digitalmars.D.learn - Calling destroy on struct pointer
- Radu (39/39) Feb 25 2017 I'm puzzled by the way destroy works when passed a pointer to a
- =?UTF-8?Q?Ali_=c3=87ehreli?= (8/12) Feb 25 2017 It's by design because setting a pointer to null can be considered as
- Radu (14/28) Feb 25 2017 I think this is BAD. Why?
- Moritz Maxeiner (23/57) Feb 25 2017 Unfortunately, I don't think it's viable to change destroy (see
- Moritz Maxeiner (11/18) Feb 25 2017 Sorry for double posting, I failed at copy-paste, here's the
- Radu (10/30) Feb 25 2017 The correct way of doing it using deref would to look like:
- Moritz Maxeiner (3/11) Feb 25 2017 Right, I read the post and immediately failed to apply the new
- Radu (41/103) Feb 25 2017 Here is sample on how destroy fails with a fwd decl error:
- Moritz Maxeiner (7/47) Feb 25 2017 Hm, that's an issue you'd best take up to the bugtracker, I
- Radu (4/14) Feb 28 2017 Made a bug report on the fwd reference error.
I'm puzzled by the way destroy works when passed a pointer to a
struct, observe:
----------------------code.d----------------------
int i;
struct C
{
this(ref int i)
{
++i;
ii = &i;
}
~this()
{
--(*ii);
}
int* ii;
}
unittest
{
C c = C(i);
C* cc = &c;
destroy(cc);
assert(i == 0); // dtor not called
destroy(*cc);
assert(i == 0); // dtor called
}
int main()
{
return 0;
}
----------------------code.d----------------------
destroy(cc) -> does c = C.init
destroy(*cc); -> calls the C dtor
Is this by design? If so - how can I destroy and get the dtor
called without dereferencing the pointer?
Currently I resort to calling delete on the pointer so dtor gets
called.
Feb 25 2017
On 02/25/2017 12:17 AM, Radu wrote:destroy(cc) -> does c = C.init destroy(*cc); -> calls the C dtor Is this by design? If so - how can I destroy and get the dtor called without dereferencing the pointer?It's by design because setting a pointer to null can be considered as destroying the pointer. Dereferencing is the right way of destroying the object through the pointer. I had added the following warning after somebody else was burnt by this feature. :) http://ddili.org/ders/d.en/memory.html#ix_memory.destroy Ali
Feb 25 2017
On Saturday, 25 February 2017 at 08:36:02 UTC, Ali Çehreli wrote:On 02/25/2017 12:17 AM, Radu wrote:I think this is BAD. Why? - it is one of those WAT?? moments that brings a RTFM slap to you. The defaults should not be surprising, and in this case straight dangerous as it can lead to leaks. - it is not always possible to dereference the pointer, think some circular structures where deref would get you one of those fwd. declaration errors. - the deprecated delete will call the dtor, destroy is suppose to replace delete - hence it should work the same. In my opinion destroy should do this: - call dtor if the pointer type has one defined - nullify the pointer This is what I was expecting anyhow to happen...destroy(cc) -> does c = C.init destroy(*cc); -> calls the C dtor Is this by design? If so - how can I destroy and get the dtorcalledwithout dereferencing the pointer?It's by design because setting a pointer to null can be considered as destroying the pointer. Dereferencing is the right way of destroying the object through the pointer. I had added the following warning after somebody else was burnt by this feature. :) http://ddili.org/ders/d.en/memory.html#ix_memory.destroy Ali
Feb 25 2017
