What does id _Nullable __autoreleasing * _Nonnull mean? - objective-c

Question

I am confused with the usage of (out id _Nullable *) in the code docs and the error my snippet throws at me.
#import <Foundation/Foundation.h>
void getvalue(char *a, bool *value){
#autoreleasepool {
NSURL *url = [[NSURL alloc] initFileURLWithFileSystemRepresentation:a
isDirectory:NO
relativeToURL:nil];
[url getResourceValue:value
forKey:NSURLIsDirectoryKey
error:nil];
}
}
void main (int argc, const char * argv[])
{
bool value[10];
char a[] = "a";
getvalue(a, value + 4);
}
Cannot initialize a parameter of type 'id _Nullable __autoreleasing * _Nonnull' with an lvalue of type 'bool *'
What does _Nullable * _Nonnull mean and what am I doing wrong in the code?
Reference:
getResourceValue:forKey:error:
- (BOOL)getResourceValue:(out id _Nullable *)value
forKey:(NSURLResourceKey)key
error:(out NSError * _Nullable *)error;

Answer 1

_Nullable * _Nonnull isn't the issue here. The problem is you're passing a pointer to a bool when you need to pass a pointer to a pointer to an object. getResourceValue:forKey:error: returns an object, not a primitive. In this case, an NSNumber.
NSNumber *result = nil;
[url getResourceValue:&result
forKey:NSURLIsDirectoryKey
error:nil];
*value = [result boolValue];
To the question, id _Nullable * _Nonnull means "a pointer, which must not be null, to an object-pointer, which may be null." This is the standard return-by-reference pattern in ObjC. id is a typedef for a pointer-to-an-object.