gistfile1.m

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

@interface MyDecimalNumber : NSDecimalNumber

@end

@implementation MyDecimalNumber


-(NSDecimal)decimalValue
{
    NSLog(@"%@ %@", NSStringFromSelector(_cmd), self);
    return [super decimalValue];
}
@end


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

    @autoreleasepool {
        short exp = -15;
        NSArray *numbers = @[ [NSDecimalNumber decimalNumberWithMantissa:1 exponent:exp isNegative:NO],
                              [NSDecimalNumber decimalNumberWithMantissa:2 exponent:exp isNegative:NO],
                              [NSDecimalNumber decimalNumberWithMantissa:3 exponent:exp isNegative:NO]];
        [numbers enumerateObjectsUsingBlock:^(NSDecimalNumber *obj, NSUInteger idx, BOOL *stop) {
            object_setClass(obj, [MyDecimalNumber class]);
        }];
        
        
        NSDecimalNumber *sum = [numbers valueForKeyPath:@"@sum.self"];
        NSLog(@"%@", sum);
        
    }
    return 0;
}