C++ - Calling virtual function from constructor
Rob Newberry
robnewberry@grouplogic.com
Thu Nov 15 18:19:00 GMT 2001
> Why can I not or how could I call a virtual function from the
> contstructor?
I don't know about the specifics of you're trying to do (you may be safe),
and I think it may depend on your compiler (and I don't know what g++
does), BUT...
The reason you don't call virtual methods inside a constructor is because
you don't know what kind of object you are -- while you're in the process
of being constructed, you're still in a _somewhat_ indeterminate state.
Only after you are constructed are you a real object.
Here's an example:
class a
{
public:
a();
virtual void v_meth();
};
class b : public a
{
public:
b();
virtual void v_meth();
};
a::a()
{
// this will _always_ call a::v_meth, because
// at this point, that's what we are -- even
// if we're in the process of constructing a
// subclass of class a
v_meth();
}
void a::v_meth()
{
printf("a::v_meth\n");
}
b::b()
{
// likewise, this will always call b::v_meth,
// because at this point, that's what we are
v_meth();
}
void b::v_meth()
{
printf("b::v_meth\n");
}
main()
{
printf("constructing an a:\n");
a an_a;
printf("constructing a b:\n");
b a_b;
}
The output of this, with gcc, is:
constructing an a:
a::v_meth
constructing a b:
a::v_meth
b::v_meth
As you can see, when you are constructing the 'b' object a_b, the
constructor for 'a' and 'b' get called in succession. But the constructor
for 'a' calls 'v_meth', it ONLY calls the 'a' method, because at that
point, that's all the object it is. This might not be the behavior you
want to have, since you're constructing a 'b' object. That's why the
compiler complains about you calling a virtual method in a constructor.
Rob
---------------------------------------------------------------------
Rob Newberry
Director of Fajita Technology
Group Logic, Inc.
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