I realize 99% of you think "what the h***…" But please help me to get my head around the this concept of using pointers. I'm sure my specific question would help lots of newbies.
I understand what pointers ARE and that they are a reference to an adress in memory and that by using the (*) operator you can get the value in that address.
Let's say:
int counter = 10;
int *somePointer = &counter;
Now I have the address in memory of counter, and I can indirectly point to its value by doing this:
int x = *somePointer;
Which makes x = 10, right?
But this is the most basic example, and for this case I could use int x = counter; and get that value, so please explain why pointers really are such an important thing in Objective-C and some other languages... in what case would only a pointer make sense?
Appreciate it.
Objective-C has pointers because it is an evolution of C, which used pointers extensively. The advantage of a pointer in an object-oriented language like Objective-C is that after you create an object, you can pass around a pointer to the object instead of passing around the object itself. In other words, if you have some object that takes up a large amount of storage space, passing around a pointer is a lot more memory-efficient than passing around a copy of the object itself. This may not be noticeable in simple cases when you’re only dealing with primitive types like ints, but when you start dealing with more complex objects the memory and time savings are enormous.
More importantly, pointers make it much easier for different parts of your code to talk to each other. If variables could only be passed to functions “by value” instead of “by reference” (which is what happens when you use pointers), then functions could never alter their inputs. They could only change the state of your program by either returning a value or by changing a global variable—the overuse of which generally leads to sloppy, unorganized code.
Here’s a concrete example. Suppose you have an Objective-C method that will parse a JSON string and return an NSDictionary:
+ (NSDictionary *)parseJsonString:(NSString *)json
error:(NSError **)error;
The method will do the parsing and return an NSDictionary if everything goes okay. But what if there’s some problem with the input string? We want a way to indicate to the user (or at least to the programmer) what happened, so we have a pointer to a pointer to an NSError, which will contain that information. If our method fails (probably returning nil), we can dereference the error parameter to see what went wrong. What we’ve effectively done is to give our method two different kinds of return values: usually, it will return an NSDictionary, but it could also return an NSError.
If you want to read more about this, you may have better luck searching for “pointers in C” rather than “pointers in Objective-C”; pointers are of course used extensively in Objective-C, but all of the underlying machinery is identical to that of C itself.
What is the biggest advantage of using pointers in ObjectiveC
I'd say the biggest advantage is that you can use Objective-C at all - all Objective-C objects are pointers are accessed using pointers (the compiler and the runtime won't let you create objects statically), so you wouldn't get any further without them...
Item:
What if I told you to write me a program that would maintain a set of counters, but the number of counters would be entered by the user when he started the program. We code this with an array of integers allocated on the heap.
int *counters = malloc(numOfCounters * sizeof(int));
Malloc works with memory directly, so it by nature returns a pointer. All Objective-C objects are heap-allocated with malloc, so these are always pointers.
Item:
What if I told you to write me a function that read a file, and then ran another function when it was done. However, this other function was unknown and would be added by other people, people I didn't even know.
For this we have the "callback". You'd write a function that looked like this:
int ReadAndCallBack(FILE *fileToRead, int numBytes, int whence, void(*callback)(char *));
That last argument is a pointer to a function. When someone calls the function you've written, they do something like this:
void MyDataFunction(char *dataToProcess);
ReadAndCallBack(myFile, 1024, 0, MyDataFunction);
Item:
Passing a pointer as a function argument is the most common way of returning multiple values from a function. In the Carbon libraries on OSX, almost all of the library functions return an error status, which poses a problem if a library function has to return something useful to the programmer. So you pass the address where you'd like the function to hand information back to you...
int size = 0;
int error = GetFileSize(afilePath,&size);
If the function call returns an error, it is in error, if there was no error, error will probably be zero and size will contain what we need.
The biggest advantage of pointers in Objective-C, or in any language with dynamic allocation, is that your program can handle more items than the names that you invent in your source code.
