FastRemoveObject in CCArray will change the positions of objects? - cocos2d-iphone

I was told that if relying on a specific ordering of objects, I should not use the fastRemoveObject methods in CCArray. Cocos2d API references don't show the contents of the method specifically. Can anyone tell me the reason?

Yes, fastRemoveObject changes the order of nodes. It is therefore not recommended unless it really doesn't matter in your case.
What it does is the following:
assign object at last index to index of object being removed
nil last object
decrease array count
That way the array will not have to perform memory operations (hence: fast). But the last object will now be at the index of the removed object.

Related

Advice on how to implement std::vector using a circular array?

I am to write a C++ program that :
"Implements the vector ADT by means of an extendable array used in a circular fashion, so that insertions and deletions at the beginning and end run in constant time. (So not O(n)). Print the circular array before and after each insertion and deletion, You cannot use the STL."
This task seems very confusing to me. A std::vector is implemented using a dynamic array that is based off the concept of a stack, correct? Performing a deletion or insertion at the front seems to me that this should be implemented as a Queue or maybe a Dequeue, not a Vector. Also, a circular array would mean that when data is pushed onto an array that is Full, old data becomes overwritten, right? So when should I know to expand the vector's capacity?
If I'm not making sense here, Basically I need help in understanding how I should go about implementing a dynamic circular array..
Yes, this is a homework assignment. No, I do not expect anyone to provide code for me, I only wish for someone to give me a push in the right direction as to how I should think about implementing this. Thank you.
I think you are actually being asked to implement deque. The point of the "circularity" is that in normal vector you cannot add an element at the beginning since there is no free space and you would have to move all other elements to the right. So what you can do is you simulate a circle by putting the element to the end the base array and remember that's where the first element is.
Example: 2, 3, -, -, 1 where 1 is first and 3 is last
So, basically you insert elements circullary, and remember where the first and the last elements are so you can add to beginning/end in O(1). Also when the array is full, you have to move all the elements to a larger one. If you double the size, you still get amortized time of O(1)
1) m_nextIn and m_nextOut - data attributes of class queue;
I find it useful to have two integers, with label m_nextIn and m_nextOut ... these identify where in the circular array you 'insert' the next (i.e. youngest) obj instance into the queue, and where you 'delete' the oldest obj instance from the queue.
These two items also provide constant time insert and delete.
Don't get confused as to where the beginning or end of the queue is. The array starts at index 0, but this is not the beginning of your queue.
The beginning of your queue is at nextIn (which probably is not 0, but may be). Technique also known as round-robin (a research term).
2) empty and full - method attributes
Determining queue full / empty can be easily computed from m_nextIn and m_nextOut.
3) extendable
Since you are prohibited from using vector (which itself is extendable) you must implement this functionality yourself.
Note about your comment: The "dynamic memory" concept is not related to stack. (another research term)
Extendable issues occur when your user code invokes the 'insert' AND the array is already full. (capture this test effort) You will need to detect this issue, then do 4 things:
3.1) allocate a new array (use new, and simply pick an appropriate size.)
Hint - std::vector() doubles it's capacity each time a push_back() would overflow the current capacity
3.2) transfer the entire contents of the array to the new array, fixing all the index's as you go. Since the new array is bigger, just insert trivially.
3.3) delete the old array - i.e. you copied from the old array to the new array, so do you 'delete' them? or simply delete the array?
3.4) finish the 'insert' - you were in the middle of inserting another instance, right?
Good luck.

