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Objective-C Cheatsheet

A quick reference for Objective-C syntax, data types, control structures, and object-oriented programming concepts. This cheat sheet provides a concise overview for both beginners and experienced developers.

Fundamentals

Basic Syntax

Header File

#import <Foundation/Foundation.h>

Implementation File

#import "YourClass.h"

Comments

// Single-line comment
/* Multi-line comment */

NSLog

NSLog(@"Hello, World!");

Variables

int age = 30;
NSString *name = @"John";

Data Types

Integer

int age = 30;

Float

float price = 99.99;

Double

double pi = 3.14159265359;

Boolean

BOOL isTrue = YES; (YES or NO)

Character

char initial = 'J';

NSString

NSString *message = @"Hello, Objective-C!";

Operators

Arithmetic

+, -, *, /, %

Assignment

=, +=, -=, *=, /=, %=

Comparison

==, !=, >, <, >=, <=

Logical

&&, ||, !

Bitwise

&, |, ^, ~, <<, >>

Control Structures

Conditional Statements

If Statement

if (condition) {
    // Code to execute if condition is true
}

If-Else Statement

if (condition) {
    // Code if condition is true
} else {
    // Code if condition is false
}

Else If Statement

if (condition1) {
    // Code if condition1 is true
} else if (condition2) {
    // Code if condition2 is true
} else {
    // Code if all conditions are false
}

Switch Statement 

switch (variable) {
    case constant1:
        // Code to execute if variable == constant1
        break;
    case constant2:
        // Code to execute if variable == constant2
        break;
    default:
        // Code to execute if variable doesn't match any constant
        break;
}

Looping

For Loop

for (int i = 0; i < 10; i++) {
    // Code to be executed
}

While Loop

while (condition) {
    // Code to be executed while condition is true
}

Do-While Loop

do {
    // Code to be executed at least once
} while (condition);

For-In Loop (Fast Enumeration)

NSArray *array = @[@"apple", @"banana", @"cherry"];
for (NSString *item in array) {
    NSLog(@"%@", item);
}

Object-Oriented Programming

Classes

Defining a Class (.h file)

@interface MyClass : NSObject
{
    // Instance variables
    int myInteger;
    NSString *myString;
}

// Methods
- (void)myMethod;
+ (int)myClassMethod;

@end

Implementing a Class (.m file)

#import "MyClass.h"

@implementation MyClass

- (void)myMethod {
    NSLog(@"Instance method called");
}

+ (int)myClassMethod {
    return 42;
}

@end

Properties

Defining Properties in .h file

@interface MyClass : NSObject

@property (nonatomic, strong) NSString *name;
@property (nonatomic, assign) int age;

@end

Synthesizing Properties in .m file (Prior to Xcode 4.4)

@implementation MyClass

@synthesize name, age;

@end

Accessing Properties

MyClass *myObject = [[MyClass alloc] init];
myObject.name = @"John";
int age = myObject.age;

Methods

Instance Method

-(returnType)methodName:(parameterType)parameterName;
Example: -(void)printName:(NSString *)name;

Class Method

+(returnType)methodName:(parameterType)parameterName;
Example: +(MyClass*)createObject;

Method Implementation

 
-(void)printName:(NSString *)name {
    NSLog(@"Name: %@", name);
}

Protocols

Defining a Protocol

@protocol MyProtocol

- (void)myMethod;
@optional
- (void)optionalMethod;

@end

Adopting a Protocol

@interface MyClass : NSObject <MyProtocol>

@end

@implementation MyClass

- (void)myMethod {
    NSLog(@"Method implemented from MyProtocol");
}

@end

Memory Management

Automatic Reference Counting (ARC)

ARC automates memory management by tracking object ownership using reference counting. The compiler inserts retain and release calls as needed.

To enable ARC, set the compiler flag -fobjc-arc.

When ARC is enabled, you no longer need to manually call retain, release, or autorelease.

Strong and Weak References

Strong Reference

A strong reference increases the retain count of an object. It keeps the object alive as long as the strong reference exists.
@property (nonatomic, strong) NSObject *myObject;

Weak Reference

A weak reference does not increase the retain count. When the object is deallocated, the weak reference is automatically set to nil. Used to prevent retain cycles.
@property (nonatomic, weak) NSObject *myObject;

Unretained Reference

An unretained reference is similar to a weak reference, but it is not set to nil when the object is deallocated. Accessing an unretained reference to a deallocated object will result in a crash (dangling pointer).
@property (nonatomic, unsafe_unretained) NSObject *myObject; (Use with caution.)

Autorelease Pool

An autoreleasepool is a mechanism to defer the release of objects. Objects sent the autorelease message are added to the pool and released when the pool is drained.

In ARC, you rarely need to use @autoreleasepool blocks directly, as the system often manages them for you (e.g., in the main event loop).

Example:

@autoreleasepool {
    // Code that creates autoreleased objects
}
// Objects are released when the pool is drained
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