Example

class Base {
public:
void fnc();
};

class Derived: public Base {
public:
void fnc(){}
};

int main(){
Derived d;

Base *basePtr = &d;
Derived *derivedPtr = &d;

basePtr->fnc();//calls Base::fnc()
derivedPtr->fnc(); //calls Derived::fnc()

return 0;
}

Solution

Do not override non-virtual functions. Change your design in order not to use the base class, or make the base class function virtual to ensure polymorphic behaviour.

class Base {
public:
virtual void fnc();
};

class Derived: public Base {
public:
void fnc(){}
};

int main(){
Derived d;

Base *basePtr = &d;
Derived *derivedPtr = &d;

basePtr->fnc();//calls Derived::fnc()
derivedPtr->fnc(); //calls Derived::fnc()

return 0;
}