What does the 'class' function mean in Objective-C? - objective-c

Question

#interface Soka : NSObject
#property (nonatomic, copy) NSString * name;
-(void)speak;
#end
#implementation Soka
-(void)speak{
NSLog(#"my name is: %#", self.name);
}
#end
int main(int argc, const char * argv[]) {
#autoreleasepool {
NSString * asdfasdfasdfsdf = #"xxxxxxx4";
id cls = [Soka class];
void * obj = &cls;
[(__bridge id)obj speak];
}
return 0;
}
The output is :
[14402:5468392] my name is: xxxxxxx4
Can anyone explain this??
How was the asdfasdfasdfsdf values set to class property?
Thanks
Thanks for all your guys' replies:
Making An Update:
#Leo
Thanks for your reply.
1# What is the meaning of Soka Class object cls, what is its memory structure. It's pointer pointing to what address?
2# find memory near the ocObj object? what if the nearest memory is an integer value? It's a part of OBJC or whatelse theory?

Answer 1

Well it is a little tricky,not useful. It is about Objective C instance memory
Your code can be converted to this
NSString * asdfasdfasdfsdf = #"xxxxxxx4";
Class cls = [Soka class];
void * obj = &cls;
id ocObj = (__bridge id)obj;
NSLog(#"%#",[ocObj name]);
Lets make it clear one by one
cls is a Soka Class object
Class cls = [Soka class];
obj is a pointer point to Soka Class object
void * obj = &cls;
This convert obj to Object C object
id ocObj = (__bridge id)obj;
When call this [ocObj name],Objective C will find memory near the ocObj object.It is #"xxxxxxx4" in the stack.
Also,I test this to make sure I am right
#interface Soka : NSObject
#property (nonatomic, copy) NSString * name;
#property (nonatomic,copy) NSString * address;
-(void)speak;
#end
#implementation Soka
-(void)speak{
NSLog(#"my name is: %#", self.name);
}
#end
int main(int argc, char * argv[]) {
#autoreleasepool {
NSString * asdfasdfasdfsdf = #"xxxxxxx4";
NSString * address = #"address";
Class cls = [Soka class];
void * obj = &cls;
id ocObj = (__bridge id)obj;
NSLog(#"%#",[ocObj name]);
NSLog(#"%#",[ocObj address]);
return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
}
}
This will log
2015-07-01 23:26:56.857 OCTest[10910:347815] address
2015-07-01 23:26:56.859 OCTest[10910:347815] xxxxxxx4
Update:
+class function will return the class object of one class and type is Class.
So,what is Class type,it is something about objective c runtime
typedef struct objc_class *Class;
This is the define of objc_class
struct objc_class {
Class isa OBJC_ISA_AVAILABILITY;
#if !__OBJC2__
Class super_class OBJC2_UNAVAILABLE;
const char *name OBJC2_UNAVAILABLE;
long version OBJC2_UNAVAILABLE;
long info OBJC2_UNAVAILABLE;
long instance_size OBJC2_UNAVAILABLE;
struct objc_ivar_list *ivars OBJC2_UNAVAILABLE;
struct objc_method_list **methodLists OBJC2_UNAVAILABLE;
struct objc_cache *cache OBJC2_UNAVAILABLE;
struct objc_protocol_list *protocols OBJC2_UNAVAILABLE;
#endif
} OBJC2_UNAVAILABLE;
When you create an Objective C object,it will auto manage the memory. In your code,it is just a trick to make an Objective C object like memory.

Answer 2

Actually objective-c's classes is structures with properties.
For your's class Soka you have hidden fields in that structure: _name and class. They placed in same order, as placed your's variables asdfasdfasdfsdf and cls.
All works fine because at address &cls placed variable of type Class, that describes class. So after casting obj to id runtime find required field, that describes class and trust that that pointer points at Soka. Of course it'f false, but by circumstances that variables placed at same order and at right places this code work and doesn't crush.
You can't alloc objective-c objects in stack and should avoid this code if you think that you can alloc objective-c object in such way. Because it can lead to unpredicted errors.

Answer 3

The class method gives you the type of an object.
For example, if you want to know if myObject is an NSString, you could do this:
[myObject isKindOfClass:[NSString class]]
isKindOfClass takes a type as its argument, so we call class on NSString to get the type.
It's like typeof in the C languages.