I am trying to write this without dot notation but can't figure it out:
[[textField text] rangeOfCharacterFromSet:someInstanceVar].location
I keep getting bad receiver type NSRange (aka '_struct NSRange')
Is this not possible?
Kind regards
You cannot call that without the dot.
rangeOfCharacterFromSet returns a NSRange, which is a plain C struct:
typedef struct _NSRange {
NSUInteger location;
NSUInteger length;
} NSRange;
and not an Objective-C object. .location accesses the first member of that struct
and is pure C syntax.
That has nothing to do with the dot-notation for properties, or with method calls.
Related
I recently discovering these classes like NSMapTable and NSPointerArray, which work like the traditional collections, but also let you store weak references or plain old C pointers. Unfortunately it looks like you can't use the for...in syntax to iterate over non-NSObject pointers. For example:
typedef struct Segment {
CGPoint bottom, top;
} Segment;
...
NSPointerArray *segments = [[NSPointerArray alloc]
initWithOptions:NSPointerFunctionsOpaqueMemory];
...
Segment *s = malloc(sizeof(Segment));
[segments addPointer: s];
...
for (Segment *s in segments) { // nope...
The compiler does not like that last line. The error:
Selector element type 'Segment *' (aka 'struct Segment *') is not a valid object
So, do I need to do this?
for (int i=0, len=segments.count; i<len; i++) {
Segment *seg = [segments pointerAtIndex:i];
...
That's not the end of the world, but I just want to make sure.
(This might be more of theoretical interest.)
NSPointerArray does conform to the NSFastEnumeration protocol, it is only the
for (id object in collection) language construct that cannot be used with arbitrary pointers which
are not Objective-C pointers.
But you can get a whole bunch of pointers from the array by calling the NSFastEnumeration
method countByEnumeratingWithState:objects:count: directly. This is a bit tricky because
that method need not fill the supplied buffer (as explained here: How for in loop works internally - Objective C - Foundation).
Here is a simple example how this would work:
__unsafe_unretained id objs[10];
NSUInteger count = [segments countByEnumeratingWithState:&state
objects:objs count:10];
// Now state.itemsPtr points to an array of pointers:
for (NSUInteger i = 0; i < count; i++) {
Segment *s = (__bridge Segment *)state.itemsPtr[i];
NSLog(#"%p", s);
}
So this does not help to make the code simpler and you probably want to stick with
your explicit loop.
But for large arrays it might improve the performance because the pointers are "fetched"
in batches from the array instead of each pointer separately.
the for (... in ...) syntax won't work in this case because Segment is a struct, not an Objective C object. Your second for loop should work.
So I understand that in ObjC everything lives in the Heap, and everything has a pointer to it. I'm reading through O'Reilys book and I'm grasping most things, but when I'm following through the tutorials/examples and something like this comes up
NSMutableArray *bar = [[[foo alloc] init] callMethod];
The * is right next to bar, but then you have things like
- (NSString *)createDeck:(NSString *)numOfCards;
Why is NSString * and not - (NSString)*createDeck:(NSString)*numOfCards;?
Any help understand things concept would be great thanks.
Edit:
NSUInteger *randomIndex = arc4random() % [deck count];
As Where
NSUInteger randomIndex = arc4random() % [deck count];
Works fine, how come removing the pointer in this case works?
tl;dr
The type is NSString * and that's why you have
- (NSString *)createDeck:(NSString *)numOfCards;
The return type and the argument type are enclosed within the parentheses.
Concerning the last question, an NSUInteger is not a object, despite the naming may suggest otherwise. Being a native type it lives on the stack and you don't need a pointer to it.
If you cmd-click on the type name, you'll find that it's actually a typedef for unsigned int (or unsigned long, depending on the architecture).
Discussion
Variables in C (and consequently in Objective-C) are declared using declarators, which are composed by a type and an identifier. In your example NSString * is the type and bar is the identifier.