Related
I'm trying to do the following, but NSValue's creation method returns nil.
Are C bitfields in structs not supported?
struct MyThingType {
BOOL isActive:1;
uint count:7;
} myThing = {
.isActive = YES,
.count = 3,
};
NSValue *value = [NSValue valueWithBytes:&myThing objCType:#encode(struct MyThingType)];
// value is nil here
First and foremost, claptrap makes a very good point in his comment: why bother using bitfield specifiers (which are mainly used to either do micro-optimization or manually add padding bits where you need them), to then wrap it all up in an instance of NSValue).
It's like buying a castle, but then living in the kitchen to not ware out the carpets...
I don't think it is, a quick canter through the apple dev-docs came up with this... there are indeed several issues to take into account when it comes to bit fields.
I've also just found this, which explains why bit-fields + NSValue don't really play well together.
Especially in cases where the sizeof a struct can lead to NSValue reading the data in an... shall we say erratic manner:
The struct you've created is padded to 8 bits. Now these bits could be read as 2 int, or 1 long or something... From what I've read on the linked page, it's not unlikely that this is what is happening.
So, basically, NSValue is incapable of determining the actual types, when you're using bit fields. In case of ambiguity, an int (width 4 in most cases) is assumed and under/overflow occurs, and you have a mess on your hands.
Since the compiler still has some liberty as to where what member is actually stored, it doesn't quite suffice to pass the stringified typedef sort of thing (objCType: #encode(struct YourStruct), because there is a good chance that you won't be able to make sense of the actual struct itself, owing to compiler optimizations and such...
I'd suggest you simply drop the bit field specifiers, because structs should be supported... at least, last time I tried, a struct with simple primitive types worked just fine.
You can solve this with a union. Simply put the structure into union that has another member with a type supported by NSValue and has a size larger than your structure. In your case this is obvious for long.
union _bitfield_word_union
{
yourstructuretype bitfield;
long plain;
};
You can make it more robust against resizing the structure by using an array whose size is calculated at compile time. (Please remember that sizeof() is a compile time operator, too.)
char plain[(sizeof(yourstructuretype)/sizeof(char)];
Then you can store the structure with the bitfield into the union and read the plain member out.
union converter = { .bitfield = yourstructuretypevalue };
long plain = converter.plain;
Use this value for NSValue instance creation. Reading out you have to do the inverse way.
I'm pretty sure that through a technical correctum of C99 this became standard conforming (called type punning), because you can expect that reading out a member's value (bitfield) through another members value (plain) and storing it back is defined, if the member being read is at least as big as the member being written. (There might be undefined bits 9-31/63 in plain, but you do not have to care about it.) However it is real-world conforming.
Dirty hack? Maybe. One might call it C99. However using bitfields in combination with NSValue sounds like using dirty hacks.
Quick question: The difference between (ReturnClass *)methodName... and (ReturnClass)methodName... in Objective-C?
I know (ReturnType *)methodName... returns an instance of ReturnClass as opposed to (ReturnClass)methodName just returning what?
Thank you.
if your ReturnClass is an object (which is most likely) then you can't use it without *. You'll get a compilation error. Otherwise *is a pointer to instance (primitive type or struct), and absence of * is instance itself
EDIT: Brilliant explanation on pointers from Joe:
Think of the instance as your house and the pointer as your address. Your house is a large object, so instead of making copies of your house and passing it to your friends when you have a party you give them your address so they can get to your house. Now if you had a small object, like a flyer, it would be easy to just make copies and pass out rather than give everyone instructions to get to that piece of paper. Think of the large object (house) as an Objective-C class or struct full of info, and the small object (flyer) as a primitive like int, BOOL, double etc
The Objective C Runtime Guide from Apple, states that you should never use objc_msgSend() in your own code, and recommends using methodForSelector: instead. However, it doesn't provide any reason for this.
