initialization - c++ member array not initialized -

i have following constellation:

class base {...};  class derived : public base { public:   unsigned int* a;    derived(size_t num = 1) : a(0), _b(0)   {      = new unsigned int[num];      _b = new unsigned char[num];   } private:   unsigned char* _b; } 

all fine doing this:

derived* instance = new derived(); 

but doing this:

base* instance = new derived(); 

_b stays 0x0 , errors occur later when try use _b.

what happening here?

update:

the problem caused windows / linux cross platform issue - values filled incorrect pointers.

first:

why need initialize a , b twice? use explicit initialization only, or standard member initialization not both...

derived(size_t num = 1) : a(new unsigned int[num]),                            _b(new unsigned char[num]) { } 

this should trick.

second: if create instance of derived base, members , functions of derived useless. except members , functions of base, common in both classes.