Reference counting in a collection

Let's have a collection of objects (say string is type of collection). I want each element of collection to have a reference count. So, on Add-Usage it should increment count for this given element.
coll.AddUsage("SomeElement"); // Type doesn't matter - but should increase count
On, Release-Usage, it should decrement the reference count for given element, and if count reaches 0, then it should remove the element from collection.
It is not important if AddUsage will allocate element (and set reference-count to 1), or would fail altogether (since element didn't exist). Important thing is RemoveUsage, which should remove given element (object) from collection.
I thought of using vector of a pair (or a custom struct), or using any kind of map/multimap. There exists no existing class in C++ library (may be out of thread-support library, one atomic classes, shared-pointer classes etc).
Question:
So, my question is how to implement such idea, using existing C++ library? It should be thread safe. Yes, C++11/14 is perfectly okay for me. If good idea is there, I would probably craft it on top of templates.
Assuming you ask for a data structure to implement your reference-counting collection...
Use a map<K,V> with K as the type of collection elements (in your example string) and V a type to keep track of meta-information about the element (e.g. reference count). The simplest case is when V is int.
Then, AddUsage is simple, just do refMap[value]++. For RemoveUsage just do a refMap[value]--, then check if the counter hit zero and remove the value from the map.
You need to add error handling too, since AddUsage / RemoveUsage may be
called with an object which is not in the map (not added to the collection)
EDIT: You tagged your question with "multithreading", so you probably want to have a mutex of some sort which guards the concurrent access to refMap.
You could implement something similar to shared_ptr class but extending it to hold collection of objects.
Like you could design a class with map/multimap as its data member. Key would be your object and value be your reference count.As far as interface is concerned just expose two methods:-
AddUsage(Object);
RemoveUsage(Object);
In your AddUsage method you would first check if element already exists in map.If yes then only increment the count. Likewise you would handle RemoveUsage.Object would be deleted from map if its reference count reaches zero.
This is just my opinion. Please let me know if there are any bottlenecks in this implementation.
You can use static member(integer) variable in the structure or class. Increment or decrement whereever you want. Remove the element if the value is zero.

C++ Deleting objects from memory

Lets say I have allocated some memory and have filled it with a set of objects of the same type, we'll call these components.
Say one of these components needs to be removed, what is a good way of doing this such that the "hole" created by the component can be tested for and skipped by a loop iterating over the set of objects?
The inverse should also be true, I would like to be able to test for a hole in order to store new components in the space.
I'm thinking menclear & checking for 0...
boost::optional<component> seems to fit your needs exactly. Put those in your storage, whatever that happens to be. For example, with std::vector
// initialize the vector with 100 non-components
std::vector<boost::optional<component>> components(100);
// adding a component at position 15
components[15].reset(component(x,y,z));
// deleting a component at position 82
componetnts[82].reset()
// looping through and checking for existence
for (auto& opt : components)
{
if (opt) // component exists
{
operate_on_component(*opt);
}
else // component does not exist
{
// whatever
}
}
// move components to the front, non-components to the back
std::parition(components.begin(), components.end(),
[](boost::optional<component> const& opt) -> bool { return opt; });
The short answer is it depends on how you store it in memmory.
For example, the ansi standard suggests that vectors be allocated contiguously.
If you can predict the size of the object, you may be able to use a function such as size_of and addressing to be able to predict the location in memory.
Good luck.
There are at least two solutions:
1) mark hole with some flag and then skip it when processing. Benefit: 'deletion' is very fast (only set a flag). If object is not that small even adding a "bool alive" flag can be not so hard to do.
2) move a hole at the end of the pool and replace it with some 'alive' object.
this problem is related to storing and processing particle systems, you could find some suggestions there.
If it is not possible to move the "live" components up, or reorder them such that there is no hole in the middle of the sequence, then the best option if to give the component objects a "deleted" flag/state that can be tested through a member function.
Such a "deleted" state does not cause the object to be removed from memory (that is just not possible in the middle of a larger block), but it does make it possible to mark the spot as not being in use for a component.
When you say you have "allocated some memory" you are likely talking about an array. Arrays are great because they have virtually no overhead and extremely fast access by index. But the bad thing about arrays is that they aren't very friendly for resizing. When you remove an element in the middle, all following elements have to be shifted back by one position.
But fortunately there are other data structures you can use, like a linked list or a binary tree, which allow quick removal of elements. C++ even implements these in the container classes std::list and std::set.
A list is great when you don't know beforehand how many elements you need, because it can shrink and grow dynamically without wasting any memory when you remove or add any elements. Also, adding and removing elements is very fast, no matter if you insert them at the beginning, in the end, or even somewhere in the middle.
A set is great for quick lookup. When you have an object and you want to know if it's already in the set, checking it is very quick. A set also automatically discards duplicates which is really useful in many situations (when you need duplicates, there is the std::multiset). Just like a list it adapts dynamically, but adding new objects isn't as fast as in a list (not as expensive as in an array, though).
Two suggestions:
1) You can use a Linked List to store your components, and then not worry about holes.
Or if you need these holes:
2) You can wrap your component into an object with a pointer to the component like so:
class ComponentWrap : public
{
Component component;
}
and use ComponentWrap.component == null to find if the component is deleted.
Exception way:
3) Put your code in a try catch block in case you hit a null pointer error.

