http://d.puremagic.com/issues/show_bug.cgi?id=2834

           Summary: Struct Destructors are not called by the GC, but called
                    on explicit delete.
           Product: D
           Version: 2.027
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Keywords: spec
          Severity: minor
          Priority: P3
         Component: DMD
        AssignedTo: bugzilla@digitalmars.com
        ReportedBy: sandford@jhu.edu


According to the spec: [Struct] "Destructors are called when an object goes out of scope. Their purpose is to free up resources owned by the struct object." And while the heap is always 'in scope', it does delete structs which arguably places them out of scope. Note that an explicit call to delete will run the destructor. At a minimum, the spec should be updated to clearly reflect this limitation. Since memory leaks, open files, etc will probably occur when a struct with a postblit is manually allocated directly on the heap, instead of on the stack or as part of an object, this issue might warrant a compiler warning/error (i.e. an explicit cast is required).


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http://d.puremagic.com/issues/show_bug.cgi?id=2834


Rob Jacques <sandford@jhu.edu> changed:

           What    |Removed                     |Added
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                 CC|                            |andrei@metalanguage.com


--- Comment #1 from Rob Jacques <sandford@jhu.edu> 2010-07-09 23:20:01 PDT ---
*** Issue 4442 has been marked as a duplicate of this issue. ***

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--- Comment #2 from Rob Jacques <sandford@jhu.edu> 2010-07-09 23:22:39 PDT ---
From Issue 4442:
Example brought up by Sean Kelly in private correspondence:

struct S1{ ~this() { writeln("dtor"); } }
void main() {
    auto a = S1();
    auto b = new S1();
    delete b;
    auto c  = new S1();
    c = null;
    GC.collect();
}

"dtor" is only printed twice.

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nfxjfg@gmail.com changed:

           What    |Removed                     |Added
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                 CC|                            |nfxjfg@gmail.com


--- Comment #3 from nfxjfg@gmail.com 2010-07-10 00:00:11 PDT ---
Gc finalization is not deterministic. You can't expect it to be called.

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Sean Kelly <sean@invisibleduck.org> changed:

           What    |Removed                     |Added
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                 CC|                            |sean@invisibleduck.org


--- Comment #4 from Sean Kelly <sean@invisibleduck.org> 2010-07-10 08:40:16 PDT ---
What about structs whose memory are freed by the GC?  Would you expect their dtors to be called?  They aren't.  Try allocating ten million structs in a loop, not one "dtor" line will print.

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--- Comment #5 from Sean Kelly <sean@invisibleduck.org> 2010-07-10 08:41:54 PDT ---
Fixing this will probably be fairly involved.  The GC will have to store a TypeInfo reference for each block that needs to be finalized.  The best approach may be to integrate this with precise scanning, since that requires detailed type info too.

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
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                 CC|                            |bearophile_hugs@eml.cc


--- Comment #6 from bearophile_hugs@eml.cc 2010-07-11 15:16:07 PDT ---
For the moment the compiler can show a warning when the code allocates on the heap a struct that has a destructor.

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--- Comment #7 from bearophile_hugs@eml.cc 2010-07-11 17:24:58 PDT ---
One case where struct destructors are not called, in this situation it seems simpler for the GC to know what destructors to call:


import core.memory: GC;
import core.stdc.stdio: printf;
struct Foo {
    int x;
    this(int xx) { this.x = xx; }
    ~this() { printf("Foo dtor x: %d\n", x); }
}
void main() {
    Foo[] a;
    a.length = 2;
    a[0].x = 1;
    a[1].x = 2;
    // delete a;
}


(I am not sure, but a type information can be useful in arrays, maybe to fix
bug 2095 too.)

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Max Samukha <samukha@voliacable.com> changed:

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                 CC|                            |samukha@voliacable.com


--- Comment #8 from Max Samukha <samukha@voliacable.com> 2010-11-18 03:39:17 PST ---
So what is the verdict? Should we simply specify that struct destructors are not automatically called except in RAII and remove the struct-in-class special case?

BTW, there are other problems (serious IMO):

auto ss = new S[10];
ss.length = 5;
delete ss;

Destructors are not called on the last 5 elements.

----
auto ss = new S[10];
ss ~= ss;
delete ss;

We have a nasty problem when destructors are called on the appended elements because postblits was not run for them during append.

etc

Essentially, operations on arrays of structs with postblits/dtors defined are currently unusable.

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--- Comment #9 from Max Samukha <samukha@voliacable.com> 2010-11-18 03:59:12 PST ---
(In reply to comment #8)
> We have a nasty problem when destructors are called on the appended elements because postblits was not run for them during append.

I meant the problem is the destructors called on objects that have not been postblitted.

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