main.m

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        Queue<NSString *> *stringQueue = [[Queue alloc] init];
        [stringQueue enqueue:@"abc"]
        
        Queue<NSNumber *> *numberQueue = [[Queue alloc] init];
        [numberQueue enqueue:@123];
        
        [numberQueue enqueue:@"abc"];		// Warning: Incompatible pointer types sending 'NSString *' to parameter of type 'NSNumber *'
    }
    
    return 0;
}