Virtual functions in template class issue

**Bo Persson** · Nov 16 '05, 09:27 PM

Re: Virtual functions in template class issue

"Chris" <chawkins@ufl.e du> skrev i meddelandet
news:049801c367 6e$f383f200$a50 1280a@phx.gbl.. .[color=blue]
> I am having a very strange problem involving virtual
> functions in template classes. First of all, here is an
> extremely simplified structure of the two classes I am
> having problems with.
>
> template<class Type> class base
> {
> public:
> base& operator/=(const base&);
> Type *image;
> // There is more stuff here, but it is not necessary
> for this problem
> }[/color]

I guess that in the real code, all the classes are terminated by a
semicolon. :-)
[color=blue]
>
> class derived : public base<complex<do uble> >
> {
> // There is stuff here, but it is not necessary for
> this problem
> }
>
> template<class Type> base& base<Type>::ope rator/=(const[/color]

I think the return value should be

base<Type>&
[color=blue]
> base& im)
> {
> for(int i=0; i<loopLength; i++)
> {
> if(im.image[i] != 0) {[/color]

And here is your missing operator!=(), since the complex types cannot
be compared to int. Change it to 0.0 and it will compile.
[color=blue]
> image[i] = image[i] / im.image[i]; }
> else {
> image[i] = 0; }
> }
> return *this;
> }
>[/color]

Bo Persson
bop2@telia.com

**Craig Powers** · Nov 16 '05, 09:27 PM

[multipost] Re: Virtual functions in template class issue

Also addressed in vc.language and vc.stl.

Please don't multipost. If you must post the same message to multiple
groups, you should crosspost.

--
Craig Powers
MVP - Visual C++

**John Madsen** · Nov 16 '05, 09:27 PM

Re: Virtual functions in template class issue

"Chris" <chawkins@ufl.e du> wrote in message news:<049801c36 76e$f383f200$a5 01280a@phx.gbl> ...[color=blue]
> I am having a very strange problem involving virtual
> functions in template classes. First of all, here is an
> extremely simplified structure of the two classes I am
> having problems with.
>
> template<class Type> class base
> {
> public:
> base& operator/=(const base&);
> Type *image;
> // There is more stuff here, but it is not necessary
> for this problem
> }
>
> class derived : public base<complex<do uble> >
> {
> // There is stuff here, but it is not necessary for
> this problem
> }
>
> template<class Type> base& base<Type>::ope rator/=(const
> base& im)[/color]

This needs to be:
template<class Type> base<Type>& base<Type>::ope rator/=(const
base<Type>& im)
[color=blue]
> {
> for(int i=0; i<loopLength; i++)
> {
> if(im.image[i] != 0) {[/color]

Your use of literal 0 here should be Type(0). As it stands, the
compiler needs to do 2 conversions to turn 0 into a
std::complex<do uble>. First, 0 (an int) needs to be converted to a
double, then the double needs to be converted to a complex<double> .
The compiler won't do 2 conversions, so you need to help it out with
one.
[color=blue]
> image[i] = image[i] / im.image[i]; }
> else {
> image[i] = 0; }
> }
> return *this;
> }
>[/color]

[snip]
[color=blue]
>
> It seems to me that this should all still work, but
> instead I get 23 strange errors when I try to compile the
> code with the virtual function. I should point out that[/color]

The 23 errors come from the fact that there are 23 operator!=()s in
scope and it's trying all of them looking for a match.

[snip]
[color=blue]
> I am completely baffled so please help me. Why does it
> work completely fine without the virtual?
>[/color]

virtual is a red-herring here. If you eliminate the derived class and
make operator/= non-virtual, the follow will still produce the 23
errors:

int main() {
base<std::compl ex<double> > x;
x /= x;
}

"x /= x" is needed to force the instantiation of base<T>::operat or/=,
where the real problem is. Using virtual also forces that
instantiation so that the compiler can generate the vtables for base
and derived.

Hope this helps,

John

Virtual functions in template class issue

Virtual functions in template class issue

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