According to this comment:
C functions inside #implementation blocks have the unique property of
being able to access private and protected ivars directly. Thus, from
my own experience, it's become a strong idiom to place C functions
that "belong" to a class inside the corresponding implementation.
My code, defining a private instance variable in the implementation only as suggested by this answer:
With the brackets:
#implementation ViewController{
MyTest *tt;
}
void testf(){
NSLog(#"hello back from c++ into obj c land");
[tt testcf: 5];
}
...
Will not build; the compiler indicates that tt in testf is undeclared. If I remove the brackets, then the C function works fine.
But... if I remove the brackets, do I understand that actually this is no longer an instance variable, but sneakily it is a global variable, disconnected from the class, despites its placement in the #implementation? It would appear that this is indeed true, since I can do this as the end:
#end
void testf2(){
NSLog(#"hello back from c++ into obj c land");
[tt testcf: 5];
}
And the compiler does not contain about tt being out of scope. So -- how to declare a private instance variable in an implementation and have it truly be an instance variable, but also have a C function be able to access it? For the purposes of this question, I am trying to get this to work based on the linked comments/answers without using the id pointer of the object itself.
You will need to pass a reference to self to the C-function:
void testf(ViewController *vc){
NSLog(#"hello back from c++ into obj c land");
[vc->tt testcf: 5];
}
and call it like this:
- (void)someMethodInViewController
{
testf(self);
}
However it's not clear why you are using C-functions at all.
Related
Can someone confirm that you cannot access instance variables defined in an Objective C #implementation block from within C style functions of the same class? The compiler is throwing errors saying "XXX undeclared' where XXX is the instance variable name.
Here's an example of what I am explaining:
#interface FontManager : NSObject {
CGFontRef fontRef;
}
static int CstyleFunction() {
NSUInteger emSize = CGFontGetUnitsPerEm(fontRef);
}
I want to verify that I cannot use "fontRef" from withing "CstyleFunction".
Any insight would be greatly appreciated.
A "C style method" doesn't really deserve the name "method", I'd call it a "function" instead as in C.
A C function has no self, so it cannot implicitly access ivars as a method can. If you pass an instance to the C function as a parameter, you can access ivars in the same manner you would access a field in a struct pointer.
#Anomie and #jlehr are correct, the C function has no concept of the FontManager object and its current state, it just happens to live in the same file.
However, if FontManager is a singleton and you make fontRef a property (or create an accessor for it), then it would be possible to access the value within your C class:
static int CstyleMethod() {
FontManager *fm = [FontManager sharedManager];
NSUInteger emSize = CGFontGetUnitsPerEm(fm.fontRef);
}
Bottom line, you can mix-and-match C and ObjC syntax within C functions & ObjC methods. But because C functions have no default reference to self (and the object's associated instance variables), you can only reference ObjC objects that are singletons, stored in a global variable, or passed in as parameters.
That's correct. You seem to be mixing up methods and functions though. Methods exist only in Objective-C. What you're referring to as a 'C style method' is really just a C function.
C is not an object-oriented programming language. Since there's no such thing as an object in C, there's also no such thing as an instance variable in C, so the fontRef instance variable would not be visible in the function you posted, nor for that matter in any other C function in your program.
Trying to make sense of MTAudioProcessingTap sample code from WWDC 2012.
This structure is used by the Tap to pass data around.
typedef struct AVAudioTapProcessorContext {
...
void *self; //Note use of word self
} AVAudioTapProcessorContext;
The word self here is used apparently as a standard variable name even though Xcode highlights it in pink. Is this just a code parsing oversight in Xcode?
Later, the Tap management class passes a reference to itself to this variable.
//Setting up Tap callbacks:
callbacks.clientInfo = (__bridge void *)self,
//And then storing it in the above struct when it's passed by the callback:
static void tap_InitCallback(MTAudioProcessingTapRef tap, void *clientInfo, void **tapStorageOut)
{
AVAudioTapProcessorContext *context = calloc(1, sizeof(AVAudioTapProcessorContext));
...
context->self = clientInfo;
*tapStorageOut = context;
}
And the confusion starts again in a later function (another Tap callback). Once again, the word self is used to refer to the Tap management class reference. Note that this occurs in a function defined after #end of the Tap management class implementation.
AudioFFTTapProcessor *self = ((__bridge AudioFFTTapProcessor *)context->self);
self has a special meaning only in the context of an Objective-C method. In all other contexts, it's just an ordinary identifier. You can think of Objective-C methods as being rewritten to take a hidden parameter with the name self (and a second hidden parameter named _cmd that's not relevant for this discussion), e.g.:
#implementation MyClass
- (void)doSomething:(int)param1 withThis:(NSString *)param2
{
// 'self' refers to the MyClass instance in this function
...
