Why ivars are safer than private properties for private data.

PlaygroundTests.m

//
//  PlaygroundTests.m
//  PlaygroundTests
//
//  Created by Borders, Heath on 3/9/16.
//  Copyright © 2016 Twitch. All rights reserved.
//

#import <XCTest/XCTest.h>

@interface Foo : NSObject

- (void)populate:(NSMutableArray<NSString *> * _Nonnull)array;

@end

@interface Bar : Foo

- (void)populate:(NSMutableArray<NSString *> * _Nonnull)array;

@end

@interface PlaygroundTests : XCTestCase

@end

@implementation PlaygroundTests

- (void)testSameNameIvar
{
    NSMutableArray<NSString *> * _Nonnull fooArray = [NSMutableArray<NSString *> new];
    Foo * _Nonnull foo = [Foo new];
    [foo populate:fooArray];
    XCTAssertEqualObjects(fooArray,
                          ([NSArray<NSString *> arrayWithObjects:
                            @"safeFoo",
                            @"unsafeFoo",
                            nil]));
    NSMutableArray<NSString *> * _Nonnull barArray = [NSMutableArray<NSString *> new];
    Bar * _Nonnull bar = [Bar new];
    [bar populate:barArray];
    XCTAssertEqualObjects(barArray,
                          ([NSArray<NSString *> arrayWithObjects:
                            @"safeFoo",
                            @"unsafeFoo", // this will actually be @"unsafeBar" because Bar.unsafe tramples Foo.unsafe
                            @"safeBar",
                            @"unsafeBar",
                            nil]));
}

@end

@interface Bar ()

@property (nonatomic) NSString * _Nonnull unsafe;

@end

@implementation Bar
{
    NSString * _Nonnull _safe;
}

- (instancetype _Nonnull)init
{
    self = [super init];
    if (self)
    {
        _safe = @"safeBar";
        self.unsafe = @"unsafeBar";
    }
    
    return self;
}

#pragma mark - Public API

- (void)populate:(NSMutableArray<NSString *> *)array
{
    [super populate:array];
    
    [array addObject:_safe];
    [array addObject:self.unsafe];
}

@end

@interface Foo ()

@property (nonatomic) NSString * _Nonnull unsafe;

@end

@implementation Foo
{
    NSString * _Nonnull _safe;
}

- (instancetype _Nonnull)init
{
    self = [super init];
    if (self)
    {
        _safe = @"safeFoo";
        self.unsafe = @"unsafeFoo";
    }
    return self;
}

#pragma mark - Public API

- (void)populate:(NSMutableArray<NSString *> *)array
{
    [array addObject:_safe];
    [array addObject:self.unsafe];
}

@end

PlaygroundTests.m: test failure: -[PlaygroundTests testSameNameIvar] failed: ((barArray) equal to (([NSArray<NSString *> arrayWithObjects: @"safeFoo", @"unsafeFoo", @"safeBar", @"unsafeBar", nil]))) failed: ("(
    safeFoo,
    unsafeBar,
    safeBar,
    unsafeBar
)") is not equal to ("(
    safeFoo,
    unsafeFoo,
    safeBar,
    unsafeBar
)")