Ceylon has several different concepts for things that might all be considered some kind of array: List, Tuple, Sequence, Sequential, Iterable, Array, Collection, Category, etc. What's is different about these these types and when should I use them?
The best place to start to learn about these things at a basic level is the Ceylon tour. And the place to learn about these things in depth is the module API. It can also be helpful to look at the source files for these.
Like all good modern programming languages, the first few interfaces are super abstract. They are built around one formal member and provide their functionality through a bunch of default and actual members. (In programming languages created before Java 8, you may have heard these called "traits" to distinguish them from traditional interfaces which have only formal members and no functionality.)
Category
Let's start by talking about the interface Category. It represents types of which you can ask "does this collection contain this object", but you may not necessarily be able to get any of the members out of the collection. It's formal member is:
shared formal Boolean contains(Element element)
An example might be the set of all the factors of a large number—you can efficiently test if any integer is a factor, but not efficiently get all the factors.
Iterable
A subtype of Category is the interface Iterable. It represents types from which you can get each element one at a time, but not necessarily index the elements. The elements may not have a well-defined order. The elements may not even exist yet but are generated on the fly. The collection may even be infinitely long! It's formal member is:
shared formal Iterator<Element> iterator()
An example would be a stream of characters like standard out. Another example would be a range of integers provided to a for loop, for which it is more memory efficient to generate the numbers one at a time.
This is a special type in Ceylon and can be abbreviated {Element*} or {Element+} depending on if the iterable might be empty or is definitely not empty, respectively.
Collection
One of Iterable's subtypes is the interface Collection. It has one formal member:
shared formal Collection<Element> clone()
But that doesn't really matter. The important thing that defines a Collection is this line in the documentation:
All Collections are required to support a well-defined notion of value
equality, but the definition of equality depends upon the kind of
collection.
Basically, a Collection is a collection who structure is well-defined enough to be equatable to each other and clonable. This requirement for a well-defined structure means that this is the last of the super abstract interfaces, and the rest are going to look like more familiar collections.
List
One of Collection's subtypes is the interface List. It represents a collection whose elements we can get by index (myList[42]). Use this type when your function requires an array to get things out of, but doesn't care if it is mutable or immutable. It has a few formal methods, but the important one comes from its other supertype Correspondence:
shared formal Item? get(Integer key)
Sequential, Sequence, Empty
The most important of List's subtypes is the interface Sequential. It represents an immutable List. Ceylon loves this type and builds a lot of syntax around it. It is known as [Element*] and Element[]. It has exactly two subtypes:
Empty (aka []), which represents empty collections
Sequence (aka [Element+]), which represents nonempty collections.
Because the collections are immutable, there are lots of things you can do with them that you can't do with mutable collections. For one, numerous operations can fail with null on empty lists, like reduce and first, but if you first test that the type is Sequence then you can guarantee these operations will always succeed because the collection can't become empty later (they're immutable after all).
Tuple
A very special subtype of Sequence is Tuple, the first true class listed here. Unlike Sequence, where all the elements are constrained to one type Element, a Tuple has a type for each element. It gets special syntax in Ceylon, where [String, Integer, String] is an immutable list of exactly three elements with exactly those types in exactly that order.
Array
Another subtype of List is Array, also a true class. This is the familiar Java array, a mutable fixed-size list of elements.
drhagen has already answered the first part of your question very well, so I’m just going to say a bit on the second part: when do you use which type?
In general: when writing a function, make it accept the most general type that supports the operations you need. So far, so obvious.
Category is very abstract and rarely useful.
Iterable should be used if you expect some stream of elements which you’re just going to iterate over (or use stream operations like filter, map, etc.).
Another thing to consider about Iterable is that it has some extra syntax sugar in named arguments:
void printAll({Anything*} things, String prefix = "") {
for (thing in things) {
print(prefix + (thing?.string else "<null>"));
}
}
printAll { "a", "b", "c" };
printAll { prefix = "X"; "a", "b", "c" };
Try online
Any parameter of type Iterable can be supplied as a list of comma-separated arguments at the end of a named argument list. That is,
printAll { "a", "b", "c" };
is equivalent to
printAll { things = { "a", "b", "c" }; };
This allows you to craft DSL-style expressions; the tour has some nice examples.