}
#end
could be imagined as if it were rewritten like this:
void MyClass_doSomething(id self, SEL _cmd, int param1, NSString *param2)
{
// 'self' refers to the MyClass instance in this function
...
}
Outside of Objective-C methods, you can use self as any other kind of identifier, whether it be a local variable, a struct member, a global function, or type name, etc., though the latter uses should be highly discouraged due to high potential for confusion.
So yes, XCode highlighting the self instance in pink in the struct definition is just a parsing anomaly.
self is not a keyword in C. It's a keyword for Objective-C, which is probably why it's getting highlighted.
self does not mean anything special as a C struct field name. Objective-C uses it as the equivalent of most other OO languages' this, which is why Xcode highlights it specially.
Do variables declared in methods get erased after the method call is done? Ie If i have the method "someMethod" and every time it is called i want to return a different NSString from a stack property will the method return the next object on the stack or will it keep returned the first index since x is erased at the end of the method call. I know if C that variables are erased after function calls, is it the same in objective c? Would using a seperate property for the variable x some this problem? Thanks
(Double) someMethod{
int x;
[self.stack objectAtIndex:x];
x++;
}
After reading the comments I tried creating a property to replace x and here is what I wrote but I get an error warning stating "local declaration of "_location" hides instance variable" What does this mean?
#property (nonatomic) int location;
#synthesize location=_location;
-(int) location{
if(!_location){
int _location = 0;
//warning is here
return _location;
}
_location++;
return _location;
}
(Double) someMethod{
int x;
[self.stack objectAtIndex:self.location];
x++;
}
Do variables declared in methods get erased after the method call is done?
Yes
Objective C methods are implemented "on top" of C functions, so the same rules apply. In particular, your code exhibits undefined behavior (reading of uninitialized variable).
To fix this issue, add an instance variable x in place of the automatic variable that your code snippet currently declares.
automatic is the "official" name of "stack" variables, i.e. variables that you declare inside your methods / functions.
Yes, the lifetime of local variables is limited to the time the enclosing function is executing (with exceptions for blocks, but you're not using those in this case).
Your counter x is probably best as an object property, if you want to maintain its value between calls to someMethod.
Answering to your question about the warning...
It is just saying that when you declare inside your if in method location()
int _location = 0;
this local variable has the same name as the property you created earlier
#syntenshize location = _location
Thus it gets confusing (for the programmer) to know which one he is using at the moment.
The compiler will understand that inside the if _location is an int ... and outside the if _location is your property.
I'm trying to get some code going that lets me display raw trackpad data from my macbook pro, like the app FingerMgmt. Unfortunately, no one seems to have the source for FingerMgmt. I did find some other source code that kind of works, however. I was able to NSLog the data I wanted to see like this:
int callback(int device, Finger *data, int nFingers, double timestamp, int frame) {
for (int i=0; i<nFingers; i++) {
Finger *f = &data[i];
NSLog(#"Frame %7d: Angle %6.2f, ellipse %6.3f x%6.3f; "
"position (%6.3f,%6.3f) vel (%6.3f,%6.3f) "
"ID %d, state %d [%d %d?] size %6.3f, %6.3f?\n",
f->frame,
f->angle * 90 / atan2(1,0),
f->majorAxis,
f->minorAxis,
f->normalized.pos.x,
f->normalized.pos.y,
f->normalized.vel.x,
f->normalized.vel.y,
f->identifier, f->state, f->foo3, f->foo4,
f->size, f->unk2);
//todo-get data from raw C to obj-C variable
}
return 0;
}
But whenever I try to store any of the data to an Obj-c string or variable, the C code does not see the variable as having been declared. Because of this, I cannot write to any text fields or graphical displays in Obj-C, and I cannot store the data to a variable that Obj-c can access.
Basically, I need a way to write to an Obj-C variable or object from within the callback.
On a side note, I had a very similar problem with an iPhone app a while back, and I ended up fixing it by somehow declaring the app delegate within the C code and writing to or reading from the variable like this-
me.delegate=(id <UIApplicationDelegate,UITabBarControllerDelegate>)[[UIApplication sharedApplication] delegate];//allows access to the delegate within C function
me.delegate.number0=5;//writes to this variable in the delegate
For some reason, I can not seem to adapt this to my current situation. I always get the error that "me" is undeclared.
A Objective-C method can access instance variables because it is automagically passed a hidden parameter with the public name self - any reference to an instance variable, say fred, is translated by the compiler into a field reference, say self->fred (and a similar translation for property references).
