Incremented variable "never used"?

Question

I'm kind of inexperienced with C++, and I'm converting a program that I wrote in C to C++. I have a RollDice function that takes numbers that I read in from a text file and uses them to generate the number. This is the function in C:

void rollDice(Move *move, GameState *game_state) {
    int diceNum1 = 0;
    int diceNum2 = 0;
    int randomNumber1 = 0;
    int randomNumber2 = 0;
    randomNumber1 = game_state->randomNums[game_state->current_roll]; //gets the random number from the array randomNum (which holds the numbers from the text file), at index "current_roll"
    game_state->current_roll++; //increments so the next random number will be the next number in the array
    diceNum1 = 1 + (randomNumber1 % (1 + 6 - 1));
    randomNumber2 = game_state->randomNums[game_state->current_roll];
    game_state->current_roll++;
    diceNum2 = 1 + (randomNumber2 % (1 + 6 - 1));
    move->dice_sum = diceNum1 + diceNum2;
    printf("You rolled a %d!\n", move->dice_sum);
}

This works just how I want it to when I run it. Now, when converting my program to C++ I had to change things around. My parameters are now pass by reference and I made a vector to store the list of random numbers from the text file:

void rollDice(Move& move, GameState& game_state) {
    std:: vector<int> randomNums = game_state.getRandomNums();
    int current_roll = game_state.getCurrentRoll();
    int diceNum1 = 0;
    int diceNum2 = 0;
    int randomNumber1 = 0;
    int randomNumber2 = 0;
    randomNumber1 = randomNums.at(current_roll);
    current_roll++;
    diceNum1 = 1 + (randomNumber1 % (1 + 6 - 1));
    randomNumber2 = randomNums.at(current_roll);
    current_roll++;   //this line is grayed out and says "this value is never used"
    diceNum2 = 1 + (randomNumber2 % (1 + 6 - 1));
    move.dice_sum = diceNum1 + diceNum2;
    std:: cout << "You rolled a " << move.dice_sum << "!\n";
}

My code is telling me that the second time I increment current_roll it is unused. This didn't happen for my C code, so why is it happening here and how can I fix it? I'm completely lost.

Answer 1

It's never used because you write to the variable, but never read from it. Having a variable that you never read is effectively meaningless.

Presumably your game_state.getCurrentRoll function returns an integer, when you store this, you store the value (rather than a reference to the value), thus incrementing it doesn't increment the current roll inside the game_state, instead you should add a function to your game_state called makeRoll for example which increments the game_states internal current_roll value.

This is different from your C code which increments the current_roll value directly using game_state->current_roll++ (alternatively you could make game_state.current_roll public and increment it the same way as in your C code).

From your comment I assume you have some class:

class GameState {
private:
    int current_roll;
    ...
public:
    int getCurrentRoll() {
        return current_roll;
    }
    ...
}

All you'd need to do is add another function to your class to increment the current_roll:

class GameState {
private:
    int current_roll;
    ...
public:
    int getCurrentRoll() {
        return current_roll;
    }
    void makeRoll() {
        current_roll++;
    }
    ...
}

Then you can call it as normal.

---

Regarding your new issue in the comments regarding the error:

parameter type mismatch: Using 'unsigned long' for signed values of type 'int'.

This is because the signature of at is std::vector::at( size_type pos ); That is, it expects a value of type size_type which is an unsigned integer type, rather than int as you're using which is signed. This post may be helpful.