Collection is, like Correspondence, fairly abstract and in my experience rarely used directly.
List sounds like it should be a often used type, but actually I don’t recall using it a lot. I’m not sure why. I seem to skip over it and declare my parameters as either Iterable or Sequential.
Sequential and Sequence are when you want an immutable, fixed-length list. It also has some syntax sugar: variadic methods like void foo(String* bar) are a shortcut for a Sequential or Sequence parameter. Sequential also allows you to do use the nonempty operator, which often works out nicely in combination with first and rest:
String commaSeparated(String[] strings) {
if (nonempty strings) {
value sb = StringBuilder();
sb.append(strings.first); // known to exist
for (string in strings.rest) { // skip the first one
sb.append(", ").append(string);
// we don’t need a separate boolean to track if we need the comma or not :)
}
return sb.string;
} else {
return "";
}
}
Try online
I usually use Sequential and Sequence when I’m going to iterate over a stream several times (which could be expensive for a generic Iterable), though List might be the better interface for that.
Tuple should never be used as Tuple (except in the rare case where you’re abstracting over them), but with the [X, Y, Z] syntax sugar it’s often useful. You can often refine a Sequential member to a Tuple in a subclass, e. g. the superclass has a <String|Integer>[] elements which in one subclass is known to be a [String, Integer] elements.
Array I’ve never used as a parameter type, only rarely as a class to instantiate and use.
I’ve found that assigning blocks behaves differently with respect to Objective-C class parameters and C++ classes parameters.
Imagine I have this simple Objective-C class hierarchy:
#interface Fruit : NSObject
#end
#interface Apple : Fruit
#end
Then I can write stuff like this:
Fruit *(^getFruit)();
Apple *(^getApple)();
getFruit = getApple;
This means that, with respect to Objective-C classes, blocks are covariant in their return type: a block which returns something more specific can be seen as a “subclass” of a block returning something more general. Here, the getApple block, which delivers an apple, can be safely assigned to the getFruit block. Indeed, if used later, it's always save to receive an Apple * when you're expecting a Fruit *. And, logically, the converse does not work: getApple = getFruit; doesn't compile, because when we really want an apple, we're not happy getting just a fruit.
Similarly, I can write this:
void (^eatFruit)(Fruit *);
void (^eatApple)(Apple *);
eatApple = eatFruit;
This shows that blocks are covariant in their argument types: a block that can process an argument that is more general can be used where a block that processes an argument that is more specific is needed. If a block knows how to eat a fruit, it will know how to eat an apple as well. Again, the converse is not true, and this will not compile: eatFruit = eatApple;.
This is all good and well — in Objective-C. Now let's try that in C++ or Objective-C++, supposing we have these similar C++ classes:
class FruitCpp {};
class AppleCpp : public FruitCpp {};
class OrangeCpp : public FruitCpp {};
Sadly, these block assignments don't compile any more:
FruitCpp *(^getFruitCpp)();
AppleCpp *(^getAppleCpp)();
getFruitCpp = getAppleCpp; // error!
void (^eatFruitCpp)(FruitCpp *);
void (^eatAppleCpp)(AppleCpp *);
eatAppleCpp = eatFruitCpp; // error!
Clang complains with an “assigning from incompatible type” error. So, with respect to C++ classes, blocks appear to be invariant in the return type and parameter types.
Why is that? Doesn't the same argument I made with Objective-C classes also hold for C++ classes? What am I missing?
This distinction is intentional, due to the differences between the Objective-C and C++ object models. In particular, given a pointer to an Objective-C object, one can convert/cast that pointer to point at a base class or a derived class without actually changing the value of the pointer: the address of the object is the same regardless.
Because C++ allows multiple and virtual inheritance, this is not the case for C++ objects: if I have a pointer to a C++ class and I cast/convert that pointer to point at a base class or a derived class, I may have to adjust the value of the pointer. For example, consider:
class A { int x; }
class B { int y; }
class C : public A, public B { }
B *getC() {
C *c = new C;
return c;
}
Let's say that the new C object in getC() gets allocated at address 0x10. The value of the pointer 'c' is 0x10. In the return statement, that pointer to C needs to be adjusted to point at the B subobject within C. Because B comes after A in C's inheritance list, it will (generally) be laid out in memory after A, so this means adding an offset of 4 bytes (
== sizeof(A)) to the pointer, so the returned pointer will be 0x14. Similarly, casting a B* to a C* would subtract 4 bytes from the pointer, to account for B's offset within C. When dealing with virtual base classes, the idea is the same but the offsets are no longer known, compile-time constants: they're accessed through the vtable during execution.
