Re: Are there any methods like isDestroyed or isDisposed to detect whether some object is destroyed or not?
On 12/13/14 2:03 AM, Andrey Derzhavin wrote:
Hello!
We have object, for example, objA.
ObjectAType objA = new ObjectAType(...);
// we have a many references to objA
void someFunction1(...)
{
// destroying the objA
destroy(one_of_the_refToObjA);
//
}
void someFunction2()
{
// segmentation fault if the objA is destroyed
another_refToObjA.method1();
// I need a safe way (without using signals) to detect whether object
// destroyed or not, like this
// if (!isDestroyed(another_refToObjA))
// another_refToObjA.method1();
}
// call functions
someFunction1(..);
// . some other operations...
// the objectA (objA) is destroyed, but debugger shows us a not null
reference
// to that objA. However, when we try to call some methods of objectA or
// try to use it's variables we get a segmentation fault error (in
// someFunction2, when objA is destroyed).
someFunction2(...);
Yes, you can tell if the typeinfo pointer is null, this is how the
runtime does it.
Unfortunately, typeid(objA) uses the pointer to look up the most derived
type and results in a segfault.
You can use the knowledge of the object layout (the typeinfo pointer is
the first item in the memory block) to ascertain whether it's null or
not, it's what the runtime does:
bool isDestroyed(Object o)
{
return o is null || *(cast(void **)o) is null;
}
Although, I highly recommend not doing this. Instead of destroying an
object, leave the object live and dispose of it using some other method
(e.g. close, dispose, etc.). Then let the GC clean up the memory later
when nothing refers to it.
-Steve
Re: Are there any methods like isDestroyed or isDisposed to detect whether some object is destroyed or not?
Thank you very much, Steven!
Re: Are there any methods like isDestroyed or isDisposed to detect whether some object is destroyed or not?
On Saturday, 13 December 2014 at 21:20:43 UTC, Andrey Derzhavin
wrote:
import std.stdio;
class ObjectAType {
bool ok;
this() {ok = true;}
}
void main()
{
auto a = new ObjectAType;
assert(a.ok);
destroy(a);
assert(!a.ok); // a has been destroyed.
}
This method of detection of collected objects is what I needed.
Thanks to everybody for your advise.
Be careful - the memory could still have been reused. For example:
assert(a.ok);
destroy(a);
// ... lots of code, maybe multi-threaded ...
assert(!a.ok);// can fail
If between `destroy()` and the second `assert()`, the memory of
the object is collected, a new object (of the same or different
type) can have been placed there. You will then read an arbitrary
piece of data from that new object, which may or may not evaluate
to `true`.
The only case where this can be safe is when you can guarantee
that there are no new memory allocations between the destruction
and the test, or that the GC doesn't run. The first condition may
also be no longer sufficient if the GC starts moving objects
around (the current one doesn't).
Really, in practice you just cannot safely access GC managed
resources from inside a GC destructor. You're just asking for
trouble if you try to hack around that fact.
Re: Are there any methods like isDestroyed or isDisposed to detect whether some object is destroyed or not?
Marc Schütz\ schue...@gmx.net writes:
On Saturday, 13 December 2014 at 21:20:43 UTC, Andrey Derzhavin wrote:
import std.stdio;
class ObjectAType {
bool ok;
this() {ok = true;}
}
void main()
{
auto a = new ObjectAType;
assert(a.ok);
destroy(a);
assert(!a.ok); // a has been destroyed.
}
This method of detection of collected objects is what I needed.
Thanks to everybody for your advise.
Be careful - the memory could still have been reused. For example:
assert(a.ok);
destroy(a);
// ... lots of code, maybe multi-threaded ...
assert(!a.ok);// can fail
If between `destroy()` and the second `assert()`, the memory of the
object is collected, a new object (of the same or different type) can
have been placed there. You will then read an arbitrary piece of data
from that new object, which may or may not evaluate to `true`.
In this case the object cannot be collected by gc because there is still
a good reference to it (variable 'a'). I agree if delete was used
instead of destroy that is would be unsafe.
--
dano
Re: Are there any methods like isDestroyed or isDisposed to detect whether some object is destroyed or not?
On Sunday, 14 December 2014 at 16:05:13 UTC, Dan Olson wrote:
Marc Schütz\ schue...@gmx.net writes:
On Saturday, 13 December 2014 at 21:20:43 UTC, Andrey
Derzhavin wrote:
import std.stdio;
class ObjectAType {
bool ok;
this() {ok = true;}
}
void main()
{
auto a = new ObjectAType;
assert(a.ok);
destroy(a);
assert(!a.ok); // a has been destroyed.
