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digitalmars.D.learn - Calling destroy on struct pointer

reply Radu <void null.pt> writes:
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
parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
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
parent reply Radu <void null.pt> writes:
On Saturday, 25 February 2017 at 08:36:02 UTC, Ali Çehreli wrote:
 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
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...
Feb 25
parent reply Moritz Maxeiner <moritz ucworks.org> writes:
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:
 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
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.
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).
 - 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
next sibling parent reply Moritz Maxeiner <moritz ucworks.org> writes:
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
parent reply Radu <void null.pt> writes:
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:
 ---
 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 ---
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);
Feb 25
parent Moritz Maxeiner <moritz ucworks.org> writes:
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
prev sibling parent reply Radu <void null.pt> writes:
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:
 On Saturday, 25 February 2017 at 08:36:02 UTC, Ali Çehreli 
 wrote:
 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
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.
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).
 - 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?
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();
Feb 25
parent reply Moritz Maxeiner <moritz ucworks.org> writes:
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
parent Radu <void null.pt> writes:
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:
 [...]
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.
Made a bug report on the fwd reference error. https://issues.dlang.org/show_bug.cgi?id=17230
Feb 28