C++ separation data and logic

The reason why I want to separate in such way - data depends from nothing, methods can have hundred dependencies. All, who depends from data shouldn`t depends from methods dependencies.

I want to do something like:

//s_class_name.h
struct structName
{
    data m_data
}

---

//class_name.h
class className : public structName
{
public:
    void method(arg1, arg2, arg3, ..., argN); //using m_data
}

---

static_cast<className *>(&structNameObject)->method(arg1, arg2, arg3, ..., argN);

There are I see two problems:

1) All acces to data is opened. I want to open data to all, who operate with struct, but not with class.

2) Static cast downcasting is undefined.

Could you explain me, how to realize pattern data&logic separation better?