Inside_NSNumber.m

#import <Foundation/Foundation.h>

@interface MyPlaceholderValue : NSObject
- (instancetype)initWithInteger:(NSInteger)number;
@end

@implementation MyPlaceholderValue

- (id)initWithInteger:(NSInteger)number {
    CFNumberRef result = CFNumberCreate(NULL, kCFNumberNSIntegerType, &number);
    
    return (id)result;
}

@end

@interface MyNumber : NSObject
- (instancetype)initWithInteger:(NSInteger)number;
@end

@implementation MyNumber

+ (instancetype)allocWithZone:(struct _NSZone *)zone {
    static id _Nullable __placeholderNumber = nil;
    
    if (!__placeholderNumber) {
        __placeholderNumber = NSAllocateObject(MyPlaceholderValue.class, 0, zone);
    }
    
    return __placeholderNumber;
}

@end


int main(int argc, const char * argv[]) {
    MyNumber *number = [[MyNumber alloc] initWithInteger:3];
    NSLog(@"%@", number); // 3
    [number release];
    NSLog(@"%@", number); // 3
    
    int count = 0;
    while (count++ < 100) {
        [number release];
    }
    
    return 0;
}