}
This method of detection of collected objects is what I
needed.
Thanks to everybody for your advise.
Be careful - the memory could still have been reused. For
example:
assert(a.ok);
destroy(a);
// ... lots of code, maybe multi-threaded ...
assert(!a.ok);// can fail
If between `destroy()` and the second `assert()`, the memory
of the
object is collected, a new object (of the same or different
type) can
have been placed there. You will then read an arbitrary piece
of data
from that new object, which may or may not evaluate to `true`.
In this case the object cannot be collected by gc because there
is still
a good reference to it (variable 'a'). I agree if delete was
used
instead of destroy that is would be unsafe.
--
dano
Right, but I see it only as a simplified example. In a real
program, that reference will most likely be a member of object
whose destructor is being run.
Re: Are there any methods like isDestroyed or isDisposed to detect whether some object is destroyed or not?
import std.stdio;
class ObjectAType {
bool ok;
this() {ok = true;}
}
void main()
{
auto a = new ObjectAType;
assert(a.ok);
destroy(a);
assert(!a.ok); // a has been destroyed.
}
This method of detection of collected objects is what I needed.
Thanks to everybody for your advise.
Are there any methods like isDestroyed or isDisposed to detect whether some object is destroyed or not?
Hello!
We have object, for example, objA.
ObjectAType objA = new ObjectAType(...);
// we have a many references to objA
void someFunction1(...)
{
// destroying the objA
destroy(one_of_the_refToObjA);
//
}
void someFunction2()
{
// segmentation fault if the objA is destroyed
another_refToObjA.method1();
// I need a safe way (without using signals) to detect whether
object
// destroyed or not, like this
// if (!isDestroyed(another_refToObjA))
// another_refToObjA.method1();
}
// call functions
someFunction1(..);
// . some other operations...
// the objectA (objA) is destroyed, but debugger shows us a not
null reference
// to that objA. However, when we try to call some methods of
objectA or
// try to use it's variables we get a segmentation fault error
(in
// someFunction2, when objA is destroyed).
someFunction2(...);
Thanks.
Re: Are there any methods like isDestroyed or isDisposed to detect whether some object is destroyed or not?
On Sat, 13 Dec 2014 07:03:22 +
Andrey Derzhavin via Digitalmars-d-learn
digitalmars-d-learn@puremagic.com wrote:
Hello!
We have object, for example, objA.
ObjectAType objA = new ObjectAType(...);
// we have a many references to objA
void someFunction1(...)
{
// destroying the objA
destroy(one_of_the_refToObjA);
//
}
void someFunction2()
{
// segmentation fault if the objA is destroyed
another_refToObjA.method1();
// I need a safe way (without using signals) to detect whether
object
// destroyed or not, like this
// if (!isDestroyed(another_refToObjA))
// another_refToObjA.method1();
}
// call functions
someFunction1(..);
// . some other operations...
// the objectA (objA) is destroyed, but debugger shows us a not
null reference
// to that objA. However, when we try to call some methods of
objectA or
// try to use it's variables we get a segmentation fault error
(in
// someFunction2, when objA is destroyed).
someFunction2(...);
Thanks.
first. you'd better to not write such code, it's completely unreliable.
second. you can check if the block is allocated by using GC.addrOf().
third. and this is completely unreliable, 'cause memory can be reused.
so the only right answer is: don't do that! use list of free objects,
for example, and write `free()` function that will move the object to
that list (and possibly set some flags to mark it as freed).
let me stress it again: you *CAN'T* reliably detect if some object was
manually destroyed. don't even think you can do this in working code,
you will never made such code fully working.
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Re: Are there any methods like isDestroyed or isDisposed to detect whether some object is destroyed or not?
On Sat, 13 Dec 2014 07:03:22 + Andrey Derzhavin via Digitalmars-d-learn digitalmars-d-learn@puremagic.com wrote: p.s. you can emulate weak links if this is what you want to have. i.e. references to object that will not stop GC to collect that object and which will be auto-nullified if the object was collected. see this module, for example: http://repo.or.cz/w/iv.d.git/blob/HEAD:/weakref.d signature.asc Description: PGP signature
Re: Are there any methods like isDestroyed or isDisposed to detect whether some object is destroyed or not?
On Sat, 13 Dec 2014 07:03:22 + Andrey Derzhavin via Digitalmars-d-learn digitalmars-d-learn@puremagic.com wrote: p.p.s. please-please-please don't invent other hacks with `rt_attachDisposeEvent()`! such hackery will not do you good in the long term. signature.asc Description: PGP signature
