How to assign and retrive the value in member variable? - c++

Question

I am put/get value in/from subclass B from object of base class A. But I am not able to assign or get the value. My code is:
class A
{
};
class B: A
{
string SID;
};
class C: A
{
string Name;
};
class D : A
{
string Name;
};
class E
{
A a;
UINT32 AccessLevel;
};
.......
main()
{
E e;
}
Using object of e am trying to get the value of subclass B.
I need to get the SID from the class B?
Thanks,

Answer 1

The C++11 standard 11/3 says:
Members of a class defined with the keyword class are private by default.
at 11.2/2
In the absence of an access-specifier for a base class [...] private is assumed when the class is defined with the class-key class.
and at 11.2/1:
If a class is declared to be a base
class for another class using the private access specifier, the public and protected members of the base
class are accessible as private members of the derived class.
So what does that mean? First of all:
class A {};
class B : A {};
Here A, by virtue of 11.2/2 is inherited privately. This may be okay if you want to inherit variables and you want to implement getter/setters for a variable only in a derived class, but that's usually considered bad style.
In your case however, as stated by 11/3, your members are not inherited at all because they are private members:
class A
{
public:
int a; // inherited
protected:
int b; // inherited
private:
int c; // NOT inherited
};
and especially
class A { int a; };
is equivalent to
class A { private: int a; };
So you could make your members accessable from within your derived classes by making them public or protected (see 11.2/1):
class A { public: int a; };
class B : A {}; // privately inherits a
and if you wanted to make it acessable from outside of your derived classes you will have to inherit as public as well:
class A { public: int a; };
class B : public A {}; // publicly inherits a
but that's not what you usually would do. It's considered better style to make variables private and expose only getters and setters for those:
class A
{
public:
int get_a() const { return a_; }
void set_a(int val) { a_ = val; }
private:
int a_;
};
class B : public A {}; // now publicly inherits the getters and setters
// but not a_ itself