digitalmars.D - Should destructors be able to tell who called them?
- Steven Schveighoffer (42/42) Aug 10 2010 One of the common problems of destructors is that they cannot assume any...
- Michel Fortin (15/30) Aug 10 2010 In Objective-C we have the dealloc method (deterministic) and the
- Steven Schveighoffer (10/32) Aug 10 2010 Good point. I'd restate it as should safeD be able to define
- Sean Kelly (23/25) Aug 10 2010 Just a note. It's already possible to do this in D2 using an interface....
- Steven Schveighoffer (10/36) Aug 10 2010 I don't like the semantics. If you inherit from Disposable, then dispos...
- Yao G. (7/33) Aug 10 2010 Thanks for this Sean. Although I concur with Steve that it looks weird, ...
- Sean Kelly (2/50) Aug 10 2010
- Mafi (15/59) Aug 10 2010 Interestingly I had exactly the same idea a few hours ago as I red the
- Steven Schveighoffer (6/76) Aug 10 2010 pushing an ignored argument on the stack doesn't matter. Basically,
- Lutger (11/14) Aug 10 2010 My first thought was that they are actually two separate functions disti...
- Steven Schveighoffer (10/27) Aug 10 2010 Hm... something I just thought of that makes this bad, destructors are
- Lutger (3/36) Aug 10 2010 clear could check if the base classes implement the interface and act on...
- Steven Schveighoffer (10/49) Aug 10 2010 Checking if a class implements an interface is not as trivial as accessi...
One of the common problems of destructors is that they cannot assume any of their GC-allocated resources are valid when inside the destrutor. The spec says: The garbage collector is not guaranteed to run the destructor for all unreferenced objects. Furthermore, the order in which the garbage collector calls destructors for unreference objects is not specified. This means that when the garbage collector calls a destructor for an object of a class that has members that are references to garbage collected objects, those references may no longer be valid. This means that destructors cannot reference sub objects. This rule does not apply to auto objects or objects deleted with the DeleteExpression, as the destructor is not being run by the garbage collector, meaning all references are valid. Let's analyze that last sentence (BTW, I think where it says auto, it should say scope). "This does not apply to objects deleted with the DeleteExpression..." Well, how the hell do you know while in the destructor whether you were called via delete/clear or from the GC? You don't. So the destructor *must* be written as if you are called from the GC, because the GC may be calling it, it is up to the user to determine whether your class will be destroyed via delete/clear or the GC. So you must assume worst case. Unless you declare your class as scope, which I don't even know if anyone does that anymore (I think it's scheduled for deprecation anyways). But, what if the destructor was given an idea of whether its resources are valid or not? Then you could do something different based on that input. For example, an object that wants to open a file can do so, and in the destructor, use the determination of validity to decide whether to close the file or not. This can be implemented by an optional parameter to the destructor that's always passed, but it's not necessary to use it. i.e. you could declare your destructor like this: ~this(bool deterministic) { if(deterministic) file.close(); } or like this: ~this() { } Scope classes (if allowed to exist) or calls to clear will set deterministic to true. The GC will set it to false. This would make destructors a lot more useful. Thoughts? -Steve
Aug 10 2010
On 2010-08-10 14:25:48 -0400, "Steven Schveighoffer" <schveiguy yahoo.com> said:~this(bool deterministic) { if(deterministic) file.close(); } or like this: ~this() { } Scope classes (if allowed to exist) or calls to clear will set deterministic to true. The GC will set it to false. This would make destructors a lot more useful. Thoughts?In Objective-C we have the dealloc method (deterministic) and the finalize method (non-deterministic). The former is used in a reference-counted environment while the second is used when memory is managed by a GC. One issue with having one destructor with a deterministic argument (as opposed to having two separate destructors) is that non-deterministic destructors should probably not be allowed in SafeD because you can then get access to dangling pointers and you can even pass them around to make them live long after the collection has terminated. -- Michel Fortin michel.fortin michelf.com http://michelf.com/
Aug 10 2010
On Tue, 10 Aug 2010 14:38:13 -0400, Michel Fortin <michel.fortin michelf.com> wrote:On 2010-08-10 14:25:48 -0400, "Steven Schveighoffer" <schveiguy yahoo.com> said:Good point. I'd restate it as should safeD be able to define deterministic destructors? i.e. a solution to this is not to allow defining class destructors at all in safeD. That might be too limiting, but not any more limiting than it is now... I don't know what the right answer is. What I was looking for was a solution that was backwards compatible with current code, and one that didn't require you to call __dtor directly. -Steve~this(bool deterministic) { if(deterministic) file.close(); } or like this: ~this() { } Scope classes (if allowed to exist) or calls to clear will set deterministic to true. The GC will set it to false. This would make destructors a lot more useful. Thoughts?In Objective-C we have the dealloc method (deterministic) and the finalize method (non-deterministic). The former is used in a reference-counted environment while the second is used when memory is managed by a GC. One issue with having one destructor with a deterministic argument (as opposed to having two separate destructors) is that non-deterministic destructors should probably not be allowed in SafeD because you can then get access to dangling pointers and you can even pass them around to make them live long after the collection has terminated.
