Difference between declaring an ivar in @interface and putting variable in @implementation?

The difference between using an ivar and declaring a variable inside the implementation is that the variable within the implementation is at file scope. That means all instances (and any static functions) will share the same variable; i.e. If one instance of your object changes the variable, it will change it for all instances.

As far as I know, putting a variable declaration inside the @implementation is no different from putting it outside the implementation: It's not an ivar, it's just a variable declared at file scope.

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