initializing array of variable size inside a class

Question

I am trying to initialize an array of size n based off the input argument of my constructor. This works:

//Inside Header
class runningAverage{
    private:
        byte n;
        float array[10];
    public:
        runningAverage(byte);
};

//Inside .cpp
runningAverage::runningAverage(byte a){
    n = a;
    for (byte i = 0; i<n; i++) {
        array[i] = 0;
    }
}

and this does not work:

//Inside Header
class runningAverage{
    private:
        byte n;
        float array[];
    public:
        runningAverage(byte);
};

//Inside .cpp
runningAverage::runningAverage(byte a){
    n = a;
    for (byte i = 0; i<n; i++) {
        array[i] = 0;
    }
}

I want to initialize the array so that is the size specified by n. This way I don't waste memory by arbitrarily specifying float array[256] or something like that. Any help is appreciated!

Answer 1

You have to actually allocate the array; and you'll want to use a pointer type, float array[] is not what you think there. As juanchopanza reminds us, you'll also want to either disable the copy constructor and assignment operator, or implement ones that do a proper deep copy.

//Inside Header
class runningAverage{
    private:
        byte n;
        float *array; // <= correct type
    public:
        runningAverage(byte);
        ~runningAverage(); // <= you'll need a destructor to cleanup
    private:
        runningAverage(const runningAverage &);
        runningAverage & operator = (const runningAverage &);
};

//Inside .cpp
runningAverage::runningAverage(byte a){
    array = new float[n]; // <= allocate array
    n = a;
    for (byte i = 0; i<n; i++) {
        array[i] = 0;
    }
}

// clean up
runningAverage::~runningAverage(){
    delete[] array;
}

However, if you have some dynamic, automatic container at your disposal (e.g. std::vector) you might want to use that instead - then you don't have to deal with copy / assignment / destructor / memory management.

Answer 2

If you only know the size at runtime, Jason C's answer is what you want. ( With juanchopanzas comment )

If the size is known at compile time, you can use templates:

template < int SIZE >
class runningAverage
{
    float array [ SIZE ];
};

runningAverage < 10 > ra;

Or use classes like std::array instead of std::vector.