What is difference between
BYTE* p;
and
BYTE^ p;
Does this declarations makes any difference in c++cli if there is any difference what is that?
thanks in advance
BYTE^ p would give you a managed handle to a single, boxed byte. You can't do much with it (you can read it, and I'm not sure you can write it even in C++/CLI; you certainly can't in C#), and there is little interest over using the byte directly in its unboxed form.
BYTE* p on the other hand, is the classic pointer to a byte, which can be the first of an array (or even any byte within an array thereof).
pin_ptr can give you the latter from the former.
Related
I have a NSString which i have to convert into NSData. After converting into NSData, I need to transpose the NSData in objective-c.
How do i transpose NSData in objective -c ?
Thanks
It seems you wish to reverse the order of the bytes, here is an outline algorithm:
Use NSData's length to get the number of bytes
Allocate a C-array of type Byte to hold the bytes. Use one of the malloc() family.
Use getBytes:length: to get a copy of the bytes
Set a C Byte * variable, say frontPtr, to point at the first byte; set another, say rearPtr to point at the last byte.
Now iterate, exchanging the bytes referenced by the two pointers, then increment frontPtr, decrement rearPtr, and keep iterating while rearPtr > frontPtr.
Create a new NSData from the working buffer using NSData's + dataWithBytesNoCopy:length:freeWhenDone: passing YES as the last argument - this will take ownership of the malloc'ed buffer so there is no need for you to free it.
The algorithm simply moves two pointers from the ends of the buffer towards the middle exchanging bytes as it goes. The termination condition make it work for even and odd lengths (in the latter case the middle byte doesn't need to be swapped).
If one the other hand you didn't wish to reverse the order of the bytes, but instead reverse the order of bits in each byte Google "C bit reverse" and follow the same general structure as the above algorithm but do bit reversing in the loop.
If after coding the above you have a problem ask a new question, include your code, and explain your issue. Someone will undoubtedly help you.
HTH
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.
I'm working on a project that has C code embedded within Objective-C code. The C code produces some void * pointers that I would like to pass between Objective-C methods, so I'd like to wrap them in Objective-C objects to make an NSSet or something to that effect.
I have looked into NSData, which seems to accept arbitrary data, but wants to know the length of the memory chunk, that I don't have.
Any help is appreciated.
The NSValue class is usually used for this task:
NSValue* value = [NSValue valueWithPointer: myPointer];
and
void* myPointer = [value pointerValue];
Note, though, that the pointer given will not receive any special treatment with respect to memory management. You (and you alone) are responsible for making sure, that the memory it points to remains valid as long as pointers to that memory region exist and are reachable in some part of your program.
Note, though, that using such a value with NSSet or as key in a NSDictionary might or might not yield the intended effect:
NSData is essentially a byte buffer. It actually represents the content of the memory in question. Comparing NSData instances for equality compares at byte level. This is one of the reasons, NSData needs to know about the length of the memory region in question.
NSValue with a pointer value is an entirely different beast. Here, that actual (numeric) pointer value is the essential thing. No consideration is given (when comparing two NSValue instances) to the actual content at the address.
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.
I want to create a custom dictionary that does not copy its keys (just retains).
Internally, I plan to use an NSMutableArray with a special Pair object, where the first object of the pair is the key, and the second is the value.
Keys are all unique.
In order to quickly retrieve objects on keys, I'd use binary search algorithm - so the array should be SORTED (attention!) on memory addresses of pairs' first objects.
(btw, this is why I refused to use CFDictionaryRef with a special set of callbacks - I suspect it to degrade to O(n) in case when a reasonable hash would not be provided)
Is this a bad idea, assuming that:
Objects used as keys may change internally (that is, I cannot use isEqual: instead of address comparison);
Objects used as keys are NOT going to be deallocated (which is reasonable - they are retained by a special Pair object, and the Pair is retained by an internal NSArray).
Why is it bad (or good)?
How do I obtain memory address from a pointer? Just cast it to a long long int or something like that?
Thank you!
To get an integer value from a pointer, cast to intptr_t or uintptr_t. These types are defined in <stdint.h> and are explicitly guaranteed to be large enough to hold the value of a pointer.
I assume that pointers in objective-c are similar to c/c++. I have built sorted arrays of memory addresses to use for looking up a stack trace. On a 64 bit machine, it should be cast to whatever scalar variable is 64 bits, probably a long int. Or if you can use a uint64_t type, it will guarantee the correct size. For a 32 bit machine, use an unsigned 32 bit integer, or a uint32_t. You can use the > < >= <= = and - operators on pointers and you can also use the + operator with a pointer and an integer.