静态与非静态变量C ++

Question

class Expression
{
    private:
       ToStringDisplay* toString;
    public:
        ......
};

   // Some code
   if (toString == NULL)
   // do something 

When I debugged, toString referred to an address, not NULL. But if I declared static ToStringDisplay* toString; to instead, toString referred to NULL ? What differ between them ?

Answer 1

as a standard in C/C++, static variables are always initialized to null if not stated otherwise. However, for nonstatic local variables there is no such a guarantee

Answer 2

Static member variables are initialized to zero according to http://www.learncpp.com/cpp-tutorial/811-static-member-variables/ where all other are undefined, unless you initialize them explicitly, so they will have whatever random value that was in memory before. For an explanation on what's the difference with static and non-static see the above link.