In Objective-C, we can put the instance variables in the @implementation section?

I have always put the instance variables in the interface .h file, but just saw some code doing this:

@implementation Fraction  {
     int numerator;
     int denominator; 
 }
 

which is to move the instance variables to the implementation side. It works too, but since when can we do it (or can we always do it with Objective-C), and I suppose it is better because what instance variables we use is not really part of an interface.

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