I'd like to pass an UninterpretedBytes to an external library, say something like this
MyLibrary>>foo: buf len: len
<C: int foo(void *buf,unsigned int len)>
MyApplication>>test
buffer := UninterpretedBytes new: 4.
^MyLibrary new foo: buffer len: buffer size.
But this fails in CPointerType>>coerceForArgument:anObject.
I know that I could use buffer gcCopyToHeap but I don't want to.
I want to handle an object in Smalltalk memory with Smalltalk 1-based address and no extra copy nor any other complication.
I find the last lines of CPointerType>>coerceForArgument:anObject suspiscious:
(anObject isString
or: [self isOopRef and: [anObject class isBits and: [anObject class isVariable]]])
ifTrue: [^anObject].
self isOopRef means that the CPointerType has been declared as _oopref *.
If I cheat and declare:
MyLibrary>>foo: buf len: len
<C: int foo(_oopref *buf,unsigned int len)>
then MyApplication new test works as expected, but I don't want to cheat, the prototypes are extracted automatically from C headers and I don't want to patch them manually.
self isOopRef semantically means is a reference to an Object Oriented Pointer.
Every Object can be viewed as an Object Oriented Pointer and can thus be passed by reference to such _oopref *, except immediate objects like SmallInteger SmallDouble Character. There is a guard above in code to check for the case when anObject isImmediate, so at this stage, anObject is definitely an oop.
Such _oopref could be manipulated from within the external C-code, for example thru some Object Engine function oe*(), and this is not limited to arrays of bytes/words/double words!
The guard and: [anObject class isBits and: [anObject class isVariable]] thus makes no sense.
If I modify the last lines of CPointerType>>coerceForArgument:anObject into:
(anObject isString
or: [self isOopRef or: [anObject class isBits and: [anObject class isVariable]]])
ifTrue: [^anObject].
Then MyApplication new test works like a charm.
Wasn't it the real intention? Pass either any kind of non immediate object if the prototype specifies a pointer to an oop, or pass a reference to an array of bytes/words/...
Related
Is it possible (and if so, safe) to create/use a block which takes a double pointer as an argument?
For instance:
- (void)methodWithBlock:(void (^)(NSError **error))block;
Additional context, research, and questions:
I'm using ARC.
When I declare the method above and attempt to call it, XCode autocompletes my method invocation as follows: [self methodWithBlock:^(NSError *__autoreleasing *error) {}];
What does __autoreleasing mean here and why is it being added? I presume it has something to do with ARC.
If this is possible and safe, can the pointer still be dereferenced in the block as it would be anywhere else?
In general, what are the important differences between doing what I'm describing, and simply passing a double pointer as a method parameter (e.g. - (void)methodWithDoublePointer:(NSError **)error;)? What special considerations, if any, should be taken into account (again assuming this is possible at all)?
yes, pointers are always just pointers. you only need to make sure you dereference it before sending it a message (assuming objc object).
Also be aware that the pointer may be nil. always check it before trying to dereference it or what have you.
As #verec mentioned if you are using ARC you should declare the parameter as __autoreleasing
according to the docs
__autoreleasing is used to denote arguments that are passed by reference (id *) and are autoreleased on return.
remember id is a pointer to an object so that is saying object**
there is no difference between passing pointers to pointers to methods or blocks.
The answers are both Yes & No...
At a base level passing pointers to pointers to blocks is no different than passing them to methods; and, with the usual proviso that your pointers must be valid, is perfectly OK.
However that __autoreleasing is very significant here and is tied up with ARC and pass-by-writeback. Whether using the block will work as expected will be dependent on context as the compiler often uses hidden variables when passing parameters of type NSError * __autoreleasing * as part of the pass-by-writeback implementation.
If pass-by-writeback is not what you need, or is unsuitable, you may wish to declare you block as taking a different type, such as NSError * __strong *. Read this answer which explains what happens under the hood, that should help you decide whether in your context the block declaration is good.
In summary (a) declaring the block is fine, but (b) you need to understand how it may be called and may need to change the signature.
warning: untested
For the sake of argument, let's start with:
typedef NSError * NSErrorPtr ;
- (void) foo: (NSErrorPtr *) errPtr {
*errorPtr = [NSError new] ;
}
errPtr isn't declared either __weak nor __strong.
So, according to ARC, even though its contents is allocated within foo the responsibility for releasing it has to reside somewhere.
