Objective-C 中的可选参数？

typedef NS_ENUM(NSInteger, CalculatorOperation) {
    CalculatorOperationAdd = 0,
    CalculatorOperationSubtract,
    CalculatorOperationMultiply,
    CalculatorOperationDivide
};

typedef NS_OPTIONS(NSInteger, CalculatorOptions) {
    CalculatorOptionsNone = 0,
    CalculatorOptionsRoundResult = 1 << 0
};

- (double)calculateWithFirstNumber:(double)firstNumber
                            secondNumber:(double)secondNumber
                                 operation:(CalculatorOperation)operation
                                   options:(CalculatorOptions)options;

- (double)calculateWithFirstNumber:(double)firstNumber
                            secondNumber:(double)secondNumber
                                 operation:(CalculatorOperation)operation
                                   options:(CalculatorOptions)options {
    double result;
    switch (operation) {
        case CalculatorOperationAdd:
            result = firstNumber + secondNumber;
            break;
        case CalculatorOperationSubtract:
            result = firstNumber - secondNumber;
            break;
        case CalculatorOperationMultiply:
            result = firstNumber * secondNumber;
            break;
        case CalculatorOperationDivide:
            result = firstNumber / secondNumber;
            break;
        default:
            return NAN;
    }
    if (options & CalculatorOptionsRoundResult) {
        result = round(result);
    }
    return result;
}