There are two functions in GLib that work with the reference counting of the GObject objects:
g_object_ref increases reference count of the object, doesn't handle floating references.
g_object_ref_sink increases reference count of the object or just removes floating flag from the object if the reference is floating.
Since we don't know if the object is floating or not, we should always use g_object_ref_sink, shouldn't we? If I am wrong, when should we use g_object_ref and when should we use g_object_ref_sink? Should we use g_object_ref_sink only for GInitiallyUnowned objects?
You should generally know the type of an object you’re handling (i.e. pointers typically have more specific types than GObject*), so you’ll know if it’s potentially floating or not. GObject-based APIs which use floating references are documented as using them. Anything which isn’t documented as using floating references, doesn’t.
Related
I am working on getting rid of shortstring.
One of the many places shortstring is currently used within our programs is in records.
Alot of these records are kept in AVL trees.
The AVL tree used is a generic one, holding a pointer to a number of bytes (ElemSize), which have worked well so far.
The memory for each record in the AVL tree is allocated with GetMem, and copied with Move.
However, with string being a pointer to a reference-counted structure, copying back the memory to a record no longer works, as the sting referenced is often freed (automatically by reference count).
With only a pointer and a size of the "data block", I assume it is not possible to have the reference count of the strings increased.
I'm looking for a way to get the reference count of the stings to be taken into account when storing the record in a AVL tree.
Can I pass the record type to the tree constructor, then cast the pointer to this type and thus get the references increased? Or a similar fix, where I can isolate the changes to primarily be in the AVL unit and calls to it's constructor.
Current code for allocation of space to store the record in AVL; XData is a pointer to the record to be stored:
New(RootPtr); { create new memory space }
GetMem(RootPtr^.TreeData, ElemSize);
WITH RootPtr^ DO BEGIN
{ copy data }
Move(XData^, RootPtr^.TreeData^, ElemSize);
In essence the question you are asking is:
How can I allocate, copy and deallocate a record when all I know about its type is its size?
The simple answer is that you can use GetMem, Move and FreeMem provided that the record does not contain managed types. You wish to work with records that contain Delphi strings, which are managed. And so your current approach using GetMem and Move does not suffice.
There are plenty of ways to solve this. You could write your own code to do reference counting, so long as you knew where in the record the managed types were. I don't recommend this. You could make your user data be a class and use polymorphism to help.
The option I'd like to discuss continues to support records and indeed allows the user to choose whatever type they like. The reasoning is as follows:
If the type contains managed types, then operating on it requires knowledge of the type. If the tree is to be generic, then it cannot have that knowledge. Ergo, the knowledge must be supplied by the user of the tree.
This leads you to events. Let the tree offer events that the user can supply handlers for. The types would look like this:
type
PTreeNodeUserData = type Pointer;
TTreeNodeCreateUserDataEvent = function: PTreeNodeUserData of object;
TTreeNodeDestroyUserDataEvent = procedure(Data: PTreeNodeUserData) of object;
TTreeNodeCopyUserDataEvent = procedure(Source, Dest: PTreeNodeUserData) of object;
Then you can arrange for your tree to publish events with these types that the user can subscribe to.
The point being that this allows the user of the tree to supply the missing knowledge about the user data type.
One of the main benefits of using records is the simplicity with which they can be copied (without using Move). So your best solution is to simply replace Move with a normal assignment operator :=. This will correctly consider the reference counts for all managed types involved.
Is there a particular reason you're not using the normal assignment operator?
PS: You need to ensure that the memory for all managed types (including long strings) is correctly initialised and finalised. I suggest you do some additional reading on the Initialize and Finalize routines.
The tree is general, it can hold a given lump of data. I hoped I could extend the functionality without making a new tree class per record.
In that case you need your "copy behaviour" to be variable depending on what it's working with. As couple of options:
If your tree is wrapped in a class you can easily modify it to use a callback event to perform the copy operation. (This option might be easiest even if you first have to work on encapsulating the tree in a class.)
Modify your nodes and/or data to be objects with polymorphic copy functionality. Then each subtype will know how to copy itself correctly, and you can write something along the lines of Root.TreeData := XData.CreateCopy;
If you are working at such a low level, and don't want compiler to help you, then you need to use PChar-strings instead of regular strings.
