Re: Static Class Variables, Inheritance, and Polymorphism
On Apr 4, 7:50 pm, "crjjrc" <crj...@gmail.com> wrote:
On Apr 3, 1:31 pm, "bjeremy" <bjer...@sbcglobal.net> wrote:
I've then got a vector declared to hold instances of Base, but it's
actually filled with various instances of Derived1 and Derived2.
No it's not. A vector always holds instances of the type it was
declared with. If you declare std::vector<Base>, it will hold
instances of Base, and nothing else. This is one of the most
important invariants of std::vector.
On
iterating through and examining each item, I'd like to be able to call
getType() and have the correct derived class's class variable
retrieved. However, I only get 0 returned. I've tried making
getType() virtual in the Base class, but that didn't help anything.
class Base {
public:
virtual int getType() { return type; }
private:
static const int type = 0;
};
class Derived1 : public Base {
private:
static const int type = 1;
public:
int getType() { return type; }
};
class Derived2 : public Base {
private:
static const int type = 2;
public:
int getType() { return type; }
};
This is almost identical to what I had, but I didn't define getType
for each derivation, and I only got 0 returned from getType(). I
don't see why getType needs to be defined for each derived class.
Because it's supposed to do something different in each class.
In Base, it returns Base::type, and in Derived1, it returns
Derived1::type. For what you seem to expect, type would have to
be virtual, which isn't possible for two reasons: data types
can't be virtual, and static members can't be virtual.
It
seems like the parent's getType should pull the variable type from the
child for instances of Derived1 and Derived2. type should be a
"virtual" member, I would think.
Why? You haven't declared it virtual. For various reasons, in
fact, data members can't be virtual; only functions. And of
course, static members can't be virtual. Perhaps the reason
data members can't be virtual is because it only makes sense for
static data members; the reason static members can't be virtual
is that virtuality is based on the actual (dynamic) type of the
object, and static members don't have an associated object, so
it doesn't make sense either.
One possible solution here (other than providing the function in
all of the subclasses, which is still probably the best idea) is
to create a non-static member type in the base class, and have
the derived classes pass its value as an argument to the
constructor of Base.
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James Kanze (GABI Software) email:james.kanze@gmail.com
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