instance-variables

I have read Where to put iVars in "modern" Objective-C? and some other questions, but I am still confused. I am reading from https://www.raywenderlich.com/913/sqlite-tutorial-for-ios-making-our-app SQLite Tutorial: .h #import <Foundation/Foundation.h> #import <sqlite3.h> @interface FailedBankDatabase : NSObject { sqlite3 *_database; } + (FailedBankDatabase*)database; - (NSArray *)failedBankInfos; @end .m #import "FailedBankDatabase.h" #import "FailedBankInfo.h" @implementation FailedBankDatabase static FailedBankDatabase *_database; + (FailedBankDatabase*)database { if (_database == nil) {

I've noticed that a lot of Apple's interfaces use @private before their instance variable declarations. Is there a good design reason for this, and is it something I should do? Private instance variables are used to separate interface from implementation. In Objective-C, since the class declaration must show all of the instance variables, there needs to be a way to prevent subclasses from accessing the ones that are part of the internal implementation. Otherwise, other programmers could write code that depends on those internal variables, which would make it impossible for the class designer

Is there any difference between declaring a private instance variable in the header vs declaring it in the implementation? in TestObj.h @interface TestObj : NSObject { int test; } @end vs in TestObj.m @interface TestObj() { int test; } @end Both seem equivalent to me, is there any actual difference between declaring an instance variable in the header vs in the implementation, if not which is preferred? The @interface within the implementation file just seems like a way to declare private properties, does it have any other purpose outside that? The preference is generally to place private

Can someone explain how a class can access the instance variables of its superclass and how that is not inheritance? I'm talking about 'The Ruby Programming Language' and the example class Point def initialize(x,y) # Initialize method @x,@y = x, y # Sets initial values for instance variables end end class Point3D < Point def initialize(x,y,z) super(x,y) @z = z end def to_s "(#@x, #@y, #@z)" # Variables @x and @y inherited? end end Point3D.new(1,2,3).to_s => "(1, 2, 3)" How can class Point3D access x and y inside to_s if they're not inherited? The book says: "The reason that they sometimes

I want to initialize an instance variable within my Rails model that will hold an array and I want to access this variable in other methods within my model. I tried this: class Participant < ActiveRecord::Base @possible_statuses = [ 'exists', 'paired', 'quiz_finished', 'quiz_results_seen', 'money_sent' ] def statuses @possible_statuses end But when I tried the following using rails console: Participant.first.statuses I am returned nil :( Why does this happen? Is there a way to accomplish what I am trying to accomplish? I would recommend using a constant for this kind of cases: class