A very basic question .. but really very important to understand the concepts..
in c++ or c languages, we usually don't use pointer variables to store values.. i.e. values are stored simply as is in:
int a=10;
but here in ios sdk, in objective c, most of the objects which we use are initialized by denoting a pointer with them as in:
NSArray *myArray=[NSArray array];
So,the question arises in my mind ,that, what are the benefit and need of using pointer-objects (thats what we call them here, if it is not correct, please, do tell)..
Also I just get confused sometimes with memory allocation fundamentals when using a pointer objects for allocation. Can I look for good explanations anywhere?
in c++ or c languages, we usually don't use pointer variables to store values
I would take that "or C" part out. C++ programmers do frown upon the use of raw pointers, but C programmers don't. C programmers love pointers and regard them as an inevitable silver bullet solution to all problems. (No, not really, but pointers are still very frequently used in C.)
but here in ios sdk, in objective c, most of the objects which we use are initialized by denoting a pointer with them
Oh, look closer:
most of the objects
Even closer:
objects
So you are talking about Objective-C objects, amirite? (Disregard the subtlety that the C standard essentially describes all values and variables as an "object".)
It's really just Objective-C objects that are always pointers in Objective-C. Since Objective-C is a strict superset of C, all of the C idioms and programming techniques still apply when writing iOS apps (or OS X apps, or any other Objective-C based program for that matter). It's pointless, superfluous, wasteful, and as such, it is even considered an error to write something like
int *i = malloc(sizeof(int));
for (*i = 0; *i < 10; ++*i)
just because we are in Objective-C land. Primitives (or more correctly "plain old datatypes" with C++ terminology) still follow the "don't use a pointer if not needed" rule.
what are the benefit and need of using pointer-objects
So, why they are necessary:
Objective-C is an object-oriented and dynamic language. These two, strongly related properties of the language make it possible for programmers to take advantage of technologies such as polymorphism, duck-typing and dynamic binding (yes, these are hyperlinks, click them).
The way these features are implemented make it necessary that all objects be represented by a pointer to them. Let's see an example.
A common task when writing a mobile application is retrieving some data from a server. Modern web-based APIs use the JSON data exchange format for serializing data. This is a simple textual format which can be parsed (for example, using the NSJSONSerialization class) into various types of data structures and their corresponding collection classes, such as an NSArray or an NSDictionary. This means that the JSON parser class/method/function has to return something generic, something that can represent both an array and a dictionary.
So now what? We can't return a non-pointer NSArray or NSDictionary struct (Objective-C objects are really just plain old C structs under the hoods on all platforms I know Objective-C works on), because they are of different size, they have different memory layouts, etc. The compiler couldn't make sense of the code. That's why we return a pointer to a generic Objective-C object, of type id.
The C standard mandates that pointers to structs (and as such, to objects) have the same representation and alignment requirements (C99 6.2.5.27), i. e. that a pointer to any struct can be cast to a pointer to any other struct safely. Thus, this approach is correct, and we can now return any object. Using runtime introspection, it is also possible to determine the exact type (class) of the object dynamically and then use it accordingly.
And why they are convenient or better (in some aspects) than non-pointers:
Using pointers, there is no need to pass around multiple copies of the same object. Creating a lot of copies (for example, each time an object is assigned to or passed to a function) can be slow and lead to performance problems - a moderately complex object, for example, a view or a view controller, can have dozens of instance variables, thus a single instance may measure literally hundreds of bytes. If a function call that takes an object type is called thousands or millions of times in a tight loop, then re-assigning and copying it is quite painful (for the CPU anyway), and it's much easier and more straightforward to just pass in a pointer to the object (which is smaller in size and hence faster to copy over). Furthermore, Objective-C, being a reference counted language, even kind of "discourages" excessive copying anyway. Retaining and releasing is preferred over explicit copying and deallocation.
Also I just get confused sometimes with memory allocation fundamentals when using a pointer objects for allocation
Then you are most probably confused enough even without pointers. Don't blame it on the pointers, it's rather a programmer error ;-)
So here's...
