objective c - instance variables in @interface; header vs implementation -

is there difference between declaring private instance variable in header vs declaring in implementation?

in testobj.h

@interface testobj : nsobject {     int test; } @end 

vs in testobj.m

@interface testobj() {     int test; } @end 

both seem equivalent me, there actual difference between declaring instance variable in header vs in implementation, if not preferred? @interface within implementation file seems way declare private properties, have other purpose outside that?

the preference place private instance variables , properties in private class extension (in .m) , leave public interface file (.h) properties , methods part of public interface.

it helps isolate implementation details public interface , makes cleaner. ensures external classes not inadvertently alter private variables of class.

see class extensions extend internal implementation.