Now, consider the effect this has on an assignment like:
C (^getC)();
B (^getB)();
getB = getC;
The getC block returns a pointer to a C. To turn it into a block that returns a pointer to a B, we would need to adjust the pointer returned from each invocation of the block by adding 4 bytes. This isn't an adjustment to the block; it's an adjustment to the pointer value returned by the block. One could implement this by synthesizing a new block that wraps the previous block and performs the adjustment, e.g.,
getB = ^B() { return getC() }
This is implementable in the compiler, which already introduces similar "thunks" when overriding a virtual function with one that has a covariant return type needing adjustment. However, with blocks it causes an additional problem: blocks allow equality comparison with ==, so to evaluate whether "getB == getC", we would have to be able to look through the thunk that would be generated by the assignment "getB = getC" to compare the underlying block pointers. Again, this is implementable, but would require a much more heavyweight blocks runtime that is able to create (uniqued) thunks able to perform these adjustments to the return value (and as well as for any contravariant parameters). While all of this is technically possible, the cost (in runtime size, complexity, and execution time) outweighs the benefits.
Getting back to Objective-C, the single-inheritance object model never needs any adjustments to the object pointer: there's only a single address to point at a given Objective-C object, regardless of the static type of the pointer, so covariance/contravariance never requires any thunks, and the block assignment is a simple pointer assignment (+ _Block_copy/_Block_release under ARC).
the feature was probably overlooked. There are commits that show Clang people caring about making covariance and contravariance work in Objective-C++ for Objective-C types but I couldn't find anything for C++ itself. The language specification for blocks doesn't mention covariance or contravariance for either C++ or Objective-C.
I wanted to create a vector of a subclass of QGraphicsRectItem, named MyRect. This vector is initialized in MyClass:
MyClass::MyClass () : myVector_(80, std::vector<MyRect>(60, MyRect(true,true)))
...
I learned that vector constructs the first element and then copies it with the copy constructor. The problem is that QGraphicsRectItem's copy constructor is private and this doesn't work. (Very long error message, one hour of googling)
Now I have three possible solutions as I see it:
1.)Make a for-loop and populate myVector myself in the constructor body.
1b.) Just use regular array because it remains static anyway.
2.)Use MyRect* instead of MyRect as content of myVector (manual memory allocation -> bad)
3.)Use QVector that uses Object* by default and manages the memory for me.
After spending at least one hour on solving this I would like to hear from you if there are other good possibilities or what you think is the best solution. I am on the verge of dropping vectors for this and just using arrays.
The vector, as you declared it, will have to manipulate instances of MyRect. This means that depending of what you do the with the elements of the vector, or if you copy the vector, the MyRect instances might be duplicated.
This is not possible, because that would mean creating a new item each time a copy occurs (this is why the QGraphicsItem constructor is private). You have to manipulate the items of your scene through a pointer.
Thus, to me the best solution is to store in your vector pointers on your items (your 2nd solution) :
std::vector<MyRect*>
Memory management shouldn't be a problem at all, as this will be handled by Qt : when you destroy the scene, all items part of this scene will be destroyed.
Your vector won't duplicate items (no instanciation), only pointers, which means you won't create new items you'd have to destroy yourself.
I've a tree-like polymorphic data-structure, where the nodes are instances of class Node (implemented by me) or any its subclass. My application heavily uses Boost and the nodes are actually represented by boost::shared_ptr type rather than Node*.
Now, I want to create a Qt model to wrap my tree data-structure. Therefore I need a way to associate any model index with a node in my internal data structure. And here comes the problem:
Qt supports two ways of doing it:
First:
QModelIndex QAbstractItemModel::createIndex ( int row, int column, void * ptr = 0 ) const
Creates a model index for the given
row and column with the internal
pointer ptr.
And second:
QModelIndex QAbstractItemModel::createIndex ( int row, int column, quint32 id ) const
Creates a model index for the given
row and column with the internal
identifier, id.