On Saturday, 25 February 2017 at 10:44:07 UTC, Radu wrote:On Saturday, 25 February 2017 at 08:36:02 UTC, Ali Çehreli wrote:Unfortunately, I don't think it's viable to change destroy (see below), it would probably be better to cover this in the dlang tour (if it isn't already).On 02/25/2017 12:17 AM, Radu wrote:I think this is BAD. Why? - it is one of those WAT?? moments that brings a RTFM slap to you. The defaults should not be surprising, and in this case straight dangerous as it can lead to leaks.destroy(cc) -> does c = C.init destroy(*cc); -> calls the C dtor Is this by design? If so - how can I destroy and get the dtorcalledwithout dereferencing the pointer?It's by design because setting a pointer to null can be considered as destroying the pointer. Dereferencing is the right way of destroying the object through the pointer. I had added the following warning after somebody else was burnt by this feature. :) http://ddili.org/ders/d.en/memory.html#ix_memory.destroy Ali- it is not always possible to dereference the pointer, think some circular structures where deref would get you one of those fwd. declaration errors.In the interest of learning, could you provide an example of such a case?- the deprecated delete will call the dtor, destroy is suppose to replace delete - hence it should work the same.AFAIK destroy isn't supposed to replace delete, since delete is destruction+deallocation and destroy is only destruction; and by that definition they cannot work the same: AFAIR multiple deletes are illegal (since that equals use after free), whereas destroy can be used on the same object as often as you want (the destructor will only be called the first time).In my opinion destroy should do this: - call dtor if the pointer type has one defined - nullify the pointer This is what I was expecting anyhow to happen...This change would be backwards-incompatible and breaks user code, especially manual memory management: --- struct A {} auto a = cast (A*) malloc(A.sizeof); // Allocate emplace(a, 42); // Construct destroy(a); // Destruct free(a); // Deallocate --- if destroy were to already nullify a, how were one supposed to deallocate a?
Feb 25 2017
On Saturday, 25 February 2017 at 13:14:24 UTC, Moritz Maxeiner
wrote:
---
struct A {}
auto a = cast (A*) malloc(A.sizeof); // Allocate
emplace(a, 42); // Construct
destroy(a); // Destruct
free(a); // Deallocate
---
Sorry for double posting, I failed at copy-paste, here's the
correct example:
---
struct A { int i; }
auto a = cast (A*) malloc(A.sizeof); // Allocate
emplace(a, 42); // Construct
destroy(a); // Destruct
free(a); // Deallocate
---
Feb 25 2017
On Saturday, 25 February 2017 at 13:18:21 UTC, Moritz Maxeiner wrote:On Saturday, 25 February 2017 at 13:14:24 UTC, Moritz Maxeiner wrote:The correct way of doing it using deref would to look like: struct A { int i; } auto a = cast (A*) malloc(A.sizeof); // Allocate emplace(a, 42); // Construct destroy(*a); // Destruct A free(a); // Deallocate destroy(a); // Destruct A* assert(a is null);--- struct A {} auto a = cast (A*) malloc(A.sizeof); // Allocate emplace(a, 42); // Construct destroy(a); // Destruct free(a); // Deallocate ---Sorry for double posting, I failed at copy-paste, here's the correct example: --- struct A { int i; } auto a = cast (A*) malloc(A.sizeof); // Allocate emplace(a, 42); // Construct destroy(a); // Destruct free(a); // Deallocate ---
Feb 25 2017
Moritz Maxeiner <moritz ucworks.org>
On Saturday, 25 February 2017 at 15:21:56 UTC, Radu wrote:
The correct way of doing it using deref would to look like:
struct A { int i; }
auto a = cast (A*) malloc(A.sizeof); // Allocate
emplace(a, 42); // Construct
destroy(*a); // Destruct A
free(a); // Deallocate
destroy(a); // Destruct A*
assert(a is null);
Right, I read the post and immediately failed to apply the new
knowledge. Bad me, thanks for the correction.