...the official documentation and memory management guide by Apple;
...the earliest related Stack Overflow question I could find;
...something you should read before trying to continue Objective-C programming #1; (i. e. learn C first)
...something you should read before trying to continue Objective-C programming #2;
...something you should read before trying to continue Objective-C programming #3;
...and an old Stack Overflow question regarding C memory management rules, techniques and idioms;
Have fun! :-)
Anything more complex than an int or a char or similar is usually passed as
pointers even in C. In C you could of course pass around a struct of data
from function to function but this is rarely seen.
Consider the following code:
struct some_struct {
int an_int;
char a_char[1234];
};
void function1(void)
{
struct some_struct s;
function2(s);
}
void function2(struct some_struct s)
{
//do something with some_struct s
}
The some_struct data s will be put on the stack for function1. When function2
is called the data will be copied and put on the stack for use in function2.
It requires the data to be on the stack twice as well as the data to be
copied. This is not very efficient. Also, note that changing the values
of the struct in function2 will not affect the struct in function1, they
are different data in memory.
Instead consider the following code:
struct some_struct {
int an_int;
char a_char[1234];
};
void function1(void)
{
struct some_struct *s = malloc(sizeof(struct some_struct));
function2(s);
free(s);
}
void function2(struct some_struct *s)
{
//do something with some_struct s
}
The some_struct data will be put on the heap instead of the stack. Only
a pointer to this data will be put on the stack for function1, copied in the
call to function2 another pointer put on the stack for function2. This is a
lot more efficient than the previous example. Also, note that any changes of
the data in the struct made by function2 will now affect the struct in
function1, they are the same data in memory.
This is basically the fundamentals on which higher level programming languages
such as Objective-C is built and the benefits from building these languages
like this.
The benefit and need of pointer is that it behaves like a mirror. It reflects what it points to. One main place where points could be very useful is to share data between functions or methods. The local variables are not guaranteed to keep their value each time a function returns, and that they’re visible only inside their own function. But you still may want to share data between functions or methods. You can use return, but that works only for a single value. You can also use global variables, but not to store your complete data, you soon have a mess. So we need some variable that can share data between functions or methods. There comes pointers to our remedy. You can just create the data and just pass around the memory address (the unique ID) pointing to that data. Using this pointer the data could be accessed and altered in any function or method. In terms of writing modular code, that’s the most important purpose of a pointer— to share data in many different places in a program.
The main difference between C and Objective-C in this regard is that arrays in Objective-C are commonly implemented as objects. (Arrays are always implemented as objects in Java, BTW, and C++ has several common classes resembling NSArray.)
Anyone who has considered the issue carefully understands that "bare" C-like arrays are problematic -- awkward to deal with and very frequently the source of errors and confusion. ("An array 'decays' to a pointer" -- what is that supposed to mean, anyway, other than to admit in a backhanded way "Yes, it's confusing"??)
Allocation in Objective-C is a bit confusing in large part because it's in transition. The old manual reference count scheme could be easily understood (if not so easily dealt with in implementations), but ARC, while simpler to deal with, is far harder to truly understand, and understanding both simultaneously is even harder. But both are easier to deal with than C, where "zombie pointers" are almost a given, due to the lack of reference counting. (C may seem simpler, but only because you don't do things as complex as those you'd do with Objective-C, due to the difficulty controlling it all.)
You use a pointer always when referring to something on the heap and sometimes, but usually not when referring to something on the stack.
Since Objective-C objects are always allocated on the heap (with the exception of Blocks, but that is orthogonal to this discussion), you always use pointers to Objective-C objects. Both the id and Class types are really pointers.
Where you don't use pointers are for certain primitive types and simple structures. NSPoint, NSRange, int, NSUInteger, etc... are all typically accessed via the stack and typically you do not use pointers.
I am able to understand immutability with python (surprisingly simple too). Let's say I assign a number to
x = 42
print(id(x))
print(id(42))
On both counts, the value I get is
505494448
My question is, does python interpreter allot ids to all the numbers, alphabets, True/False in the memory before the environment loads? If it doesn't, how are the ids kept track of? Or am I looking at this in the wrong way? Can someone explain it please?
What you're seeing is an implementation detail (an internal optimization) calling interning. This is a technique (used by implementations of a number of languages including Java and Lua) which aliases names or variables to be references to single object instances where that's possible or feasible.
