Inherit member variables?

[Joseph Turian]

I'm having a little difficultly with inheritance.
I want to have a class "SubT" inherit from class "T".
SubT contains all the information in T, plus a little more based upon
the member variables of T.

For efficiency, I cannot merely have a T object in SubT.
What I'd like is something as follows:
class T {
T(int a) : _a(a) {}
protected:
int _a;
};

class SubT : public T {
SubT(int a) : _a(a), _b(a+a) {}
SubT(T foo) : _a(foo._a), _b(foo._a+foo._a) {}
public:
int _b;
};

Unfortunately, SubT doesn't seem to contain member variable _a. Why
not?
Could someone suggest an efficient way to write the constructors for
SubT?
[Assume that SubT needs to contain both _a, and _b, and that you can't
optimize one out.]

Joseph

 

[John Harrison]

I'm having a little difficultly with inheritance.
I want to have a class "SubT" inherit from class "T".
SubT contains all the information in T, plus a little more based upon
the member variables of T.

For efficiency, I cannot merely have a T object in SubT.
What I'd like is something as follows:
class T {
T(int a) : _a(a) {}
protected:
int _a;
};

class SubT : public T {
SubT(int a) : _a(a), _b(a+a) {}
SubT(T foo) : _a(foo._a), _b(foo._a+foo._a) {}
public:
int _b;
};

Unfortunately, SubT doesn't seem to contain member variable _a. Why
not?

It does, but not directly, it contains T, which contains a.

Could someone suggest an efficient way to write the constructors for
SubT?

You should use the T cosntructor, like this

SubT(int a) : T(a), _b(a+a) {}
SubT(T foo) : T(foo._a), _b(foo._a+foo._a) {}

[Assume that SubT needs to contain both _a, and _b, and that you can't
optimize one out.]

Joseph

john