Feb 25 2017
On Saturday, 25 February 2017 at 13:14:24 UTC, Moritz Maxeiner wrote:On Saturday, 25 February 2017 at 10:44:07 UTC, Radu wrote:Here is sample on how destroy fails with a fwd decl error: struct A { B b; C c; } struct B { Wrap!A val; } struct C { Wrap!A val; } struct Wrap(T) { this(bool b) { t = cast(T*) malloc(T.sizeof); } ~this() { destroy(*t); // Error: struct app.A no size because of forward reference } T* t; } Manual management fails now with the current construct, inst't it? auto a = cast (A*) malloc(A.sizeof); // Allocate emplace(a, 42); // Construct destroy(a); // Destruct |-- here a becomes null assert(a is null); // :} free(a); // Deallocate |- free null... You need to save a into a temp, then call free on temp. A nice to have enhancement would be to return the destroyed pointer from destroy, enabling something like: destroy(a).free();On Saturday, 25 February 2017 at 08:36:02 UTC, Ali Çehreli wrote:Unfortunately, I don't think it's viable to change destroy (see below), it would probably be better to cover this in the dlang tour (if it isn't already).On 02/25/2017 12:17 AM, Radu wrote:I think this is BAD. Why? - it is one of those WAT?? moments that brings a RTFM slap to you. The defaults should not be surprising, and in this case straight dangerous as it can lead to leaks.destroy(cc) -> does c = C.init destroy(*cc); -> calls the C dtor Is this by design? If so - how can I destroy and get the dtorcalledwithout dereferencing the pointer?It's by design because setting a pointer to null can be considered as destroying the pointer. Dereferencing is the right way of destroying the object through the pointer. I had added the following warning after somebody else was burnt by this feature. :) http://ddili.org/ders/d.en/memory.html#ix_memory.destroy Ali- it is not always possible to dereference the pointer, think some circular structures where deref would get you one of those fwd. declaration errors.In the interest of learning, could you provide an example of such a case?- the deprecated delete will call the dtor, destroy is suppose to replace delete - hence it should work the same.AFAIK destroy isn't supposed to replace delete, since delete is destruction+deallocation and destroy is only destruction; and by that definition they cannot work the same: AFAIR multiple deletes are illegal (since that equals use after free), whereas destroy can be used on the same object as often as you want (the destructor will only be called the first time).In my opinion destroy should do this: - call dtor if the pointer type has one defined - nullify the pointer This is what I was expecting anyhow to happen...This change would be backwards-incompatible and breaks user code, especially manual memory management: --- struct A {} auto a = cast (A*) malloc(A.sizeof); // Allocate emplace(a, 42); // Construct destroy(a); // Destruct free(a); // Deallocate --- if destroy were to already nullify a, how were one supposed to deallocate a?
Feb 25 2017
On Saturday, 25 February 2017 at 15:13:27 UTC, Radu wrote:Here is sample on how destroy fails with a fwd decl error: struct A { B b; C c; } struct B { Wrap!A val; } struct C { Wrap!A val; } struct Wrap(T) { this(bool b) { t = cast(T*) malloc(T.sizeof); } ~this() { destroy(*t); // Error: struct app.A no size because of forward reference } T* t; }Thanks for the example.Manual management fails now with the current construct, inst't it?Hm, that's an issue you'd best take up to the bugtracker, I think. Maybe there's a way around that, but I don't know.auto a = cast (A*) malloc(A.sizeof); // Allocate emplace(a, 42); // Construct destroy(a); // Destruct |-- here a becomes null assert(a is null); // :} free(a); // Deallocate |- free null... You need to save a into a temp, then call free on temp. A nice to have enhancement would be to return the destroyed pointer from destroy, enabling something like: destroy(a).free();Well, yes, but that is still backwards-incompatible and breaking user code is something I was under the impression was a big NO currently.
Feb 25 2017
On Saturday, 25 February 2017 at 16:39:18 UTC, Moritz Maxeiner wrote:On Saturday, 25 February 2017 at 15:13:27 UTC, Radu wrote:Made a bug report on the fwd reference error. https://issues.dlang.org/show_bug.cgi?id=17230[...]Thanks for the example.[...]Hm, that's an issue you'd best take up to the bugtracker, I think. Maybe there's a way around that, but I don't know.[...]Well, yes, but that is still backwards-incompatible and breaking user code is something I was under the impression was a big NO currently.
Feb 28 2017