objective-c

Objective-C

Objective-C is in maintenance mode. New features are rare, but it powers the Apple ecosystem's foundation. It allows mixing C++ (.mm) loosely.

When to Use

Legacy iOS and macOS Codebase Maintenance: Maintaining established Apple applications built before Swift.
C/C++ and Swift Bridging (Objective-C++): Acting as a high-performance interoperability bridge between native C++ engines and modern Swift.
Dynamic Runtime Introspection: Leveraging the dynamic Objective-C runtime for method swizzling and dynamic message forwarding.
High-Performance Audio & Core Audio Frameworks: Interfacing with low-level Apple media frameworks where C pointer interop is required.

Quick Start

#import <Foundation/Foundation.h>

@interface Person : NSObject @property (nonatomic, copy) NSString *name; @property (nonatomic, assign) NSInteger age;

(void)sayHello; @end

@implementation Person

(void)sayHello { NSLog(@"Hello, my name is %@", self.name); } @end


## Core Concepts

### Dynamic Message Passing (`[receiver message]`)

Method calls in Objective-C are dynamic messages resolved at runtime via `objc_msgSend`:

```objc
#import <Foundation/Foundation.h>

@interface OrderService : NSObject
- (BOOL)processPayment:(double)amount forCustomer:(NSString *)customer;
@end

@implementation OrderService
- (BOOL)processPayment:(double)amount forCustomer:(NSString *)customer {
    NSLog(@"Charging %@: $%.2f", customer, amount);
    return amount > 0.0;
}
@end

// Invocation syntax
OrderService *service = [[OrderService alloc] init];
[service processPayment:150.0 forCustomer:@"Alice"];

Automatic Reference Counting (ARC) & Nullability Annotations

Manages object memory automatically at compile time with strong/weak ownership semantics:

@property (nonatomic, strong) NSString *accountNumber;
@property (nonatomic, weak) id<OrderDelegate> delegate; // Prevents retain cycles
@property (nonatomic, copy) void (^completionHandler)(BOOL success);

Objective-C++ (`.mm`) Bridging

Seamlessly mixes C++ standard library types with Cocoa objects in the same file:

// ProcessorBridge.mm (Objective-C++)
#include <vector>
#import "ProcessorBridge.h"

@implementation ProcessorBridge {
    std::vector<double> _signalBuffer;
}
- (void)addSample:(double)sample {
    _signalBuffer.push_back(sample);
}
@end

Common Patterns

Safe Block Self-Capture to Prevent Retain Cycles

Problem: Strong reference cycles in asynchronous blocks cause permanent memory leaks under ARC.

Solution: Use the weakSelf / strongSelf dance in completion blocks:

__weak typeof(self) weakSelf = self;
[self.networkClient fetchProfileWithCompletion:^(NSDictionary *data) {
    __strong typeof(weakSelf) strongSelf = weakSelf;
    if (!strongSelf) return;
    [strongSelf updateUIWithData:data];
}];

Best Practices

Do:

Add Nullability Annotations (_Nonnull, _Nullable): Ensure smooth, idiomatic bridging into modern Swift codebases.
Use Weak References for Delegates: Mark all delegate properties as weak to eliminate memory retain cycles.
Use Objective-C Generics: Parameterize collections (NSArray<NSString *> *) for compile-time type safety.
Wrap Bridged Code with Modern Swift Interfaces: Plan incremental migrations by creating Swift wrapper classes.

Don't:

Start new greenfield Apple applications in Objective-C: Build all new iOS/macOS applications in Swift and SwiftUI.
Invoke methods on deallocated objects without weak-strong dancing: Use __weak typeof(self) weakSelf = self; in asynchronous blocks.
Use manual retain/release (MRR): Ensure modern ARC (-fobjc-arc) is enabled across all targets.

Troubleshooting

Error	Cause	Solution
`unrecognized selector sent to instance ...`	Invoking a method not implemented by the receiving object.	Verify method name and colons (e.g. `doAction:` vs `doAction`), or check with `respondsToSelector:`.
`EXC_BAD_ACCESS (code=1, address=...)`	Dereferencing deallocated memory or zombie object.	Enable Zombie Objects in Xcode scheme diagnostics to locate deallocated reference.
`Property with 'copy' attribute is not copied`	Custom setter failed to call `[newValue copy]`.	Implement custom setters using `_prop = [newValue copy];`.

References

Apple Developer Documentation