Aug 10 2010
Steven Schveighoffer Wrote:One of the common problems of destructors is that they cannot assume any of their GC-allocated resources are valid when inside the destrutor.Just a note. It's already possible to do this in D2 using an interface. Here's a thread about it: http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D&artnum=89443&header And some sample code: import core.runtime; interface Disposable { void dispose(); } bool handler( Object o ) { auto d = cast(Disposable) o; if( d !is null ) { d.dispose(); return false; } return true; } static this() { Runtime.collectHandler = &handler; }
Aug 10 2010
On Tue, 10 Aug 2010 14:51:56 -0400, Sean Kelly <sean invisibleduck.org> wrote:Steven Schveighoffer Wrote:I don't like the semantics. If you inherit from Disposable, then dispose is the nondeterministic, and ~this() becomes deterministic? How does that work if you inherit from a non-Disposable object? I guess it doesn't hurt, but I like my solution better for two other reasons: 1. you are not doing a dynamic cast (i.e. linear search) on every collected object 2. it costs next to nothing to put a bool on the stack. -SteveOne of the common problems of destructors is that they cannot assume any of their GC-allocated resources are valid when inside the destrutor.Just a note. It's already possible to do this in D2 using an interface. Here's a thread about it: http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D&artnum=89443&header And some sample code: import core.runtime; interface Disposable { void dispose(); } bool handler( Object o ) { auto d = cast(Disposable) o; if( d !is null ) { d.dispose(); return false; } return true; } static this() { Runtime.collectHandler = &handler; }
Aug 10 2010
Thanks for this Sean. Although I concur with Steve that it looks weird, at least is usable. Yao G. On Tue, 10 Aug 2010 13:51:56 -0500, Sean Kelly <sean invisibleduck.org> wrote:Steven Schveighoffer Wrote:-- Using Opera's revolutionary e-mail client: http://www.opera.com/mail/One of the common problems of destructors is that they cannot assume any of their GC-allocated resources are valid when inside the destrutor.Just a note. It's already possible to do this in D2 using an interface. Here's a thread about it: http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D&artnum=89443&header And some sample code: import core.runtime; interface Disposable { void dispose(); } bool handler( Object o ) { auto d = cast(Disposable) o; if( d !is null ) { d.dispose(); return false; } return true; } static this() { Runtime.collectHandler = &handler; }
Aug 10 2010
That hook was really added for other purposes anyway. I mostly wanted to note that it's possible to try this out now without any language changes. Yao G. Wrote:Thanks for this Sean. Although I concur with Steve that it looks weird, at least is usable. Yao G. On Tue, 10 Aug 2010 13:51:56 -0500, Sean Kelly <sean invisibleduck.org> wrote:Steven Schveighoffer Wrote:-- Using Opera's revolutionary e-mail client: http://www.opera.com/mail/One of the common problems of destructors is that they cannot assume any of their GC-allocated resources are valid when inside the destrutor.Just a note. It's already possible to do this in D2 using an interface. Here's a thread about it: http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D&artnum=89443&header And some sample code: import core.runtime; interface Disposable { void dispose(); } bool handler( Object o ) { auto d = cast(Disposable) o; if( d !is null ) { d.dispose(); return false; } return true; } static this() { Runtime.collectHandler = &handler; }
Aug 10 2010
Am 10.08.2010 20:25, schrieb Steven Schveighoffer:One of the common problems of destructors is that they cannot assume any of their GC-allocated resources are valid when inside the destrutor. The spec says: The garbage collector is not guaranteed to run the destructor for all unreferenced objects. Furthermore, the order in which the garbage collector calls destructors for unreference objects is not specified. This means that when the garbage collector calls a destructor for an object of a class that has members that are references to garbage collected objects, those references may no longer be valid. This means that destructors cannot reference sub objects. This rule does not apply to auto objects or objects deleted with the DeleteExpression, as the destructor is not being run by the garbage collector, meaning all references are valid. Let's analyze that last sentence (BTW, I think where it says auto, it should say scope). "This does not apply to objects deleted with the DeleteExpression..." Well, how the hell do you know while in the destructor whether you were called via delete/clear or from the GC? You don't. So the destructor *must* be written as if you are called from the