Ok, and how exactly should I associate the node in my case? There is no possibility to associate a shared_ptr with the model index... Yes, I know, I can receive a raw pointer from my shared_ptr and supply it to CreateIndex(), but it smells bad - seems too unsafe to me.
Any ideas?
By the way, I feel that in general Boost / Qt integration seems to be not trivial at least in the area of memory management.
10x a lot.
If you want to do an easy association without passing a raw pointer, put the shared memory in a container and pass the ID value for that container element into the model index. For example, you could created declare
QMap< quint32, boost::shared_ptr< Foo > > index_map;
and use that. You'd have to be careful to not duplicate IDs for existing pointers, perhaps. It seems somewhat overly complicated to me....
You could also just keep a list of the pointers (to ensure continued availability as you need them) and then use the actual address of the pointer in the QModelIndex as well. This is probably what I would do.
When using reference counting, what are possible solutions/techniques to deal with circular references?
The most well-known solution is using weak references, however many articles about the subject imply that there are other methods as well, but keep repeating the weak-referencing example. Which makes me wonder, what are these other methods?
I am not asking what are alternatives to reference counting, rather what are solutions to circular references when using reference counting.
This question isn't about any specific problem/implementation/language rather a general question.
I've looked at the problem a dozen different ways over the years, and the only solution I've found that works every time is to re-architect my solution to not use a circular reference.
Edit:
Can you expand? For example, how would you deal with a parent-child relation when the child needs to know about/access the parent? – OB OB
As I said, the only good solution is to avoid such constructs unless you are using a runtime that can deal with them safely.
That said, if you must have a tree / parent-child data structure where the child knows about the parent, you're going to have to implement your own, manually called teardown sequence (i.e. external to any destructors you might implement) that starts at the root (or at the branch you want to prune) and does a depth-first search of the tree to remove references from the leaves.
It gets complex and cumbersome, so IMO the only solution is to avoid it entirely.
Here is a solution I've seen:
Add a method to each object to tell it to release its references to the other objects, say call it Teardown().
Then you have to know who 'owns' each object, and the owner of an object must call Teardown() on it when they're done with it.
If there is a circular reference, say A <-> B, and C owns A, then when C's Teardown() is called, it calls A's Teardown, which calls Teardown on B, B then releases its reference to A, A then releases its reference to B (destroying B), and then C releases its reference to A (destroying A).
I guess another method, used by garbage collectors, is "mark and sweep":
Set a flag in every object instance
Traverse the graph of every instance that's reachable, clearing that flag
Every remaining instance which still has the flag set is unreachable, even if some of those instances have circular references to each other.
I'd like to suggest a slightly different method that occured to me, I don't know if it has any official name:
Objects by themeselves don't have a reference counter. Instead, groups of one or more objects have a single reference counter for the entire group, which defines the lifetime of all the objects in the group.
In a similiar fashion, references share groups with objects, or belong to a null group.
A reference to an object affects the reference count of the (object's) group only if it's (the reference) external to the group.
If two objects form a circular reference, they should be made a part of the same group. If two groups create a circular reference, they should be united into a single group.
Bigger groups allow more reference-freedom, but objects of the group have more potential of staying alive while not needed.
Put things into a hierarchy
Having weak references is one solution. The only other solution I know of is to avoid circular owning references all together. If you have shared pointers to objects, then this means semantically that you own that object in a shared manner. If you use shared pointers only in this way, then you can hardly get cyclic references. It does not occur very often that objects own each other in a cyclic manner, instead objects are usually connected through a hierarchical tree-like structure. This is the case I'll describe next.
Dealing with trees
If you have a tree with objects having a parent-child relationship, then the child does not need an owning reference to its parent, since the parent will outlive the child anyways. Hence a non-owning raw back pointer will do. This also applies to elements pointing to a container in which they are situated. The container should, if possible, use unique pointers or values instead of shared pointers anyways, if possible.
Emulating garbage collection
If you have a bunch of objects that can wildly point to each other and you want to clean up as soon as some objects are not reachable, then you might want to build a container for them and an array of root references in order to do garbage collection manually.
