Why can't I assign this int a value?

Why can't I assign this int a value?

I have a class:
    class A
    {
    public:
        A();
        ~A();

        void IncrementNum();

    private:
        int num;
    };

    A::A() : num(0)
    {

    }

    A::~A()
    {
    }

    void A::IncrementNum()
    {
        num++;
    }

    int main()
    {
        A obj;
        obj.IncrementNum();
    }
When I set a breakpoint in the constructor, it shows that num is equal to some random value (such as -2483290483), which I take as meaning it's unassigned. And sure enough, when I call IncerementNum(), and set a breakpoint on the line after num++, it shows the exact same thing (num equals some random number). Repeated calls to IncrementNum() do not change anything, num doesn't change.
So I decided to instead change num++ to num = 1, thinking surely this would force num to be set. Nope. num still shows as being some random number even after discretely setting it to 1. Again, successive calls to the new version of IncrementNum() fail to change its value.
Any idea what could be causing this?
Other info:
I'm using Windows 7 Home Edition and Visual Studio 2010