GC, because the GC may be calling it, it is up to the user to determine whether your class will be destroyed via delete/clear or the GC. So you must assume worst case. Unless you declare your class as scope, which I don't even know if anyone does that anymore (I think it's scheduled for deprecation anyways). But, what if the destructor was given an idea of whether its resources are valid or not? Then you could do something different based on that input. For example, an object that wants to open a file can do so, and in the destructor, use the determination of validity to decide whether to close the file or not. This can be implemented by an optional parameter to the destructor that's always passed, but it's not necessary to use it. i.e. you could declare your destructor like this: ~this(bool deterministic) { if(deterministic) file.close(); } or like this: ~this() { } Scope classes (if allowed to exist) or calls to clear will set deterministic to true. The GC will set it to false. This would make destructors a lot more useful. Thoughts? -SteveInterestingly I had exactly the same idea a few hours ago as I red the discussions about clear. I just forgot about it without posting. I think it's a good idea. The ignorant destructor (ie w/o parameter) defenition should be inetrnally rewritten to accept an ignored bool parameter. This would make the following thing a doubled and therfore incorrect definition. class X { ~this(){} ~this(bool det){} } If the compiler then adds an overload for 'X.__dtor()' which simply calls 'X.__dtor(false)' [1], we should be totally compatible with existing code. [1] IMO this should be false and not true but I'm not sure.
Aug 10 2010
On Tue, 10 Aug 2010 14:56:54 -0400, Mafi <mafi example.org> wrote:Am 10.08.2010 20:25, schrieb Steven Schveighoffer:pushing an ignored argument on the stack doesn't matter. Basically, __dtor would always have a bool argument, and if the definition of the class had ~this(), it doesn't matter. It's the reason you can have main either accept a string array or nothing. -SteveOne of the common problems of destructors is that they cannot assume any of their GC-allocated resources are valid when inside the destrutor. The spec says: The garbage collector is not guaranteed to run the destructor for all unreferenced objects. Furthermore, the order in which the garbage collector calls destructors for unreference objects is not specified. This means that when the garbage collector calls a destructor for an object of a class that has members that are references to garbage collected objects, those references may no longer be valid. This means that destructors cannot reference sub objects. This rule does not apply to auto objects or objects deleted with the DeleteExpression, as the destructor is not being run by the garbage collector, meaning all references are valid. Let's analyze that last sentence (BTW, I think where it says auto, it should say scope). "This does not apply to objects deleted with the DeleteExpression..." Well, how the hell do you know while in the destructor whether you were called via delete/clear or from the GC? You don't. So the destructor *must* be written as if you are called from the GC, because the GC may be calling it, it is up to the user to determine whether your class will be destroyed via delete/clear or the GC. So you must assume worst case. Unless you declare your class as scope, which I don't even know if anyone does that anymore (I think it's scheduled for deprecation anyways). But, what if the destructor was given an idea of whether its resources are valid or not? Then you could do something different based on that input. For example, an object that wants to open a file can do so, and in the destructor, use the determination of validity to decide whether to close the file or not. This can be implemented by an optional parameter to the destructor that's always passed, but it's not necessary to use it. i.e. you could declare your destructor like this: ~this(bool deterministic) { if(deterministic) file.close(); } or like this: ~this() { } Scope classes (if allowed to exist) or calls to clear will set deterministic to true. The GC will set it to false. This would make destructors a lot more useful. Thoughts? -SteveInterestingly I had exactly the same idea a few hours ago as I red the discussions about clear. I just forgot about it without posting. I think it's a good idea. The ignorant destructor (ie w/o parameter) defenition should be inetrnally rewritten to accept an ignored bool parameter. This would make the following thing a doubled and therfore incorrect definition. class X { ~this(){} ~this(bool det){} } If the compiler then adds an overload for 'X.__dtor()' which simply calls 'X.__dtor(false)' [1], we should be totally compatible with existing code.
