Question

I am in the beginning stages of writing a fairly large code. I have defined one class as such:

class GPUMD {
    private:
        double xhi, xlo, yhi, ylo, zhi, zlo;
        int numAtoms;
        Atom *atoms;
    public:
        GPUMD();
        ~GPUMD();
};

The destructor is defined as below:

GPUMD::~GPUMD() {
    if(atoms != NULL)
        delete [] atoms;
}

Right now, the code does this:

int main(int argc, char *argv[]) {
    GPUMD gpumd;
    exit(0);
}

I get a glibc detected error: trying to free invalid pointer. Using valgrind, I see that this error traces to my destructor for GPUMD. For some reason, the atoms != NULL test is returning true even though I have not assigned anything to that pointer. Why is that?

EDIT: The constructor is defined as:

GPUMD::GPUMD() {}

Answer 1

Because atoms has not been explicitly initialised to NULL or a valid pointer in the constructor. Change constructor to:

GPUMD::GPUMD() : numAtoms(0), atoms(NULL) {}

Note that the atoms != NULL check prior to the delete[] is superfluous as delete[], or delete, on a NULL pointer is no-op. The following is safe:

GPUMD::~GPUMD() {
    delete [] atoms;
}

As there is a dynamically allocated member in GPUMD you need to prevent copying of instances of GPUMD or implement the assignment operator and copy constructor (see What is The Rule of Three?).

As C++, consider using a vector<Atom> (or a vector of smart pointers) instead which will manage dynamic memory for you.