Could the S of S.O.L.I.D be extended for every single element of the code?

The S of the famous Object Oriented Programming design stands for:
Single responsibility principle, the notion that an object should have
only a single responsibility.
I was wondering, can this principle, be extended even to arrays, variables, and all the elements of a program?
For example, let's say we have:
int A[100];
And we use it to store the result of a function, but somehow we use the same A[100] to check, for example, what indexes of A have we already checked and elaborated.
Could this be considered wrong? Shouldn't we create another element to store, for example, the indexes that we have already checked? Isn't this an hint of future messy code?
PS: I'm sorry if my question is not comprehensible but English is not my primary language. If you have any problem understanding the point of it please let me know in a comment below.
If same A instance is used in different program code portions you must follow this principle. If A is a auxiliary variable, local one for example, I think you don't need to be care about it.
If you are tracking the use of bits of the array that have been updated, then you probably shouldn't be using an array, but a map instead.
In any case, if you need that sort of extra control over the array, then basically, you should be considering a class that contains both the contents of the array and the various information about what has and hasn't been done. So your array becomes local to the class object, as do your controls, and voila. You have single responsibility again.

Getting Unique Numbers and Knowing When They're Freed

I have a physics simulation (using Box2D) where bodies with identical integer IDs do not collide, for instance, bodies that belong to the same character. I have a problem though in that I need to be able to get a unique number for each possible entity, so that no two characters accidentally get the same ID. There's a finite number of bodies, but they are created and destroyed as the simulation dictates, so it's necessary to free unique IDs once the body they belonged to is gone.
A class World is responsible for creating and destroying all bodies, and is also the entity that manages the unique number generation, and anything else where physics simulation is concerned.
I thought of two methods so far but I'm not sure which would be better, if either of them at all:
Keep a vector<short>, with the data being the number of references floating around, and the position in the vector being the ID itself. This method has the disadvantage of creating unneeded complexity when coding entities that manipulate group IDs, since they would need to ensure they tell the World how many references they're taking out.
Keep a vector<bool>, with the data being if that ID is free or not, and the position in the vector being the ID itself. The vector would grow with every new call for a unique ID, if there exist no free slots. The disadvantage is that once the vector reaches a certain size, an audit of the entire simulation would need to be done, but has the advantage of entities being able to grab unique numbers without having to help manage reference counting.
What do you folks think, is there a better way?
You could maintain a "free" list of unused IDs as a singly linked list inside your master World object.
When an object is destroyed by World (making its ID unused) you could push that ID onto the head of the free list.
When you are creating a new object you could do the following:
If the free list is non-empty: pop the head item and take that ID.
Else increment a global ID counter and assign it's current value.
While you could still run out of IDs (if you simultaneously had more objects than the max value of your counter), this strategy will allow you to recycle IDs, and to do everything with O(1) runtime complexity.
EDIT: As per #Matthieu's comments below, a std::deque container could also be used to maintain the "free" list. This container also supports the push_front, pop_front operations with O(1) complexity .
Hope this helps.
How many bodies are there? Is it realistic that you'd ever run out of integers if you didn't reassign them? The simplest solution is to just have one integer storing the next ID -- you would increment this integer when you assign a new ID to a body.