What is the different between variable declared in class extension and variable in implementation class

Question

@interface TestViewController ()
{
    NSString *a;
}
@end

@implementation TestViewController
{
    NSString *b;
}

Seems like *a and *b are the same, both are private global variables in this file.
Can anyone tell me there difference?

Answer 1

Both are instance variables, not global variables, but otherwise the two declaration methods are the same. Being able to declare instance variables in the @implementation is a more recent feature of Objective-C and is the better location - instance variables are part of the implementation of a class and (generally) not part of the (public) interface.

Answer 2

@properties declared in Class Extension in not visible in other classes.