Interview Bite

Despite Swift’s dominance, Objective-C remains crucial for maintaining legacy iOS/macOS apps. Over 20% of existing iOS apps still use Objective-C, making it a valuable skill for developers working with older codebases at companies like Apple, Facebook, and Bloomberg.

This comprehensive guide covers:

• Core Objective-C concepts
• Memory management
• Runtime features
• Comparison with Swift
• Common pitfalls

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30 Essential Objective-C Interview Questions

1. What is Objective-C and why is it still used?

Answer:
Objective-C is:

• A superset of C with Smalltalk-style messaging
• The original language for macOS/iOS development
• Still used for:
  • Maintaining legacy apps
  • Interoperating with Swift via bridging
  • Low-level performance-critical code

2. Explain the difference between #import and #include

Answer:

objective-c

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#import "Header.h"  // Prevents duplicate inclusions (recommended)
#include "Header.h" // May cause duplicates (C-style)

3. What are categories in Objective-C?

Answer:
Categories add methods to existing classes without subclassing:

objective-c

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@interface NSString (MyCategory)
- (BOOL)isValidEmail;
@end

4. How does message passing work in Objective-C?

Answer:
Unlike C++’s direct method calls, Objective-C uses dynamic dispatch:

objective-c

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[object message:param]; // Compiled to objc_msgSend(object, @selector(message:), param)

5. What is the difference between atomic and nonatomic properties?

Answer:

Atomic	Nonatomic
Thread-safe (slower)	Not thread-safe (faster)
Default for @property	Common in iOS development

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Memory Management

6. Explain Manual Retain-Release (MRR) vs ARC

Answer:

objective-c

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// MRR (pre-2011)
NSObject *obj = [[NSObject alloc] init]; // retainCount = 1
[obj retain]; // retainCount = 2
[obj release]; // retainCount = 1

// ARC (automatic)
NSObject *obj = [[NSObject alloc] init]; // Compiler inserts releases

7. What is a retain cycle and how do you break it?

Answer:
Occurs when two objects hold strong references to each other. Solutions:

objective-c

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@property (weak) id delegate; // Use weak for delegates
__weak typeof(self) weakSelf = self; // In blocks

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Runtime Features

8. What is the Objective-C runtime?

Answer:
A C library that enables:

• Dynamic method resolution
• Method swizzling
• Introspection (class inspection at runtime)

9. How would you implement method swizzling?

Answer:

objective-c

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Method original = class_getInstanceMethod([self class], @selector(viewDidLoad));
Method swizzled = class_getInstanceMethod([self class], @selector(swizzled_viewDidLoad));
method_exchangeImplementations(original, swizzled);

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Comparison with Swift

10. Compare Objective-C protocols vs Swift protocols

Answer:

Feature	Objective-C	Swift
Optional methods	Yes	No (use @objc)
Protocol extensions	No	Yes
Associated types	No	Yes

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Advanced Concepts

11. What are class clusters?

Answer:
A design pattern where a public abstract class (e.g., NSArray) hides private concrete subclasses:

objective-c

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NSArray *arr = @[@1, @2]; // Actually creates __NSArrayI

12. Explain toll-free bridging

Answer:
Seamless casting between Core Foundation and Foundation types:

objective-c

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CFArrayRef cfArray = (__bridge CFArrayRef)nsArray;
NSArray *nsArray = (__bridge NSArray*)cfArray;

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People Also Ask

Q1: Is Objective-C dead in 2024?

A: No! While Swift is preferred for new projects, Objective-C is still maintained and used in:

• Legacy codebases (e.g., banking apps)
• Low-level frameworks
• Projects requiring C interoperability

Q2: Should I learn Objective-C or Swift first?

A: Learn Swift first for new projects, but Objective-C is valuable for:

• Maintaining older apps
• Understanding Apple’s legacy APIs
• Working with mixed codebases

Q3: What’s the salary for Objective-C developers?

A: In the US:

• Junior: 85K–85K–120K
• Senior: 130K–130K–200K
• Legacy system specialists: Up to $250K

Q4: How do Objective-C and Swift interoperate?

A: Through:

• Bridging headers (MyProject-Bridging-Header.h)
• @objc annotations in Swift
• Framework targets with DEFINES_MODULE = YES

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Conclusion

Mastering Objective-C interview questions requires understanding:

1. Memory management (ARC vs MRR)
2. Runtime features (method swizzling, messaging)
3. Legacy patterns (class clusters, categories)
4. Swift interoperability