C++ array value randomly increases by one

Question

MultipleStack is a class the supports multiple stacks, but stores all their values in a single one-dimensional array. push(item, k) pushes the item onto the k stack. In order to do this correctly, the class stores an array top that stores the index of the top most element of every stack. push is supposed to increment top[k - 1] by one after pushing item so that the following items get pushed correctly. However, sometimes, push increases top[k - 1] by two and I am unable to figure out why.

#include <cassert>

class MultipleStack {
    int * _s;
    size_t * _top;
    size_t _nStacks, _size;

public:
    MultipleStack(size_t nStacks, size_t size) {
        _nStacks = nStacks;
        _size = size * nStacks;
        _s = new int[size];
        _top = new size_t[nStacks];
        //Simply sets top = {0, 10, 20, ...} no error here
        for (size_t i = 0; i < nStacks; i++) {
            _top[i] = (_size / _nStacks) * i;
        }
    }

    //This is the buggy method
    void push(int item, size_t k) {
        size_t beforeSet = _top[k - 1];
        _s[_top[k - 1]] = item;
        size_t afterSet = _top[k - 1];
        assert(beforeSet ==  afterSet); // Fails when beforeSet = 14
        _top[k - 1] = _top[k - 1] + 1;
    }

    static void testImplementation() {
        //Both, nStacks and size, must be >= 9
        int nStacks = 10;
        int size = 10;
        MultipleStack ms(nStacks, size);
        for (int i = 1; i <= nStacks; i++) {
            for (int j = 0; j < size; j++) {
                ms.push(1, i);
            }
        }
    }
};

int main(int argc, const char * argv[]) {
    MultipleStack::testImplementation();
    return 0;
}

I apologise for the lengthy code sample. I stripped out as much as I could, but I was unable to simply it further.

Answer 1

I see one bug in this line in c-tor:

_s = new int[size];

If i understand correctly, the _s represents the memory block for all the stacks, so it' size should be: size * nStacks instead. This possibly leads to writing outside of the allocated memory, which overwrites the subsequent data ( _top array is one of those data). Fix the _s allocation and you should be good.

And next time try naming your variables in a more careful way, so you don't mistake size and _size so easily. This would not have been an issue if you renamed _size -> _sizeOfAllStacks or something along those lines.