Use unique pointers, raw pointers and values
In the real world I have found that the actual use cases of shared pointers are very limited and they should be avoided in favor of unique pointers, raw pointers, or -- even better -- just value types. Shared pointers are usually used when you have multiple references pointing to a shared variable. Sharing causes friction and contention and should be avoided in the first place, if possible. Unique pointers and non-owning raw pointers and/or values are much easier to reason about. However, sometimes shared pointers are needed. Shared pointers are also used in order to extend the lifetime of an object. This does usually not lead to cyclic references.
Bottom line
Use shared pointers sparingly. Prefer unique pointers and non-owning raw pointers or plain values. Shared pointers indicate shared ownership. Use them in this way. Order your objects in a hierarchy. Child objects or objects on the same level in a hierarchy should not use owning shared references to each other or to their parent, but they should use non-owning raw pointers instead.
No one has mentioned that there is a whole class of algorithms that collect cycles, not by doing mark and sweep looking for non-collectable data, but only by scanning a smaller set of possibly circular data, detecting cycles in them and collecting them without a full sweep.
To add more detail, one idea for making a set of possible nodes for scanning would be ones whose reference count was decremented but which didn't go to zero on the decrement. Only nodes to which this has happened can be the point at which a loop was cut off from the root set.
Python has a collector that does that, as does PHP.
I'm still trying to get my head around the algorithm because there are advanced versions that claim to be able to do this in parallel without stopping the program...
In any case it's not simple, it requires multiple scans, an extra set of reference counters, and decrementing elements (in the extra counter) in a "trial" to see if the self referential data ends up being collectable.
Some papers:
"Down for the Count? Getting Reference Counting Back in the Ring" Rifat Shahriyar, Stephen M. Blackburn and Daniel Frampton
http://users.cecs.anu.edu.au/~steveb/downloads/pdf/rc-ismm-2012.pdf
"A Unified Theory of Garbage Collection" by David F. Bacon, Perry Cheng and V.T. Rajan
http://www.cs.virginia.edu/~cs415/reading/bacon-garbage.pdf
There are lots more topics in reference counting such as exotic ways of reducing or getting rid of interlocked instructions in reference counting. I can think of 3 ways, 2 of which have been written up.
I have always redesigned to avoid the issue. One of the common cases where this comes up is the parent child relationship where the child needs to know about the parent. There are 2 solutions to this
Convert the parent to a service, the parent then does not know about the children and the parent dies when there are no more children or the main program drops the parent reference.
If the parent must have access to the children, then have a register method on the parent which accepts a pointer that is not reference counted, such as an object pointer, and a corresponding unregister method. The child will need to call the register and unregister method. When the parent needs to access a child then it type casts the object pointer to the reference counted interface.
When using reference counting, what are possible solutions/techniques to deal with circular references?
Three solutions:
Augment naive reference counting with a cycle detector: counts decremented to non-zero values are considered to be potential sources of cycles and the heap topology around them is searched for cycles.
Augment naive reference counting with a conventional garbage collector like mark-sweep.
Constrain the language such that its programs can only ever produce acyclic (aka unidirectional) heaps. Erlang and Mathematica do this.
Replace references with dictionary lookups and then implement your own garbage collector that can collect cycles.
i too am looking for a good solution to the circularly reference counted problem.
i was stealing borrowing an API from World of Warcraft dealing with achievements. i was implicitely translating it into interfaces when i realized i had circular references.
Note: You can replace the word achievements with orders if you don't like achievements. But who doesn't like achievements?
There's the achievement itself:
IAchievement = interface(IUnknown)
function GetName: string;
function GetDescription: string;
function GetPoints: Integer;
function GetCompleted: Boolean;
function GetCriteriaCount: Integer;
function GetCriteria(Index: Integer): IAchievementCriteria;
end;
And then there's the list of criteria of the achievement:
IAchievementCriteria = interface(IUnknown)
function GetDescription: string;
function GetCompleted: Boolean;
function GetQuantity: Integer;
function GetRequiredQuantity: Integer;
end;
All achievements register themselves with a central IAchievementController:
IAchievementController = interface
{
procedure RegisterAchievement(Achievement: IAchievement);
procedure UnregisterAchievement(Achievement: IAchievement);
}
And the controller can then be used to get a list of all the achievements:
IAchievementController = interface
{
procedure RegisterAchievement(Achievement: IAchievement);
procedure UnregisterAchievement(Achievement: IAchievement);
function GetAchievementCount(): Integer;
function GetAchievement(Index: Integer): IAchievement;
}
The idea was going to be that as something interesting happened, the system would call the IAchievementController and notify them that something interesting happend:
IAchievementController = interface
{
...