You should not depend on this behavior. It's not part of the language's formal specification and there are no guarantees that separate literal references to a string or integer will be interned nor that a given set of operations (string or numeric) yielding a given object will be interned against otherwise identical objects.
I've heard that the C Python implementation does include a set of the first hundred or so integers as statically instantiated immutable objects. I suspect that other very high level language run-time libraries are likely to include similar optimizations: the first hundred integers are used very frequently by most non-trivial fragments of code.
In terms of how such things are implemented ... for strings and larger integers it would make sense for Python to maintain these as dictionaries. Thus any expression yielding an integer (and perhaps even floats) and strings (at least sufficiently short strings) would be hashed, looked up in the appropriate (internal) object dictionary, added if necessary and then returned as references to the resulting object.
You can do your own similar interning of any sorts of custom object you like by wrapping the instantiation in your own calls to your own class static dictionary.
I have an application that searches files on the computer (configurable path, type etc). Currently it adds information to a database as soon as a matching file is found. Rather than that I want to hold the information in memory for further manipulation before inserting to database. The list may contain a lot of items. I consider performance as important factor. I may need iterating thru the items, so a structure that can be coded easily is another key issue. and how can I achieve php style associative arrays for this job?
If you're using Delphi 2009, you can use a TDictionary. It takes two generic parameters. The first should be a string, for the filename, and the second would be whatever data type you're associating with. It also has three built-in enumerators, one for key-value pairs, one for keys only and one for values only, which makes iterating easy.
Another solution would be to use just a standard TStringList.
As long as it's sorted and has some duplicate setting other than dupAccept, you can use indexof or indexofname to find items in the list quickly.
It also has the Objects addition which allows you to store object information attached to the name. Starting with D2009, TStringList has the OwnsObject property which allows you to delegate object cleanup to the TStringList. Prior to D2009 you have to handle that yourself.
Much of this will depend on how you are going to use the list and to what scale. If you are going to use it as a stack, or queue, then a TList would work fine. If your needing to search through the list for a specific item then you will need something that allows faster retrieval. TDictionary (2009) or TStringList (pre 2009) would be the most likely choice.
Dynamic arrays are also a possiblity, but if you use them you will want to minimize the use of SetLength as each time it is called it will re-allocate memory. TList manages this for you, which is why I suggested using a TList. if you KNOW how many you will deal with in advance, then use a dynamic array, and set its length on the onset.
If you have more items than will fit in memory then your choices also change. At that point I would either use a database table, or a tFileStream to store the records to be processed, then seek to the beginning of the table/stream for processing.
Try using the AVL-Tree by http://sourceforge.net/projects/alcinoe/ as your associative Array. It has an iterate-method for fast iteration. You may need to derive from his baseclass and implement your own comparator, but it's easy to use.
Examples are included.
I am refactoring some existing Delphi code into a class.
The current code uses a global variable defined as a dynamic array array of byte. At initialization time the code figures out the size of the array and uses SetLength to allocate it. It is convenient both as the buffer to obtain the data and as the runtime container for a later processing.
I want to move this variable as one of the object attributes.
But I am not sure if it is ok to maintain its type. Is it considered good practice?
The alternative I am considering is to tranform it to a dynamic container like a TList. I will keep the very same code for obtaining the data, with a local dynamic array but moving it to the container for the rest of its lifespan. Is it worth the effort? I know that elegance always pay off at the end, but I don't really see the value of the effort at this moment. Any thoughts?
Dynamic arrays are great, but really only for fixed dimentions. If they have to grow, especially in single record increments, this can cause eventual errors from the memory manager (and possible performance issues) since the array has to be reallocated and copied to the new bigger destination. TList does at least have a 'growing' mechanism that is called less frequently.
I know that elegance always pay off at the end,
Is that so? Note that changing working code always includes the risk to break something. It must IMHO be decided in every situation if the gained elegance is worth the risk.
In your case, if you add and remove items during runtime, I would use a TList since it is much easier for these operations. If you just initialize the length once and the arrays is constant after initialization you can just keep the dynamic array. There's definitely no "good practice" saying that you shouldn't use dynamic arrays.