What are the dangers of calling objc_msgSend() in your code?
Reason #1: Bad style - it's redundant and unreadable.
The compiler automatically generates calls to objc_msgSend() (or some variant thereof) when it encounters Objective-C messaging expressions. If you know the class and the selector to be sent at compile-time, there's no reason to write
id obj = objc_msgSend(objc_msgSend([NSObject class], #selector(alloc)), #selector(init));
instead of
id obj = [[NSObject alloc] init];
Even if you don't know the class or the selector (or even both), it's still safer (at least the compiler has a chance to warn you if you are doing something potentially nasty/wrong) to obtain a correctly typed function pointer to the implementation itself and use that function pointer instead:
const char *(*fptr)(NSString *, SEL) = [NSString instanceMethodForSelector:#selector(UTF8String)];
const char *cstr = fptr(#"Foo");
This is especially true when the types of the arguments of a method are sensitive to default promotions - if they are, then you don't want to pass them through the variadic arguments objc_msgSend() takes, because your program will quickly invoke undefined behavior.
Reason #2: dangerous and error-prone.
Notice the "or some variant thereof" part in #1. Not all message sends use the objc_msgSend() function itself. Due to complications and requirements in the ABI (in the calling convention of functions, in particular), there are separate functions for returning, for example, floating-point values or structures. For example, in the case of a method that performs some sort of searching (substrings, etc.), and it returns an NSRange structure, depending on the platform, it may be necessary to use the structure-returning version of the messenger function:
NSRange retval;
objc_msgSend_stret(&retval, #"FooBar", #selector(rangeOfString:), #"Bar");
And if you get this wrong (e. g. you use the inappropriate messenger function, you mix up the pointers to the return value and to self, etc.), your program will likely behave incorrectly and/or crash. (And you will most probably get it wrong, because it's not even that simple - not all methods returning a struct use this variant, since small structures will fit into one or two processor registers, eliminating the need for using the stack as the place of the return value. That's why - unless you are a hardcore ABI hacker - you rather want to let the compiler do its job, or there be dragons.)
You ask "what are the dangers?" and #H2CO3 has listed some ending with "unless you are a hardcore ABI hacker"...
As with many rules there are exceptions (and possibly a few more under ARC). So your reasoning for using msgSend should go something along the lines of:
[ 1] I think I should use msgSend - don't.
[2] But I've a case here... - you probably haven't, keep looking for another solution.
...
[10] I really think I should use it here - think again.
...
[100] Really, this looks like a case for msgSend, I can't see any other solution! OK, go read Document.m in the TextEdit code sample from Apple. Do you know why they used msgSend? Are you sure... think again...
...
[1000] I understand why Apple used it, and my case is similar... You've found and understood the exception that proves the rule and your case matches, use it!
HTH
I can make a case. We used msgSend in one of our C++ files (before we switched to ARC) that's in a cross-platform project (Windows, Mac and Linux). We use it to ref count a reference in the backed (the shared code) that's used later to go from frontend to backend and vice versa. Very special case, admittedly.
If I declare an NSMutableString
NSMutableString *str_value;
Why do I have to declare this as a pointer (using *)? If I don't, I get a compilation error.
Could someone explain this clearly?
Recall that Objective C is a superset of C. When you declare a variable without * in C, it is an indication that the memory for that variable is allocated either in the automatic storage if it is a local variable, as part of its outer structure if it is a member of a structure, or in the static memory if it is a static or a global. Using the typename or a structure tag without * in a parameter list of a function indicates passing by value.
The designers of the Objective C language could have taken the Java-like route, making every class instance is a pointer without the pointer syntax, but then the readers of programs in Objective C would need to know if a name represents a typedef based on a struct or an id type to answer even the most basic questions about objects of that type, such as if it is implicitly passed by pointer or by value, if it is allocated as part of the owning structure or as a heap object pointed to by a pointer inside the structure, and so on.