procedure Notify(eventType: Integer; gParam: TGUID; nParam: Integer);
}
And when an event happens, the controller will iterate through each child and notify them of the event through their own Notify method:
IAchievement = interface(IUnknown)
function GetName: string;
...
function GetCriteriaCount: Integer;
function GetCriteria(Index: Integer): IAchievementCriteria;
procedure Notify(eventType: Integer; gParam: TGUID; nParam: Integer);
end;
If the Achievement object decides the event is something it would be interested in it will notify its child criteria:
IAchievementCriteria = interface(IUnknown)
function GetDescription: string;
...
procedure Notify(eventType: Integer; gParam: TGUID; nParam: Integer);
end;
Up until now the dependancy graph has always been top-down:
IAchievementController --> IAchievement --> IAchievementCriteria
But what happens when the achievement's criteria have been met? The Criteria object was going to have to notify its parent `Achievement:
IAchievementController --> IAchievement --> IAchievementCriteria
^ |
| |
+----------------------+
Meaning that the Criteria will need a reference to its parent; the who are now referencing each other - memory leak.
And when an achievement is finally completed, it is going to have to notify its parent controller, so it can update views:
IAchievementController --> IAchievement --> IAchievementCriteria
^ | ^ |
| | | |
+----------------------+ +----------------------+
Now the Controller and its child Achievements circularly reference each other - more memory leaks.
i thought that perhaps the Criteria object could instead notify the Controller, removing the reference to its parent. But we still have a circular reference, it just takes longer:
IAchievementController --> IAchievement --> IAchievementCriteria
^ | |
| | |
+<---------------------+ |
| |
+-------------------------------------------------+
The World of Warcraft solution
Now the World of Warcraft api is not object-oriented friendly. But it does solve any circular references:
Do not pass references to the Controller. Have a single, global, singleton, Controller class. That way an achievement doesn't have to reference the controller, just use it.
Cons: Makes testing, and mocking, impossible - because you have to have a known global variable.
An achievement doesn't know its list of criteria. If you want the Criteria for an Achievement you ask the Controller for them:
IAchievementController = interface(IUnknown)
function GetAchievementCriteriaCount(AchievementGUID: TGUID): Integer;
function GetAchievementCriteria(Index: Integer): IAchievementCriteria;
end;
Cons: An Achievement can no longer decide to pass notifications to it's Criteria, because it doesn't have any criteria. You now have to register Criteria with the Controller
When a Criteria is completed, it notifies the Controller, who notifies the Achievement:
IAchievementController-->IAchievement IAchievementCriteria
^ |
| |
+----------------------------------------------+
Cons: Makes my head hurt.
i'm sure a Teardown method is much more desirable that re-architecting an entire system into a horribly messy API.
But, like you wonder, perhaps there's a better way.
If you need to store the cyclic data, for a snapShot into a string,
I attach a cyclic boolean, to any object that may be cyclic.
Step 1:
When parsing the data to a JSON string, I push any object.is_cyclic that hasn't been used into an array and save the index to the string. (Any used objects are replaced with the existing index).
Step 2: I traverse the array of objects, setting any children.is_cyclic to the specified index, or pushing any new objects to the array. Then parsing the array into a JSON string.
NOTE: By pushing new cyclic objects to the end of the array, will force recursion until all cyclic references are removed..
Step 3: Last I parse both JSON strings into a single String;
Here is a javascript fiddle...
