How to fix EXC_BAD_ACCESS (code=1, address=0x0) error?

Question

C++ / C#

I want to make merge sort visualization in SFML. I have already made a visualization of quick sort, everything works fine there, and the actions seem to be the same. I have an array of structures, the structure looks like this:

struct elem2
{
    sf::RectangleShape shape;
    int value;
};

Here is the basic sort function:

void merge_sort (elem2 a[], int p, int r)
{
    if (p < r)
    {
        int q = (p + r)/2;
        merge_sort(a, p, q);
        merge_sort(a, q + 1, r);
        merge(a, p, q, r);
    }
}

it seems nothing unusual, recursion is also used in quick sorting, everything works there. At the very beginning of the auxiliary function, the first time it is called, everything crashes with an EXC_BAD_ACCESS (code=1, address=0x0) error, which, it seems, usually says that I went beyond the bounds of the array. It flies out here:

int merge (elem2* a, int p, int q, int r)
{
    elem2* pq = (elem2*)malloc(sizeof(elem2) * ((q - p) + 1));
    elem2* qr = (elem2*)malloc(sizeof(elem2) * (r - q));
    sf::Vector2f pos;
    
    int j = p;
    for (int i = 0; i < q - p + 1; i++)
    {
        pq[i] = a[j];
        j++;
    }
    
    j = q + 1;
    for (int i = 0; i < r - q; i++)
    {
        qr[i] = a[j];
        j++;
    }
...

At the first call, space is allocated for two arrays with a length of one element, I checked through the debugger, everything is in order with the indexes, nothing goes beyond the boundaries. At the same time, it crashes in the first cycle, then in the second, more often in the first. Although, in fact, there seems to be nothing strange - we just copy the contents from this array to the newly created one. Next, I tried changing the structure to something like this:

struct elem2
{
    int shape;
    int value;
};

Everything worked. At the same time, the problem is precisely with the shapes, that is, for example, sf::CircleShape also does not work, but if you enter sf::Vector2f there, then everything starts. What could be the problem?

Answer 1

You "create" the sf::RectangleShape class via malloc as part of temporary pq, qr arrays. It is filled with garbage. Some of its internal invariants are violated. There can be a lot of problems: starting from the fact that the assignment operator is redefined there, which should clean up something somewhere, ending with the virtual method table of the class being corrupted. Any attempt to do anything with such a class instance is most likely undefined behavior.
If you already use classes from C ++, then you need to allocate memory through new[], and not malloc. Then the class instance will be created normally and the constructor will be called.
One would think that memmove would solve your problem - but again, this violates internal invariants. Any unique_ptr inside will no longer be unique. Such operations with memory can be done only for POD (Plain old data) - structures consisting of structures, arrays and basic types.
Or, better yet, use vector for temporary arrays. Then you don't have to worry about freeing memory. Also, instead of copying, you can use std::move, it may work faster if these sf classes have more efficient move operators.