For your C function callback to access the fields of any object (or call an object's methods) you need to pass the function a reference to the object. Two simple ways:
Add an argument to the function. Many C callback protocols include a general "user defined" values which is passed around as void *, if you are calling one of these pass your object reference as this value and cast it within the C function back to the correct Objective-C type.
Pass the object via a global (or file static) variable, e.g. static NSSomeType *objectForCallback;. This method works when you're stuck with an existing C callback protocol which doesn't support a user defined value. However it is not thread or re-entrant safe as you are sharing a single static variable.
In both cases make sure the objected is retain'ed if you're not using garbage collection.
In response to comment
Case 1: You will see C functions declared which (a) take a callback function and (b) a user-defined value to pass to that function on every call. For example:
typedef T ...;
T findMatching(T *buffer, // an array of T to search
size_t count, // number of items in array
int (*matcher)(T item, void *user), // user function for match, 1st arg is item, 2nd user-supplied value
void *userValue); // user-supplied value to pass to matcher
If you are faced with C function like this you can pass a (retain'ed if needed) Objective-C object as userValue and cast it back to its Objective-C type inside matcher. For example:
int myMatcher(T item, void *user)
{
NSMutableDictionary *myDictionary = (NSMutableDictionary *)user;
...
}
- (void) someMethod
{
NSMutableDictionary *sharedWithC = ...;
...
T found = findMatching(buffer, count, myMatcher, (void *)sharedWithC);
...
}
Case 2: Objective-C is (a superset of) C. You declare a global just as you would in C. For example (little checking, not thread safe):
static NSMutableDictionary *myGlobalDictionary = nil; // "static" makes the variable only visible to code in the same file
- (void) setupTheSharedDictionary
{
myGlobalDictionary = [[[NSMutableDictionary alloc] init] retain];
}
- (void) releaseTheSharedDictionary
{
if(myGlobalDictionary != nil)
{
[myGlobalDictionary release];
myGlobalDictionary = nil;
}
}
In response to second comment
I'm guessing you are trying to use some third party (Google?) code. That code defines a callback protocol - a C function type. You cannot just redefine that C function type adding an extra argument and expect the third party code to magically cope!
So unless you intend to change the C you can use the second approach - store the reference to Objective-C object in a global. In your case this will be something like:
static MT2AppDelegate *sharedWithCAppDelegateReference;
int callback(...)
{
...
[sharedWithCAppDelegateReference->L1 setStringValue:#"Hellofff"];
...
}
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
sharedWithCAppDelegateReference = self; // store so C can pick it up
...
MTRegisterContactFrameCallback(dev, callback);
...
}
But remember this is not thread or re-entrant safe - you are effectively passing a function parameter via a global variable. If you need it to be thread/re-entrant safe you need to get a bit more involved.
Can someone confirm that you cannot access instance variables defined in an Objective C #implementation block from within C style functions of the same class? The compiler is throwing errors saying "XXX undeclared' where XXX is the instance variable name.
Here's an example of what I am explaining:
#interface FontManager : NSObject {
CGFontRef fontRef;
}
static int CstyleFunction() {
NSUInteger emSize = CGFontGetUnitsPerEm(fontRef);
}
I want to verify that I cannot use "fontRef" from withing "CstyleFunction".
Any insight would be greatly appreciated.
A "C style method" doesn't really deserve the name "method", I'd call it a "function" instead as in C.
A C function has no self, so it cannot implicitly access ivars as a method can. If you pass an instance to the C function as a parameter, you can access ivars in the same manner you would access a field in a struct pointer.
#Anomie and #jlehr are correct, the C function has no concept of the FontManager object and its current state, it just happens to live in the same file.
However, if FontManager is a singleton and you make fontRef a property (or create an accessor for it), then it would be possible to access the value within your C class:
static int CstyleMethod() {
FontManager *fm = [FontManager sharedManager];
NSUInteger emSize = CGFontGetUnitsPerEm(fm.fontRef);
}
Bottom line, you can mix-and-match C and ObjC syntax within C functions & ObjC methods. But because C functions have no default reference to self (and the object's associated instance variables), you can only reference ObjC objects that are singletons, stored in a global variable, or passed in as parameters.
That's correct. You seem to be mixing up methods and functions though. Methods exist only in Objective-C. What you're referring to as a 'C style method' is really just a C function.
C is not an object-oriented programming language. Since there's no such thing as an object in C, there's also no such thing as an instance variable in C, so the fontRef instance variable would not be visible in the function you posted, nor for that matter in any other C function in your program.