that cast(int[])null and > cast(int[])[] are the same thing for D. That's a mistake. Well, if there's no differnce for you, you can use either of them. What's the problem?
Re: null Vs [] return arrays
Jonathan M Davis: > the compiler can likely make > assumptions about null that it can't make about [], since it probably treats > [] more generally without worrying about the fact that it happens to be empty > as far as optimizations go - that and there _is_ a semantic difference > between > null and [] if you're messing with the ptr property, so Walter may think that > it's best for null to not be turned into the same thing as [] automatically. Thank you for your answer. I have added a low-priority enhancement request. Bye, bearophile
Re: null Vs [] return arrays
On 2011-03-27 11:42, bearophile wrote:
> Kagamin:
> > [] is not null, it's an array of 0 elements, what is done exactly.
> > edx points to the allocated array.
>
> I don't understand what you say. I think the caller of foo() and bar()
> receive the same thing, two empty registers. I think that cast(int[])null
> and cast(int[])[] are the same thing for D.
>
> void main() {
> assert(cast(int[])null == cast(int[])null);
> auto a1 = cast(int[])null;
> a1 ~= 1;
> auto a2 = 1 ~ cast(int[])null;
> }
What I would _expect_ the difference between a null array and an empty one to
be would be that the null one's ptr property would be null, whereas the empty
one wouldn't be. But dmd treats them pretty much the same. empty returns true
for both. You can append to both. The null one would be a guaranteed memory
reallocation when you append to it whereas the empty one may not be, but their
behavior is almost identical.
How that affects the generated assembly code, I don't know. Particularly if
you're compiling with -inline and and -O, the compiler can likely make
assumptions about null that it can't make about [], since it probably treats
[] more generally without worrying about the fact that it happens to be empty
as far as optimizations go - that and there _is_ a semantic difference between
null and [] if you're messing with the ptr property, so Walter may think that
it's best for null to not be turned into the same thing as [] automatically.
- Jonathan M Davis
Re: null Vs [] return arrays
Kagamin:
> [] is not null, it's an array of 0 elements, what is done exactly.
> edx points to the allocated array.
I don't understand what you say. I think the caller of foo() and bar() receive
the same thing, two empty registers. I think that cast(int[])null and
cast(int[])[] are the same thing for D.
void main() {
assert(cast(int[])null == cast(int[])null);
auto a1 = cast(int[])null;
a1 ~= 1;
auto a2 = 1 ~ cast(int[])null;
}
Bye,
bearophile
Re: null Vs [] return arrays
bearophile Wrote:
> I have compiled this little D2 program:
>
>
> int[] foo() {
> return [];
> }
> int[] bar() {
> return null;
> }
> void main() {}
>
>
>
> Using DMD 2.052, dmd -O -release -inline test2.d
>
> This is the asm of the two functions:
>
> _D5test23fooFZAicomdat
> L0: pushEAX
> mov EAX,offset FLAT:_D11TypeInfo_Ai6__initZ
> push0
> pushEAX
> callnear ptr __d_arrayliteralT
> mov EDX,EAX
> add ESP,8
> pop ECX
> xor EAX,EAX
> ret
>
> _D5test23barFZAicomdat
> xor EAX,EAX
> xor EDX,EDX
> ret
>
> Is this expected and desired? Isn't it better to compile the foo() as bar()?
>
> Bye,
> bearophile
[] is not null, it's an array of 0 elements, what is done exactly.
edx points to the allocated array.
null Vs [] return arrays
I have compiled this little D2 program:
int[] foo() {
return [];
}
int[] bar() {
return null;
}
void main() {}
Using DMD 2.052, dmd -O -release -inline test2.d
This is the asm of the two functions:
_D5test23fooFZAicomdat
L0: pushEAX
mov EAX,offset FLAT:_D11TypeInfo_Ai6__initZ
push0
pushEAX
callnear ptr __d_arrayliteralT
mov EDX,EAX
add ESP,8
pop ECX
xor EAX,EAX
ret
_D5test23barFZAicomdat
xor EAX,EAX
xor EDX,EDX
ret
Is this expected and desired? Isn't it better to compile the foo() as bar()?
Bye,
bearophile