Where?
Note that this not a property of double pointers per se. But of your pattern of allocation.
consider:
int ** arrayOfarrayOfInts = {
{1, 2, 3, 4}
, {5, 6, 7, 8}
} ;
- (void) incrementElement: (int **) elem {
++(**elem) ;
}
- (void) bumpFirstColByOne {
for (int i = 0 ; i < 2 ; ++ i) {
int * arrayOfInt = arrayOfarrayOfInts[i] ;
int ** second = &arrayOfInt[0] ;
[self incrementElement: second] ;
}
}
No __autoreleasing needed here because no allocation is taking place ...
I'm trying to implement the countByEnumeratingWithState:objects:count: method from the NSFastEnumeration protocol on a custom class.
So far I have it iterating through my objects correctly, but the objects that are returned aren't Objective-C objects but rather the core foundation equivalents.
Here's the part of the code that sets the state->itemsPtr:
MyCustomCollection.m
- (NSUInteger) countByEnumeratingWithState: (NSFastEnumerationState *)state
objects: (id __unsafe_unretained *)buffer
count: (NSUInteger)bufferSize {
// ... skip details ...
NSLog(#"Object inside method: %#", someObject);
state->itemsPtr = (__unsafe_unretained id *)(__bridge void *)someObject;
// ... skip details ...
}
Then I call the 'for..in' loop somewhere else on like this
SomeOtherClass.m
MyCustomCollection *myCustomCollection = [MyCustomCollection new];
[myCustomCollection addObject:#"foo"];
for (id object in myCustomCollection) {
NSLog(#"Object in loop: %#", object);
}
The console output is:
Object inside method: foo
Object in loop: __NSCFConstantString
As you can see, inside the NSFastEnumeration protocol method the object prints fine, but as soon as it gets cast to id __unsafe_unretained * I lose the original Objective-C corresponding class.
To be honest I'm not quite sure how the (__unsafe_unretained id *)(__bridge void *) casting works in this case. The (__unsafe_unretained id *) seems to cast to match the right type itemsPtr needs. The (__bridge void *) seems to cast to a pointer of type void with __bridge used to bridge the obj-c world to the CF world. As per the llvm docs, for __bridge:
There is no transfer of ownership, and ARC inserts no retain operations
Is that correct?
From my understanding __NSCFConstantString is just the core foundation equivalent of NSString. I also understand that with ARC you need to bridge from Objective-C objects to CoreFoundation equivalents because ARC doesn't know how to manage the memory of the latter.
How can I get this working so that the objects in my 'for..in' loop are of the original type?
Also note that in this case I'm adding NSStrings to my collection but in theory it should support any object.
UPDATE
Rob's answer is on the right track, but to test that theory I changed the for loop to this:
for (id object in myCustomCollection) {
NSString *stringObject = (NSString *)object;
NSLog(#"String %# length: %d", stringObject, [stringObject length]);
}
In theory that should work since the objects are equivalent but it crashes with this error:
+[__NSCFConstantString length]: unrecognized selector sent to class
It almost looks like the objects returned in the for loop are classes and not instances. Something else might be wrong here... Any thoughts on this?
UPDATE 2 : SOLUTION
It's as simple as this: (thanks to CodaFi
state->itemsPtr = &someObject;
You're incorrectly casting someObject. What you meant is:
state->itemsPtr = (__unsafe_unretained id *)(__bridge void *)&someObject;
(Let's get rid of those awful casts as well)
state->itemsPtr = &someObject;
Without the address-of, your variable is shoved into the first pointer, which is dereferenced in the loop. When it's dereferenced (basically, *id), you get the underlying objc_object's isa class pointer rather than an object. That's why the debugger prints the string's value inside the enumerator call, and the class of the object inside the loop, and why sending a message to the resulting pointer throws an exception.
Your code is fine the way it is. Your debug output is revealing an implementation detail.
NSString is toll-free-bridged with CFString. This means that you can treat any NSString as a CFString, or vice versa, simply by casting the pointer to the other type.
In fact, under the hood, compile-time constant strings are instances of the type __NSCFConstantString, which is what you're seeing.
If you put #"hello" in your source code, the compiler treats it as a NSString * and compiles it into an instance of __NSCFConstantString.
If you put CFSTR("hello") in your source code, the compiler treats it as a CFStringRef and compiles it into an instance of __NSCFConstantString.
