Re: calling a pure virtual from base constructor
christian.bongiorno@gmail.com writes:
However, I get a linker error: unresolved symbol. The pure virtual is
defined in the grandparent,
No, you only declared it. It's undefined. Since it's a virtual
function, it shouldn't be the linkers concern though.
the parent invokes the virtual from the
constructor, the child implements it:
I find it slightly odd that you get a linker error. What exactly did
you compile? It was not the code you posted, after all. What compiler
are you using?
class GrandParent {
virtual getClass() = 0;
};
You forgot the return value. Also, this function is private so you
can't call it from the derived class.
class Parent : public GrandParent {
Parent() { getClass();}
};
And you need to have the contructor be protected to be used by Child.
class Child : public Parent {
int getClass() { return 10;}
}
And public here to be testable. There also is a semicolon
missing.
How can I do this? I guess I can pass the value in the constructor, but
that's not what I want to do
I don't know what the linker error is about, but your code is
undefined behaviour, since you call an undefined virtual function. On
my platform (gcc) this prints "pure virtual method called" at runtime
and then aborts.
Unfortunately, what you're trying to do is impossible. The subobjects
(bases and members) are constructed first, which means there is a
problem with calling to the yet unconstructed more derived objects.
For some reason I can't tell (can someone?), the standard specifies,
AFAIK, that the virtual dispatch mechanism should still be defined but
work on the current constructor call's object as the most derived
type. In you're case, this means you're calling a pure virtual
function.
Note that you even can define it, you just can't put it in the class
definition itself (because there is no syntax for it).
--
Gru??, Jens
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