Why variable with __weak qualifier retains an object?

Question

Here is my code:

extern void _objc_autoreleasePoolPrint();

int main(int argc, const char * argv[])
{

    NSArray __weak *tmp = nil;

    @autoreleasepool {         


        

Answer 1

It seems that [[NSArray alloc] init] returns a "shared instance" of an empty NSArray:

NSArray *a = [[NSArray alloc] init];
NSArray *b = [[NSArray alloc] init];
NSLog(@"a &: %p", a);
NSLog(@"b &: %p", b);

Output:

    a &: 0x100103110
    b &: 0x100103110

This "shared instance" continues to exist even if your strong reference obj is gone, therefore the weak pointer is not set to nil.

Obviously, [[NSMutableArray alloc] init] cannot return a shared instance.