At run-time, there is no difference between these objects in memory, even though you used different syntax to create them in your source code.
Just when I think I'm getting comfortable with Objective-c the mentioned symbols totally throw me down a rabbit hole...
** a double pointer??
& what sort of things can I do with &reference, is a #property? a full on object? weird pointer razzledazzle?
± I see both a + or a - before method declarations; I've seen Java annotate some datatype declarations by manually typing the + and the magic of compiling in Eclipse would change them to a -
I'm likely asking repetitious questions and/or way outta the ballpark on my guesses; thanks for answers/edits.
You're getting into the C portion that objective-c is built on top of.
** is a pointer to a pointer. Since functions in C take arguments by value, it means you can't change the value of the argument in that function. But, by providing a level of indirection and passing a pointer to the pointer, you can change the value.
& means it's a reference. If an argument takes a ** and you have a * variable, pass a reference to it.
Foo *foo;
[self changeFoo: &foo];
- (BOOL)changeFoo: (Foo **)foo
{
// dereference the double pointer and assign a val = alloc init returns a *
*foo = [[Foo alloc] init];
return YES;
}
A common usage in objective-c / cocoa is NSError. It's essentially an out argument.
NSError *err;
BOOL success = [self doSomething:#"Foo" error:&err];
- (BOOL)doSomething:(NSString*)withData error:(NSError**)error
{
}
As you might know, a pointer points to the address of an object and is the way you reference an object. A double pointer is sometimes used in Objective-C, mainly for returning NSErrors, where you want to get back the address, i.e. a pointer, to an error object (NSError) if an error occurred, thus you pass in a pointer assigned to null and the caller can change that pointer so that it points to the address of another pointer which in turn points to an NSError object.
The ampersand (&) is mostly used by the lower level C APIs, e.g. Core Graphics. They are used to reference things, like the current context. As long as most of your code uses square brackets around its method calls you won't see these very often.
Using a + or a - before a method declarations is used to differentiate between class (+) and instance (-) methods. A class methods is called on the class itself (such as alloc), while a instance method is called on an instance of that object (such as init).
- and + before a method declaration designate an instance method and a static class method. To use an instance method you have to create an object of your class before you can call its method, a static method can be called directly from a class type
I have read the memory management guide from Apple and I don't see where this case is explained...
Many times, especially when writing a class method to return an instance of a class, I'll start it out like this, because that's how I've seen it done, and it works.
[NOTE] This code is from memory - I'll update it when I get home to show an example that really works (I made this up to illustrate it, but obviously I don't recall it well enough to construct something that makes sense...
[EDIT] Here's my actual method - of course everyone was right that I must be calling alloc which I am.
+ (id)player
{
Player *player = nil;
if ((player = [[[super alloc] initWithFile:#"rocket.png"] autorelease])) {
[player setProjectileType:kProjectileBullet];
[player setProjectileLevel:1];
[player setInvincible:YES];
[player setEmitter:[CCParticleSystemQuad particleWithFile:#"exhaust.plist"]];
[[player emitter] setPosition:ccp(0.0, player.contentSize.height/2)];
[player addChild:player.emitter];
}
return player;
}
So what I got from the responses is:
* Declaring the instance just gets me a pointer to a memory location and tells Xcode what class the object will be.
* Setting the pointer to nil pretty much just sets it to zero - keeping it from having garbage in it (right?)
* Since I'm autoreleasing the instance, the object that is returned is also autoreleased.
Thanks for helping me understand this!
Can someone explain what the compiler does when it sees this?
DooDad* aDooDad = nil;
If you are really interested in what the compiler does, the answer is: the compiler will reserve some memory on the stack for the local variable aDooDad, which is a pointer type (it is generally 64 or 32 bits in size depending on the processor). That pointer is then initialized to contain nil (usually 0x00..00).
A statement like this:
DooDad* aDooDad = [[DooDad alloc] init...];
makes use of pointer variable aDooDad to store the address in memory of the object that is further allocated (which is the address of memory reserved by alloc).
So, in the end,
DooDad* aDooDad = nil;
is not declaring an object, just a variable whose content is interpreted as the address of an object of DooDad type. Such declaration, therefore, is just like any other declaration you know, e.g. when initializing an int to 0, so that later you can assign it some value in an if statement.
