Re: pure virtual functions and runtime id
On Feb 19, 12:14 am, Martin York <Martin.YorkAma...@gmail.com> wrote:
On Feb 18, 1:40 pm, cerenoc <julian....@gmail.com> wrote:
[...]
But isn't this the whole point of polymorphism? I want to be able to
have a function like someFunc and then be able to call the right
method with late binding?
Yes. But you can only call methods on an object that is still in scope
(alive).
Just a nit, but it really isn't a question of scope, but rather
of lifetime. In the case of a non-static local variable (an
"auto" variable), lifetime may correspond to scope, but this
isn't generally the case. The rule is that you can only call
member functions (virtual or not) on an object that exists.
After the object goes out of scope it is unusable. You could
re-factor you code to make sure the objects do not go out of
scope.
int main(int argc, char *argv[]) {
Base * var;
Derived1 v1;
Derived2 v2;
if (...) {
var = &v1;
} else if (...) {
var = &v2;
}
someFunc(var); // Both v1 and v2 are still in scope so now you can
use var
return 0;
}
I'd guess that 99% of the time, polymorphic objects will be
allocated dynamically. There are a few cases where you might
want to do something like:
Base const& obj(
someCondition
? static_cast< Base const& >( Derived1() )
: static_cast< Base const& >( Derived2() ) ;
but this doesn't work with all compilers; the object will be
destructed at the end of the full expression. (I think that
that's what the standard requires as well.) So you probably
have to use dynamic allocation here as well.
And of course, as soon as you start keeping the object alive,
and it can only be reached through pointers to Base or
references to Base, Base really should have a virtual
destructor.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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