To avoid this kind of confusion, designers of Objective C decided to preserve the explicit pointer syntax for id objects.
A pointer means you are pointing / referencing to that class. Yes it will cause a compilation error, the reason for a pointer is mainly for memory. One data type (int or BOOL or float etc...) is only a few bytes, therefore it is not necessary to have a pointer. But NSMutableString and other Objective-C classes have a lot of properties and methods, with a lot of code. Therefore, since in your apps will have a lot of objects, which will use a lot of memory and thus slow down your app/decrease performance. Of course you should release the object once you make a pointer.
As the title suggests, I'm having trouble understanding exactly what a pointer is and why they're used. I've searched around a bit but still don't really understand. I'm working in Objective-C mainly, but from what I've read this is really more of a C topic (so I added both tags).
From what I understand, a variable with an asterisks in front points to an address in memory? I don't quite understand why you'd use a pointer to a value instead of just using the value itself.
For example:
NSString *stringVar = #"This is a test.";
When calling methods on this string, why is it a pointer instead of just using the string directly? Why wouldn't you use pointers to integers and other basic data types?
Somewhat off topic, but did I tag this correctly? As I was writing it I thought that it was more of a programming concept rather than something language specific but it does focus specifically on Objective-C so I tagged it with objective-c and c.
I don't quite understand why you'd use a pointer to a value instead of
just using the value itself.
You use a pointer when you want to refer to a specific instance of a value instead of a copy of that value. Say you want me to double some value. You've got two options:
You can tell me what the value is: "5": "Please double 5 for me." That's called passing by value. I can tell you that the answer is 10, but if you had 5 written down somewhere that 5 will still be there. Anyone else who refers to that paper will still see the 5.
You can tell me where the value is: "Please erase the number I've written down here and write twice that number in its place." That's called passing by reference. When I'm done, the original 5 is gone and there's a 10 in its place. Anyone else who refers to that paper will now see 10.
Pointers are used to refer to some piece of memory rather than copying some piece of memory. When you pass by reference, you pass a pointer to the memory that you're talking about.
When calling methods on this string, why is it a pointer instead of just using the string directly?
In Objective-C, we always use pointers to refer to objects. The technical reason for that is that objects are usually allocated dynamically in the heap, so in order to deal with one you need it's address. A more practical way to think about it is that an object, by definition, is a particular instance of some class. If you pass an object to some method, and that method modifies the object, then you'd expect the object you passed in to be changed afterward, and to do that we need to pass the object by reference rather than by value.
Why wouldn't you use pointers to integers and other basic data types?
Sometimes we do use pointers to integers and other basic data types. In general, though, we pass those types by value because it's faster. If I want to convey some small piece of data to you, it's faster for me to just give you the data directly than it is to tell you where you can find the information. If the data is large, though, the opposite is true: it's much faster for me to tell you that there's a dictionary in the living room than it is for me to recite the contents of the dictionary.
I think maybe you have got a bit confused between the declaration of a pointer variable, and the use of a pointer.
Any data type with an asterisk after it is the address of a value of the data type.
So, in C, you could write:
char c;
and that means value of c is a single character. But
char *p;
is the address of a char.
The '*' after the type name, means the value of the variable is the address of a thing of that type.
Let's put a value into c:
c = 'H';
So
char *p;
means the value of p is the address of a character. p doesn't contain a character, it contains the address of a character.
The C operator & yields the address of a value, so
p = &c;
means put the address of the variable c into p. We say 'p points at c'.
Now here is the slightly odd part. The address of the first character in a string is also the address of the start of the string.
So for
char *p = "Hello World. I hope you are all who safe, sound, and healthy";
p contains the address of the 'H', and implicitly, because the characters are contiguous, p contains the address of the start of the string.
To get at the character at the start of the string, the 'H', use the 'get at the thing pointed to' operator, which is '*'.