Aug 10 2010
Steven Schveighoffer wrote: ...This would make destructors a lot more useful. Thoughts? -SteveMy first thought was that they are actually two separate functions distinguished by a boolean, then Michel also mentioned the SafeD argument. atm I think it is better to let go of the destructor entirely for anything else than the GC collecting non-gc owned data as we now have. (the unsafe version, not compilable in SafeD). Rather provide a standard interface to implement and base deterministic release of resources on that. A much more simple version of IDisposable. clear() can call this one and leave ~this alone. Anything that needs more reliability will need to use structs / reference counting.
Aug 10 2010
On Tue, 10 Aug 2010 15:08:30 -0400, Lutger <lutger.blijdestijn gmail.com> wrote:Steven Schveighoffer wrote: ...Hm... something I just thought of that makes this bad, destructors are special in that they automatically call base destructors. You can't do that with a simple function. But it could be done the way you say (with the caveat that you have to manually call the base method). I think clear should call ~this() in addition to the dispose method because you don't want to have to duplicate code. -SteveThis would make destructors a lot more useful. Thoughts? -SteveMy first thought was that they are actually two separate functions distinguished by a boolean, then Michel also mentioned the SafeD argument. atm I think it is better to let go of the destructor entirely for anything else than the GC collecting non-gc owned data as we now have. (the unsafe version, not compilable in SafeD). Rather provide a standard interface to implement and base deterministic release of resources on that. A much more simple version of IDisposable. clear() can call this one and leave ~this alone.
Aug 10 2010
Steven Schveighoffer wrote:On Tue, 10 Aug 2010 15:08:30 -0400, Lutger <lutger.blijdestijn gmail.com> wrote:clear could check if the base classes implement the interface and act on that. It already does the same for destructors.Steven Schveighoffer wrote: ...Hm... something I just thought of that makes this bad, destructors are special in that they automatically call base destructors. You can't do that with a simple function. But it could be done the way you say (with the caveat that you have to manually call the base method). I think clear should call ~this() in addition to the dispose method because you don't want to have to duplicate code. -SteveThis would make destructors a lot more useful. Thoughts? -SteveMy first thought was that they are actually two separate functions distinguished by a boolean, then Michel also mentioned the SafeD argument. atm I think it is better to let go of the destructor entirely for anything else than the GC collecting non-gc owned data as we now have. (the unsafe version, not compilable in SafeD). Rather provide a standard interface to implement and base deterministic release of resources on that. A much more simple version of IDisposable. clear() can call this one and leave ~this alone.
Aug 10 2010
On Tue, 10 Aug 2010 16:03:42 -0400, Lutger <lutger.blijdestijn gmail.com> wrote:Steven Schveighoffer wrote:Checking if a class implements an interface is not as trivial as accessing a member of the classinfo. It's a linear search through the interfaces. Plus, I don't know quite how it would work, I'm not sure its doable. BTW, I assumed that the most derived destructor simply called the base destructor implicitly, I didn't realize this was handled via the runtime, that seems like a low-hanging fruit opportunity for performance improvement... -SteveOn Tue, 10 Aug 2010 15:08:30 -0400, Lutger <lutger.blijdestijn gmail.com> wrote:clear could check if the base classes implement the interface and act on that. It already does the same for destructors.Steven Schveighoffer wrote: ...Hm... something I just thought of that makes this bad, destructors are special in that they automatically call base destructors. You can't do that with a simple function. But it could be done the way you say (with the caveat that you have to manually call the base method). I think clear should call ~this() in addition to the dispose method because you don't want to have to duplicate code. -SteveThis would make destructors a lot more useful. Thoughts? -SteveMy first thought was that they are actually two separate functions distinguished by a boolean, then Michel also mentioned the SafeD argument. atm I think it is better to let go of the destructor entirely for anything else than the GC collecting non-gc owned data as we now have. (the unsafe version, not compilable in SafeD). Rather provide a standard interface to implement and base deterministic release of resources on that. A much more simple version of IDisposable. clear() can call this one and leave ~this alone.
Aug 10 2010