Objective-C run-time is used to add a new method to an existing class.

Add a method to an existing class

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

int main(int argc, const char * argv[])
{

    @autoreleasepool {

        NSString *temp = @"world";
        int (^impyBlock)(id, int, int) = ^(id _self, int a, int b)
        {
            return a + b;
        };
        int (*impyFunct)(id, SEL, int, int) = (void*) imp_implementationWithBlock(impyBlock);
        class_addMethod([temp class], @selector(test:param:), (IMP)impyFunct, "i@:ii");
        
        int result = [temp test:1 param:2];
        NSLog(@"result = %i", result);
    }
    return 0;
}