So *p is 'H'
p = &c;
if (*p == c) { ... is true ... }
When a function or method is called, to use the string of characters, the only the start address of the string (typically 4 or 8 bytes) need be handed to the function, and not the entire string. This is both efficient, and also means the function can act upon the string, and change it, which may be useful. It also means that the string can be shared.
A pointer is a special variable that holds the memory location of an other variable.
So what is a pointer… look at the definition mentioned above. Lets do this one step at a time in the three step process below:
A pointer is a special variable that holds the memory location of an
other variable.
So a pointer is nothing but a variable… its a special variable. Why is it special, because… read point 2
A pointer is a special variable that holds the memory location of an
other variable.
It holds the memory location of another variable. By memory location I mean that it does not contain value of another variable, but it stores the memory address number (so to speak) of another variable. What is this other variable, read point 3.
A pointer is a special variable that holds the memory location of an
other variable.
Another variable could be anything… it could be a float, int, char, double, etc. As long as its a variable, its memory location on which it is created can be assigned to a pointer variable.
To answer each of your questions:
(1) From what I understand, a variable with an asterisks in front points
to an address in memory?
You can see it that way more or less. The asterisk is a dereference operator, which takes a pointer and returns the value at the address contained in the pointer.
(2) I don't quite understand why you'd use a pointer to a value instead of
just using the value itself.
Because pointers allow different sections of code to share information, better than copying the value here and there, and also allows pointed variables or objects to be modified by called function. Further, pointers enabled complex linked data structures. Read this short tutorial Pointers and Memory.
(3) Why wouldn't you use pointers to integers and other basic data types?
String is a pointer, unlike int or char. A string is a pointer that points to the starting address of data that contains the string, and return all the value from the starting address of the data until an ending byte.
string is a more complex datatype than char or int, for example. In fact, don't think sting as type like int of char. string is a pointer that points to a chunk of memory. Due to its complexity, having a Class like NSString to provide useful functions to work with them becomes very meaningful. See NSString.
When you use NSString, you do not create a string; you create an object that contains a pointer to the starting address of the string, and in addition, a collection of methods that allows you to manipulate the output of the data.
I have heard the analogy that an object is like a ballon, an the string you're holding it with is the pointer. Typically, code is executed like so:
MyClass *someObj = [[MyClass alloc] init];
The alloc call will allocate the memory for the object, and the init will instantiate it with a defined set of default properties depending on the class. You can override init.
Pointers allow references to be passed to a single object in memory to multiple objects. If we worked with values without pointers, you wouldn't be able to reference the same object in memory in two different places.
NSString *stringVar = #"This is a test.";
When calling methods on this string, why is it a pointer instead of just using the string directly?
This is a fairly existential question. I would posit it this way: what is the string if not its location? How would you implement a system if you can't refer to objects somehow? By name, sure... But what is that name? How does the machine understand it?
The CPU has instructions that work with operands. "Add x to y and store the result here." Those operands can be registers (say, for a 32-bit integer, like that i in the proverbial for loop might be stored), but those are limited in number. I probably don't need to convince you that some form of memory is needed. I would then argue, how do you tell the CPU where to find those things in memory if not for pointers?
You: "Add x to y and store it in memory."
CPU: OK. Where?
You: Uh, I dunno, like, where ever ...
At the lowest levels, it doesn't work like this last line. You need to be a bit more specific for the CPU to work. :-)
So really, all the pointer does is say "the string at X", where X is an integer. Because in order to do something you need to know where you're working. In the same way that when you have an array of integers a, and you need to take a[i], i is meaningful to you somehow. How is that meaningful? Well it depends on your program. But you can't argue that this i shouldn't exist.
In reality in those other languages, you're working with pointers as well. You're just not aware of it. Some people would say that they prefer it that way. But ultimately, when you go down through all the layers of abstraction, you're going to need to tell the CPU what part of memory to look at. :-) So I would argue that pointers are necessary no matter what abstractions you end up building.