NSString * bar;
^^^^^^^^^^ ^^^
type identifier
Declarators without an identifier are called abstract declarators, and are commonly used in C in three cases:
casting
float x = (float)2/4;
argument of sizeof()
sizeof(int *);
declaring argument types of a function
void foo(int *, float);
In Objective-C they are also used for return and argument types of methods, and that's why you have
- (NSString *)createDeck:(NSString *)numOfCards;
(Most of the information about declarators are adapted from http://nilsou.com/blog/2013/08/21/objective-c-blocks-syntax/)
Concerning the position of the asterkisk,
NSString *bar;
NSString * bar;
NSString* bar;
are all valid ways in to declare a variable of type pointer to NSString, aka NSString *.
Which one to use is a pure matter of personal taste, even though I believe the first one is the most common.
Since there is new NSNumber literals in Objective-C that you can use, for instance:
NSNumber *n1 = #1000; // [NSNumber numberWithInt:1000]
But it doesn't seem to be possible to use together with enums? I have tried:
typedef enum {
MyEnumA = 0,
MyEnumB,
MyEnumC
} MyEnum;
NSNumber *n2 = #MyEnumA; // [NSNumber numberWithInt:MyEnumA]
But I get a compiler error saying:
Unexpected '#' in program
I don't understand why it doesn't work since an enum is an int?
Is there a way to make this work?
For named constants, you need to use #(MyEnumA).
You need to use:
NSNumber *n2 = #(MyEnumA);
I know it's odd, but it's just the way it is. I can't think off the top of my head but I assume the parser needs the parentheses in order to distinguish between different syntax.
What I tend to do is to use parentheses always. That works with normal numbers as well as enums as well as equations like:
int a = 2;
int b = 5;
NSNumber *n = #(a*b);
Others have explained what the proper syntax is. Here's why:
#blah is called the "literal" syntax. You use it to make objects wrapping a literal, like a char, BOOL, int, etc. that means:
#42 is a boxed int
#'c' is a boxed char
#"foo" is a boxed char*
#42ull is a boxed unsigned long long
#YES is a boxed BOOL
All of the things following the at sign are primitive values. MyEnumValue is not a literal. It's a symbol. To accommodate this, generic boxing syntax was introduced:
#(MyEnumValue)
You can put a bunch of things inside the parentheses; for the most part, any sort of variable or expression ought to work.
I have an NSString.
NSString *str;
And I need to store it in a struct.
struct {
int *s;
} st;
And set it.
st.s = str;
So, how should I go about retrieving it?
return (__bridge_retained NSString *)st.s;
I've tried the above, and it gives the error: Incompatible types casting 'int *' to 'NSString *' with a __bridge_retained cast.
Answered the question. Simply define the NSString in the struct like this.
struct {
__unsafe_unretained NSString *s;
} st;
Thanks, Carl Veazey!
To store an Objective-C object in an struct you have a couple of options, the one I see most is to store it in the struct as __unsafe_unretained and then maintain a strong reference to it elsewhere.
From the "Common Issues While Converting a Project" section of the ARC Transition Notes:
If using Objective-C objects is sub-optimal, (maybe you want a dense
array of these structs) then consider using a void* instead. This
requires the use of the explicit casts...
They seem to imply __bridge is the way to cast void * to id but are not 100% clear on this.
The other option, which makes more sense to me personally and I've seen more often I think:
Mark the object reference as __unsafe_unretained. ... You declare the
structure as: struct x { NSString * __unsafe_unretained S; int X; }
Hope this helps!
I want to write a function or a directive like NSLog() that takes any kind of variable, primitives and objects. In that function I want to distinguish those.
I know how it works for objects:
- (void)test:(id)object {
if ([object isKindOfClass:[NSString class]])
...
but how do I distinguish objects from structs or even integer or floats.
Something like:
"isKindOfStruct:CGRect" or "isInt"
for example?
Is this possible?
I thought since you can send everything to NSLog(#"...", objects, ints, structs) it must be possible?
Thanks for any help!
EDIT
My ultimate goal is to implement some kind of polymorphism.
I want to be able to call my function:
MY_FUNCTION(int)
MY_FUNCTION(CGRect)
MY_FUNCTION(NSString *)
...
or [self MYFUNCTION:int]...
and in MY_FUNCTION
-(void)MYFUNCTION:(???)value {
if ([value isKindOf:int])
...
else if ([value isKindOf:CGRect])
...
else if ([value isKindOfClass:[NSString class]])
...
