Composite Objects in Objective-C

1. NSArray

NSArray represents an ordered collection of objects. Each object is stored at a specific index, starting from 0.

Key Characteristics

• Ordered
• Immutable (cannot be modified after creation)
• Stores only Objective-C objects (not primitive types directly)

Syntax

NSArray *array = [[NSArray alloc] initWithObjects:object1, object2, object3, nil];

• alloc → allocates memory
• initWithObjects → initializes the array
• nil → marks the end of the list

Example: NSArray

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // Create an array of fruit names
        NSArray *fruits = @[@"Apple", @"Mango", @"Banana", @"Grapes"];

        // Display each fruit in the array
        for (NSString *fruit in fruits) {
            NSLog(@"%@", fruit);
        }
    }
    return 0;
}

Approach

• An NSArray named fruits stores four string objects.
• The fast enumeration loop iterates through the array.
• Each element is printed using NSLog.

Output

Apple
Mango
Banana
Grapes

NSDictionary stores data in key–value pairs, similar to a map or hash table.

Key Characteristics

• Keys are unique
• Values are accessed using keys
• Keys must conform to the NSCopying protocol
• Unordered collection

Syntax

NSDictionary *dictionary = [[NSDictionary alloc] initWithObjectsAndKeys:
                            value1, key1,
                            value2, key2,
                            value3, key3,
                            nil];

Each value is immediately followed by its corresponding key.

Example: NSDictionary

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // Create a dictionary of student details
        NSDictionary *studentInfo = [[NSDictionary alloc] initWithObjectsAndKeys:
                                      @"John", @"name",
                                      @"21", @"age",
                                      @"Computer Science", @"major",
                                      nil];

        // Access values in the dictionary
        NSString *name = [studentInfo objectForKey:@"name"];
        NSString *age = [studentInfo objectForKey:@"age"];
        NSString *major = [studentInfo objectForKey:@"major"];

        // Print the values
        NSLog(@"Name: %@", name);
        NSLog(@"Age: %@", age);
        NSLog(@"Major: %@", major);
    }
    return 0;
}

Approach

• A dictionary named studentInfo stores student attributes.
• Keys (name, age, major) uniquely identify values.
• Values are retrieved using objectForKey:.

Output

Name: John
Age: 21
Major: Computer Science

NSSet stores a collection of unique objects with no defined order.

Key Characteristics

• Unordered
• No duplicate elements
• Fast membership checks
• Ideal when uniqueness matters

Syntax

NSSet *mySet = [NSSet setWithObjects:obj1, obj2, obj3, nil];

Example: NSSet

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // Create a set of vehicle types
        NSSet *vehicles = [NSSet setWithObjects:@"Car", @"Bike", @"Truck", nil];

        // Display each vehicle in the set
        for (NSString *vehicle in vehicles) {
            NSLog(@"%@", vehicle);
        }

        // Check if a specific object exists in the set
        BOOL containsBike = [vehicles containsObject:@"Bike"];
        if (containsBike) {
            NSLog(@"The set contains Bike");
        } else {
            NSLog(@"The set does not contain Bike");
        }
    }
    return 0;
}

Approach

• An NSSet named vehicles stores unique vehicle names.
• Order is not guaranteed when iterating.
• containsObject: is used for fast membership testing.

Output

Car
Bike
Truck
The set contains Bike

(Order may vary)

• Enable modular design
• Improve code reusability
• Model real-world relationships naturally
• Simplify complex object structures

Composite objects form the backbone of Objective-C application design and are heavily used in MVC architecture, data modeling, and UI development.

Composite objects allow Objective-C objects to contain and manage other objects efficiently.

• NSArray → ordered collections
• NSDictionary → key-value relationships
• NSSet → unique unordered collections