Composite Objects in Objective-C

In Objective-C, composite objects are objects that are composed of other objects. In simple terms, one object can contain, manage, or reference other objects as its instance variables. This concept is fundamental to building complex and scalable software systems.

Composite objects are widely used in:

• User interfaces (views containing buttons, labels, images)
• Data models (a student object containing address, courses, grades)
• Games (levels containing enemies, weapons, scores)
• Application architecture (controllers managing multiple model objects)

Objective-C provides several built-in collection classes that naturally support composite object design.

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Common Types of Composite Objects

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)

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Syntax

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

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

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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

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2. NSDictionary

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

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Syntax

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

Each value is immediately followed by its corresponding key.

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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

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3. NSSet

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

Key Characteristics

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

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Syntax

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

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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)

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Comparison of Composite Collection Classes

Class	Ordered	Unique Elements	Key-Based Access
NSArray	✅ Yes	❌ No	❌ No
NSDictionary	❌ No	Keys unique	✅ Yes
NSSet	❌ No	✅ Yes	❌ No

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Why Composite Objects Matter

• 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.

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Summary

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

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