objective c - Why NSNumber points to the same address when value are equals?

Question

Given the following code:

int firstInt, secondInt;

firstInt = 5;
secondInt = 5;

NSNumber *firstNumber = [NSNumber numberWithInt:firstInt];
NSNumber *secondNumber = [NSNumber numberWithInt:secondInt];

Why on Earth do those two NSNumber instances are pointing to the same address?

This drives me crazy!

Of course, if you change secondInt to, say '4', all works as expected.

Thanks, Jérémy

Answer 1

This is likely either a compiler optimisation or an implementation detail: as NSNumber is immutable there's no need for them be separate instances.

EDIT: probably an implementation optimisation thinking about it. Likely numberWithInt returns a singleton when called subsequently with the same integer.