A statement like:
[aDooDad doSomething];
is interpreted by the Objective-C runtime system like: send message doSomething to the object whose address is stored in aDooDad. If that address is nil no message is sent. On the other hand, if you dereference a nil pointer: *aDooDad you'll get undefined behavior.
Pointers are pretty low level stuff. I hope this helps.
If you're familiar with C or C++, variables can be created in one of two ways, statically on the call stack, or dynamically on the heap. Variable memory created on the stack is is reclaimed when the current stack frame goes out of scope, so you never need to worry about creating or destroying it. In Objective-C, objects are always dynamically created. Primitives (like int, float, pointers, etc), can either be statically or dynamically created. For illustration:
- (id)something {
NSObject myObject; // Illegal static object allocation
NSObject* myObject; // Legal primitive (pointer) static allocation
int myInt; // Legal primitive static allocation
int* myIntPtr; // Legal primitive (pointer) static allocation
}
So when you say DooDad* dodad = nil;, you're creating a primitive (pointer to a DooDad) on the stack. Being a stack variable, you don't alloc or dealloc it, just like you wouldn't worry about alloc'ing or dealloc'ing any of the memory in the following method:
- (id)allStackVariables {
int myInt = 0;
float myFloat = 0.0f;
float* myFloatPtr = NULL;
NSObject* myObject = nil;
}
Setting it to nil simply sets the contents of the variable to whatever the compiler defines to be nil, something like 0x000000 in hex. Saying DooDad* dooDad = nil; is conceptually identical to saying something like int myInt = 0;
Declaring simple gives you a pointer you can use later. No memory is allocated.
Not sure what the intent of the method you posted, but it seems wrong on many levels. It will return nil, always. Unless it's an initializer method, it should not call [self init]. If it is an initializer method, it should return self and be named something like "init..."
Sometimes I encounter code that has *, sometimes **. Can anyone explain what they mean in Objective C? (I used to be a Java programmer, with experience in C/C++.)
The * denotes that you are using a pointer to a variable, and is most commonly used to store a reference to an Objective-C object, objects which can only live on the heap and not the stack.
Pointers are not a part of Objective-C exclusively, but rather a feature of C (and therefore its derived languages, of which Objective-C is one of them).
If you are questioning the difference between * and **, the first denotes a pointer, whereas the second denotes a pointer to a pointer; the advantage of the latter to the former is that when passing in an object using ** in a method parameter, the method can then change this parameter and the new value is accessible in the calling method.
Perhaps the most common use of ** in Cocoa is when using NSError objects. When a method is called that can return an NSError object on failure, the method signature would look something like this:
- (id)someMethodThatUsesObject:(id)object error:(NSError**)error;
What this means is that the calling function can pass in a pointer to an NSError object, but someMethodThatUsesObject: can change the value of error to another NSError object if it needs to, which can then be accessed by the calling method.
This is often used as a workaround for the fact that functions can only return one value.
A * in Objective-C means exactly the same as in C; and you'll usually see it (or not) in these situations:
// Method signatures:
// Here the asterisk (*) shows that you have a pointer to an NSString instance.
+ (NSString *)stringWithString:(NSString *)aString;
// Method signatures, part two:
// Here the double asterisk (**) signifies that you should pass in a pointer
// to an area of memory (NSError *) where outError can be written.
- (BOOL)writeToURL:(NSURL *) atomically:(BOOL) error:(NSError **)outError;
// Method signatures make for good examples :)
// Here the asterisk is hidden; id is a typedef for void *; and it signifies that
// a pointer to some object of an indeterminate class will be returned
- (id)init;
// And a forth example to round it all out: C strings!
// Here the asterisk signifies, you guessed it, a pointer! This time, it's a
// pointer to the first in a series of const char; terminated by a \0, also known
// as a C string. You probably won't need to work with this a lot.
- (const char *)UTF8String;
// For a bit of clarity, inside example two, the outError is used as follows:
// Here the asterisk is used to dereference outError so you can get at and write
// to the memory it points to. You'd pass it in with:
// NSError *anError;
// [aString writeToURL:myURL atomically:YES error:&anError];
- (BOOL)writeToURL:(NSURL *)url atomically:(BOOL)atom error:(NSError **)outError {
// do some writing, and if it went awry:
if (outError != NULL)
*outError = [NSError errorWithName:#"NSExampleErrorName"];
return NO;
}