Hi Valery,
On 16 Aug 2018, at 08:06, Valery Kotov
<[email protected]<mailto:[email protected]>> wrote:
Hello all,
I'm trying to wrap my head around SimpleArrayData API. And I might actually
need a bit of help here.
Instead of using ValueArray now I declare myValues to be SimpleArrayData*:
#define MyObjectMembers(class, Member) \
Member(class, Pointer, SimpleArrayData*, myValues)
DECLARE_HEAP_OBJECT(MyObject, Object) {
DECLARE_MARKOBJECTS(MyObject);
In MyObjects' init function I'm creating and initializing myValues:
const size_t size = sizeof(Heap::ArrayData) + length*sizeof(Value); // I still
can have 0 length array, right?
Heap::SimpleArrayData *d =
e->memoryManager->allocManaged<QV4::SimpleArrayData>(size);
d->init();
d->type = Heap::ArrayData::Simple;
d->offset = 0;
d->values.alloc = length;
d->values.size = length;
myValues.set(e, d);
Later I'm trying to add a new item to myValues:
ScopedArrayObject a(scope,
scope.engine->memoryManager->allocate<QV4::ArrayObject>());
a->d()->arrayData.set(scope.engine, d()->myValues);
Scoped<SimpleArrayData> sa(scope, d()->myValues);
sa->push_front(a, newValue, 1);
Clearly I'm doing something wrong. push_front adds a new item to array indeed,
but not to myValues SimpleArrayData memeber. Internally, from what I could
observe, realloc creates a new SimpleArrayData and new item actually added to
it.
What I have a little bit of trouble to understand, is how to use
SimpleArrayData API. It has to be used in conjunction with ArrayObject somehow,
right? Or shall I store ArrayObject pointer instead of SimpleArrayData?
Yes, you’re right that will be problematic, as SimpleArrayData is meant to be
used together with the Object it belongs to. So either use an ArrayObject, or a
QVector then.
>> If you have a fully dynamic array, then the alternative would be to store a
>> pointer to your QVector in the heap object.
Yes, that is a good point. Thank you. Though, I have a small concern here. From
what I could see, QQmlSequence does not store JS heap objects in QVector (or
did I miss it somehow?).
Whereas I would like my container to:
* be dynamically resizable, since amount of it's items is not known in advance
* be able to store JS heap objects
What would you suggest to use in this case? Shall I still go for
SimpleArrayData and make it work? Or shall I use QVector instead? In case of
QVector would it be safe to store JS Object pointers or it should be QVector of
PersistentValues?
You can use a QVector<QV4::Value>, but then you need to reimplement the
markObjects() method manually to loop over the vector and call mark() on all
the values.
Best regards,
Lars
Thank you!
Sincerely yours,
Valery Kotov
On Thu, 9 Aug 2018 at 08:20, Valery Kotov
<[email protected]<mailto:[email protected]>> wrote:
Hi Lars, Simon,
I think I got it. Thanks a lot for your tips!
Sincerely yours,
Valery Kotov
On 8 August 2018 at 09:32, Simon Hausmann
<[email protected]<mailto:[email protected]>> wrote:
Hi,
If you want to use the ValueArray, then you need to make sure that it's the
last member of your type. Then encapsulate the creation if the type in a
factory function that works a bit like ExecutionContext::newCallContext:
(1) Calculate how much memory you're going to need for the type _and_ the
value array (minus one Value).
(2) Allocate the memory with one call to allocManaged.
(3) call init() manually.
(4) Set the alloc and size members of the value array to the amount of
allocated values.
While this approach is clever in the sense that it gives a compact memory
representation and allocates object and array data in one shot, it also means
that it's not so suitable for frequent dynamic resizing as it will require
re-allocating the entire object.
If you have a fully dynamic array, then the alternative would be to store a
pointer to your QVector in the heap object. Heap::QQmlSequence does that, for
example. Just make sure to use V4_NEEDS_DESTROY and provide a destroy()
function to delete the vector.
I'd probably go for the first approach with perhaps an indirect type (similar
to how Object has MemberData) if the array contains references to the JS heap.
If your array has only references to the C heap, then you're probably better
off using a pointer to a regular vector.
Simon
________________________________
From: Development
<[email protected]<mailto:[email protected]>>
on behalf of Valery Kotov
<[email protected]<mailto:[email protected]>>
Sent: Wednesday, August 8, 2018 9:10:43 AM
To: Qt development mailing list
Subject: [Development] Storing and managing ValueArray inside QV4::Heap objects
Hello all,
I have a question about QV4 heap objects and what could be stored in them.
I would like to move data structure from c++ heap to GC heap. Unfortunately, my
data structure stores a pair of QLists internally.
As far as I'm aware, QV4::Heap objects should be "trivially constructible", and
thus, cannot store QList(s).
One option would be to store QList pointer and manage it's lifecycle manually.
But that does not sound like a good idea.
I've noticed though, that some of qv4 types are using DECLARE_MARKOBJECTS
macro. By using DECLARE_MARKOBJECTS macro and ObjectMember(s) define I can
actually store a ValueArray member in my QV4 heap object.
#define MyObjectMembers(class, Member) \
Member(class, ValueArray, ValueArray, myValues)
DECLARE_HEAP_OBJECT(MyObject, Object) {
DECLARE_MARKOBJECTS(MyObject);
In this case I can refer to myValues ValueArray type inside of MyObject.
Unfortunately, it is a little bit unclear to me how to manage ValueArrays.
From what I can see from some examples in the code (qv4generatorobject,
qv4arraydata), it seems that I need to manage ValueArray manually.
What I'm missing at the moment a little bit is how the actually allocation for
ValueArray.values is done.
I suspect that it is probably not done via plain "new" call. Otherwise it is a
little bit confusing why not just to store the pointer directly.
Could you guys please give me a few hints how to go about ValueArray management?
Thank you!
Sincerely yours,
Valery Kotov
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