https://jsfiddle.net/7uondjhe/5/
function my_json(item) {
var parse_key = 'restore_reference',
stringify_key = 'is_cyclic';
var referenced_array = [];
var json_replacer = function(key,value) {
if(typeof value == 'object' && value[stringify_key]) {
var index = referenced_array.indexOf(value);
if(index == -1) {
index = referenced_array.length;
referenced_array.push(value);
};
return {
[parse_key]: index
}
}
return value;
}
var json_reviver = function(key, value) {
if(typeof value == 'object' && value[parse_key] >= 0) {
return referenced_array[value[parse_key]];
}
return value;
}
var unflatten_recursive = function(item, level) {
if(!level) level = 1;
for(var key in item) {
if(!item.hasOwnProperty(key)) continue;
var value = item[key];
if(typeof value !== 'object') continue;
if(level < 2 || !value.hasOwnProperty(parse_key)) {
unflatten_recursive(value, level+1);
continue;
}
var index = value[parse_key];
item[key] = referenced_array[index];
}
};
var flatten_recursive = function(item, level) {
if(!level) level = 1;
for(var key in item) {
if(!item.hasOwnProperty(key)) continue;
var value = item[key];
if(typeof value !== 'object') continue;
if(level < 2 || !value[stringify_key]) {
flatten_recursive(value, level+1);
continue;
}
var index = referenced_array.indexOf(value);
if(index == -1) (item[key] = {})[parse_key] = referenced_array.push(value)-1;
else (item[key] = {})[parse_key] = index;
}
};
return {
clone: function(){
return JSON.parse(JSON.stringify(item,json_replacer),json_reviver)
},
parse: function() {
var object_of_json_strings = JSON.parse(item);
referenced_array = JSON.parse(object_of_json_strings.references);
unflatten_recursive(referenced_array);
return JSON.parse(object_of_json_strings.data,json_reviver);
},
stringify: function() {
var data = JSON.stringify(item,json_replacer);
flatten_recursive(referenced_array);
return JSON.stringify({
data: data,
references: JSON.stringify(referenced_array)
});
}
}
}
Here are some techniques described in Algorithms for Dynamic Memory Management by R. Jones and R. Lins. Both suggest that you can look at the cyclic structures as a whole in some way or another.
Friedman and Wise
The first way is one suggested by Friedman and Wise. It is for handling cyclic references when implementing recursive calls in functional programming languages. They suggest that you can observe the cycle as a single entity with a root object. To do that, you should be able to:
Create the cyclic structure all at once
Access the cyclic structure only through a single node - a root, and mark which reference closes the cycle
If you need to reuse just a part of the cyclic structure, you shouldn't add external references but instead make copies of what you need
That way you should be able to count the structure as a single entity and whenever the RC to the root falls to zero - collect it.
This should be the paper for it if anyone is interested in more details - https://www.academia.edu/49293107/Reference_counting_can_manage_the_circular_invironments_of_mutual_recursion
Bobrow's technique
Here you could have more than one reference to the cyclic structure. We just distinguish between internal and external references for it.
Overall, all allocated objects are assigned by the programmer to a group. And all groups are reference counted. We distinguish between internal and external references for the group and of course - objects can be moved between groups. In this case, intra-group cycles are easily reclaimed whenever there are no more external references to the group. However, inter-group cyclic structures should still be an issue.
If I'm not mistaken, this should be the original paper - https://dl.acm.org/doi/pdf/10.1145/357103.357104
I'm in the search for other general purpose alternatives besides those algorithms and using weak pointers.
Y Combinator
I wrote a programming language once in which every object was immutable. As such, an object could only contain pointers to objects that were created earlier. Since all reference pointed backwards in time, there could not possibly be any cyclic references, and reference counting was a perfectly viable way of managing memory.
The question then is, "How do you create self-referencing data structures without circular references?" Well, functional programming has been doing this trick forever using the fixed-point/Y combinator to write recursive functions when you can only refer to previously-written functions. See: https://en.wikipedia.org/wiki/Fixed-point_combinator
That Wikipedia page is complicated, but the concept is really not that complicated. How it works in terms of data structures is like this:
Let's say you want to make a Map<String,Object>, where the values in the map can refer to other values in the map. Well, instead of actually storing those objects, you store functions that generate those objects on-demand given a pointer to the map.
So you might say object = map.get(key), and what this will do is call a private method like _getDefinition(key) that returns this function and calls it:
Object get(String key) {
var definition = _getDefinition(key);
return definition == null ? null : definition(this);
}
So the map has a reference to the function that defines the value. This function doesn't need to have a reference to the map, because it will be passed the map as an argument.
When the definition called, it returns a new object that does have a pointer to the map in it somewhere, but the map it self does not have a pointer to this object, so there are no circular references.
There are couple of ways I know of for walking around this:
The first (and preferred one) is simply extracting the common code into third assembly, and make both references use that one
The second one is adding the reference as "File reference" (dll) instead of "Project reference"
Hope this helps