Can you explain the concept of Side Table and its application?

Side Table is an auxiliary data structure used in iOS (Objective-C and Swift) to dynamically add properties to objects without modifying their original class.

It is used when you need to extend the functionality of an existing class, for example, to add a new property to an object, but do not have the ability or desire to create a subclass.

In implementation, Side Table typically uses the mechanism of associated objects, which allows linking additional data to an object at runtime.

Example of usage in Objective-C:

#import <objc/runtime.h>

static char kAssociatedObjectKey;

@implementation NSObject (SideTable)

- (void)setCustomProperty:(NSString *)value {
    objc_setAssociatedObject(self, &kAssociatedObjectKey, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (NSString *)customProperty {
    return objc_getAssociatedObject(self, &kAssociatedObjectKey);
}

@end

Thus, Side Table allows flexible extension of objects without changing their original code.