}
I know that isKindOf doesn't exists and you can't even perform such methods on primitives. I'm also not sure about the "???" generic type of "value" in the function header.
Is that possible?
#define IS_OBJECT(T) _Generic( (T), id: YES, default: NO)
NSRect a = (NSRect){1,2,3,4};
NSString* b = #"whatAmI?";
NSInteger c = 9;
NSLog(#"%#", IS_OBJECT(a)?#"YES":#"NO"); // -> NO
NSLog(#"%#", IS_OBJECT(b)?#"YES":#"NO"); // -> YES
NSLog(#"%#", IS_OBJECT(c)?#"YES":#"NO"); // -> NO
Also, check out Vincent Gable's The Most Useful Objective-C Code I’ve Ever Written for some very handy stuff that uses the #encode() compiler directive (that) returns a string describing any type it’s given..."
LOG_EXPR(x) is a macro that prints out x, no matter what type x is, without having to worry about format-strings (and related crashes from eg. printing a C-string the same way as an NSString). It works on Mac OS X and iOS.
A function like NSLog() can tell what types to expect in its parameter list from the format string that you pass as the first parameter. So you don't query the parameter to figure out it's type -- you figure out what type you expect based on the format string, and then you interpret the parameter accordingly.
You can't pass a C struct or primitive as a parameter of type id. To do so, you'll have to wrap the primitive in an NSNumber or NSValue object.
e.g.
[self test: [NSNumber numberWithInt: 3.0]];
id is defined as a pointer to an Objective-C object.
#alex gray answer did not work(or at least did not work on iOS SDK 8.0). You can use #deepax11 answer, however I want to point how this 'magic macro' works. It relies on type encodings provided from the system. As per the Apple documentation:
To assist the runtime system, the compiler encodes the return and argument types for each method in a character string and associates the string with the method selector. The coding scheme it uses is also useful in other contexts and so is made publicly available with the #encode() compiler directive. When given a type specification, #encode() returns a string encoding that type. The type can be a basic type such as an int, a pointer, a tagged structure or union, or a class name—any type, in fact, that can be used as an argument to the C sizeof() operator.
To break the macro apart, we first get "typeOf" our variable, then call #encode() on that type, and finally compare returned value to 'object' and 'class' types from encoding table.
Full example should look like:
const char* myType = #encode(typeof(myVar));//myVar declared somewhere
if( [#"#" isEqualToString:#(myType)] || [#"#" isEqualToString:#(myType)] )
{
//myVar is object(id) or a Class
}
else if ( NSNotFound != [[NSString stringWithFormat:#"%s", myType] rangeOfCharacterFromSet:[NSCharacterSet characterSetWithCharactersInString:#"{}"]].location )
{
//myVar is struct
}
else if ( [#"i" isEqualToString:#(myType)] )
{
//my var is int
}
Please note that NSInteger will return int on 32-bit devices, and long on 64-bit devices. Full list of encodings:
‘c’ - char
‘i’ - int
’s’ - short
‘l’ - long
‘q’ - long long
‘C’ - unsigned char
‘I’ - unsigned int
’S’ - unsigned short
‘L’ - unsigned long
‘Q’ - unsigned long long
‘f’ - float
‘d’ - double
‘B’ - C++ bool or a C99 _Bool
‘v’ - void
‘*’ - character string(char *)
‘#’ - object(whether statically typed or typed id)
‘#’ - class object(Class)
‘:’ - method selector(SEL)
‘[<some-type>]’ - array
‘{<some-name>=<type1><type2>}’ - struct
‘bnum’ - bit field of <num> bits
‘^type’ - pointer to <type>
‘?’ - unknown type(may be used for function pointers)
Read more about Type Encodings at Apple
#define IS_OBJECT(x) ( strchr("##", #encode(typeof(x))[0]) != NULL )
This micro works which I got somewhere in stack overflow.
It's important to note that id represents any Objective-C object. And by Objective-C object, I mean one that is defined using #interface. It does not represent a struct or primitive type (int, char etc).
Also, you can only send messages (the [...] syntax) to Objective-C objects, so you cannot send the isKindOf: message to a normal struct or primitive.
But you can convert a integer etc to a NSNumber, a char* to a NSString and wrap a structure inside a NSObject-dervied class. Then they will be Objective-C objects.