C++ Properly deallocate multidimensional array with underlying contiguous memory

Question

I have a function which allocates memory for a 3d array. The goal is for the memory to be guaranteed contiguous but also have the convenience of a[k][j][i] syntax:

double *** allocate_3d(const int& sz, const int& sy, const int& sx){
    double * aMem = new double[sz*sy*sx];
    double *** array = new double**[sz];
    for(int k=0; k<sz; k++){
        array[k] = new double*[sy];
        for(int j=0; j<sy; j++){
            array[k][j]= aMem + k*sy*sx + j*sx;
        }
    }
    return array;
}

Supposing that I call the function like this

x   = allocate_3d(wdz,wdy,wdx);

How can I properly deallocate x? I tried the following:

void delete_3d(double *** array, const int& sz, const int& sy, const int& sx){
    for(int k=0; k<sz; k++){    
        for(int j=0; j<sy; j++){
            delete[] array[k][j];
        }
        delete[] array[k];
    }
    delete[] array;
}

But running Valgrind gives an error and seems to indicate that the memory is not deallocated properly.

Answer 1

Should actually be like this:

void delete_3d(double *** array, const int& sz, const int& sy, const int& sx) {
    // first, restore the pointer to the actual aMem
    double * aMem = array[0][0];
    // only one dimension was allocated in the loop,
    // so only one loop should be deallocating things
    for(int k = 0; k < sz; k++) {
        delete [] array[k];
    }
    delete[] array;
    delete[] aMem;
}

---

A way-better solution

Wrap std::vector to get both: contiguous memory and a reasonably simple access syntax and as a bonus no additional arrays which is better for memory, performance and maintenance:

class Array3d {
public:
    Array3d(size_t sx, size_t sy, size_t sz)
        : m_sx(sx), m_sy(sy) {
        m_array.resize(sx * sy * sz);
    }

    // Can't overload operator[] with more that one parameter
    // so we'll have to do with operator()
    double &operator()(size_t x, size_t y, size_t z) {
        size_t index = x * m_sx * m_sy + y * m_sy + z;
        return m_array[index];
    }

private:
    size_t m_sx, m_sy;
    std::vector<double> m_array;
};

Usage example:

Array3d array(3, 3, 3);
array(0, 1, 2) = 3.14;