Sven Barth schrieb:
On 20.09.2014 13:42, Sven Barth wrote:
On 20.09.2014 13:11, Peter Popov wrote:
- to remedy this TObject is extended with non-virtual methods that allow
manual reference counting and would rely on the RTTI data I mentioned
(let's call the methods AddRef, Release, IsReferenceCounted and RefCount
for now, which can also be used to hook up the reference counting of
IUnknown interfaces);
I'd add a _RefCount field to TObject, regardless of whether it's really
used later; this will fix the field offset - just like the VMT reference
is fixed in TObject, but not in Object types. This will eliminate
problems with class helpers.
This approach also would allow to switch any object from managed to
unmanaged on the fly, by setting the counter to -1, because the special
value -1 already indicates an unmanaged/const memory object (like with
string literals).
In my first draft I considered virtual _AddRef/_Release methods, but
calling a virtual method is more expensive than calling or inlining a
static method.
the code from above would then look like this to
make it safe:
=== code begin ===
function CreateObject: TObject;
begin
Result := TARCObject.Create;
Result.AddRef;
end;
=== code end ===
Here the compiler would always insert _AddRef, just like with
interfaces, eventually optimized (inlined?) like:
if Result._RefCounter <> -1 then
Result._AddRef; //or InterlockedIncrement(Result._RefCounter);
- TObject.Free would be extended to take reference counting into account
as well. If the object is reference counted (IsReferenceCounted returns
true) it will call Release and otherwise it will continue to Destroy.
- there would be a TARCObject declared in System which is a direct
descendant of TObject, but with reference counting enabled; same maybe
also for TInterfacedObject
The convention, of -1 meaning unmanaged, favors managed objects by
default, when InitInstance zeroes all fields of the instance just
created. But when the VMT reference must be excluded or inserted
afterwards afterwards, then _RefCount can be initialized at the same
time (to -1 for the unmanaged default). Later on a TARCObject base class
constructor/initializer will reset _RefCount to zero again.
- all classes can now have operator overloads as well though it should
be warned in the documentation that non-reference counted objects might
result in memory leaks there
...unless operators also test _RefCount
- this now only leaves the problems of cycles; take this code:
=== code begin ===
type
TSomeClass = class(TARCObject)
Children: specialize TList<TSomeClass>;
Owner: TSomeClass;
constructor Create(aOwner: TSomeClass);
end;
constructor TSomeClass.Create(aOwner: TSomeClass);
begin
Children := specialize TList<TSomeClass>.Create;
Owner := aOwner;
if Assigned(Owner) then
Owner.Children.Add(Self);
end;
Here I'd prefer
Owner.AddChild(Self);
so that the Owner can implement any decent/appropriate child management
under the hood.
procedure Test;
var
t1, t2: TSomeClass;
begin
t1 := TSomeClass.Create(Nil);
t2 := TSomeClass.Create(t1);
// do something
end;
=== code end ===
Now once Test is left it would leave the instances which were assigned
to t1 and t2 hanging, because they have references to each other.
This depends on the implementation of TOwner.Children[] and
TChild.Owner. Is a stored TChild.Owner reference really required in a
managaged environment? IMO a (strong) unidirectional reference from
Owner to Child will do it all. Then no child will be destroyed, as long
as its owner holds a reference to it. That's the intended purpose of
both owner/child and automatic memory management.
When it's desireable to definitely destroy an owned object at will, then
its owner must be known, of course. In this case two different
management approaches conflict with each other. In this case I'd accept
a weak Owner reference, because the referenced Owner will stay alive
longer than it's listed children.
More problematic are circular references without a decicated owner/child
relationship.
There are (as far as I see) three ways to solve this:
* provide a way to break the circle (in this example e.g. setting Owner
to Nil before leaving Test; this is what Delphi provides with the
DisposeOf virtual method)
* introduce weak references which would disable reference counting, e.g.:
=== code begin ===
type
TSomeClass = class(TARCObject)
// ...
Owner: TSomeClass weak;
// ...
end;
=== code end ===
Now the "TSomeClass.Create(t1)" line in "Test" wouldn't increase the
reference count of "t1" further and thus both class instances would be
destroyed after "Test" is left.
This IMO is the preferable way to go, in a definite owner/child
relationship. The lifetime of an owner can not depend on the existence
of owned children, so that the owner will survive until it has
destroyed/released all his children himself. A child-to-owner reference
is not required in automatic management, it only is required when it
must be possible to definitely destroy an owned child object.
Otherwise cyclic references *without* a definite owner/child
relationship are really problematic:
* provide a possibilty to execute a cycle detection algorithm during the
Release part of the reference counting; this has the benefit of avoiding
the need for "weak", but there would be the problem that the algorithm
can be potentially expensive especially with large object instance
hierarchies (think LCL here) and this would also need to be executed for
*each* decrement of the reference count, thus for both the automated one
(which till now could have been rather efficient) and the manual one.
That's the classic mark/sweep approach, with all its known drawbacks; in
detail its asynchronous destruction of objects is a known troublemaker
in OOP, see "Why a garbage collector never should call an destructor...".
In such cases IMO the developer should establish a definite owner/child
relationship, by e.g. making all but one reference Weak, or by a special
(intelligent) _Release method.
Even if I mentioned the use of Weak references as possible solutions of
some problems, I still don't favor such solutions for the already
mentioned reasons.
DoDi
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