C++ Bus error when using `^=` and `<<` on a class member `unsigned long`

Question

I am trying to implement the random number generator defined in this answer. There is some ambiguity, at least from my knowledge, as to how the first line, static unsigned long x=123456789, y=362436069, z=521288629; should be implemented, since it is shown outside the function. I assumed that it was intended as a class member and implemented it thus:

class rng2{

public:    

    unsigned long x, y, z;
    rng2() : x(123456789), y(362436069), z(521288629) {}

    unsigned long xorshf96(void) {          //period 2^96-1

        //static unsigned long x=123456789, y=362436069, z=521288629;

        unsigned long t;
        x ^= x << 16;          //BUS ERROR, debug mode
        x ^= x >> 5;
        x ^= x << 1;

        t = x;
        x = y;                 //SEG FAULT, release mode
        y = z;
        z = t ^ x ^ y;

        return z;
    }

};

int main () 
{
    rng2 rand2;
    rng2 * prand;

    for(long d =0; d < 10000; d++)
        cout << "\n" << (*prand).xorshf96();
}

For some reason, this gives me errors at the noted locations, depending on which mode I compile with. However, if I comment out the member variable and the constructor and use the static variable instead, everything works. If this is the correct code, I don't see why it was shown differently at the link, and either way, I don't know why the error is happening.

Answer 1

You're using *prand, but not initializing prand.

Answer 2

rng2 * prand;

Are you perfectly sure this is the real code? Considering you are not initializing this pointer and dereferencing it later, the error is pretty obvious.

Answer 3

prand is a wild pointer.

Change:

int main () 
{
    rng2 rand2;
    rng2 * prand;

    for(long d =0; d < 10000; d++)
        cout << "\n" << (*prand).xorshf96();
}

to:

int main () 
{
    rng2 rand2;
    rng2 * prand = &rand2;

    for(long d =0; d < 10000; d++)
        cout << "\n" << (*prand).xorshf96();
}

or better just:

int main () 
{
    rng2 rand2;

    for(long d =0; d < 10000; d++)
        cout << "\n" << rand2.xorshf96();
}

Answer 4

This is because the prand pointer is never assigned but only used. When using a static the variable no datamember is accessed and that's why you do not get a bus error. You should definately assign your pointer a valid value in your main function. Like this

rng2 * prand = new rng2();