Bowling Game: State Machine: Std::Variant: Step 23

Checking that the rolls in a frame sum up to within range looks like this:

        struct WaitingForSecondRollWith1Bonus : SecondRollUtils
        {
            WaitingForSecondRollWith1Bonus(int first) : SecondRollUtils(first) {}
            State Update(int pins, int& score, int& frame) const
            {
                if (sumOfRolls(pins) > 10)
                    throw std::invalid_argument("sum of rolls in a frame must be <= 10");
                score += pins*2;
                ++frame;
                if (isSpare(pins))  return WaitingForFirstRollWith1Bonus{};
                                    return WaitingForFirstRollWith0Bonuses{};
            }
        };

That precondition is used twice though, so let’s extract that precondition into its own method and put it on the SecondRollUtils class, and then click next.