Re: Likely causes of Unresolved external symbol in pure virtual function
Dilip wrote:
Victor Bazarov wrote:
Dilip wrote:
I can't understand why I am getting the linker error -- what is my
mistake?
Isn't this in the FAQ? See section on templates.
Victor
Thanks for the pointer. I looked up the FAQ and read this (and others
too):
http://www.parashift.com/c++-faq-lite/templates.html#faq-35.15
While I understood the motivation, Philp's idea seems to be easier
because I am getting confused with 2 issues here.
#1) Cline's faq basically asks you to do a empty instantiation in the
.cpp file. So if my main code creates several different instances of
the derived template class with different template parameters, should
I include a empty instantiation for every such parameter?
There is no such thing as "empty instantiation". You probably mean
an explicit instantiation. It can help, but it's not the best solution.
An explicit instantiation causes the compiler to generate code for some
combination of the template arguments. If some user of your template
wants to have a different combination of template arguments than any of
those that you predicted, the user's compiler won't be able to generate
the instantiation if it does not see the code for the template itself.
#2) I had a situation like this that I couldn't immediately figure out
what to do:
// file a.h
class base
{
virtual void dosomething() = 0;
// I need the type parameter passed to derv for _only_
// this method in base
template<typename T>
void somefunc();
};
template<typename T>
class derv1 : public base
{
virtual void dosomething();
};
// derv1.cpp
template<typename T>
void derv<T>::dosomething()
{
somefunc<T>();
}
// file main.cpp
base* pBase = new derv1<int>();
pBase->dosomething();
how should I now define the empty instantiation for somefunc()?
is it:
template class base::somefunc<???>;
I think you'd need to put
template void derv<int>::dosomething();
in the 'derv1.cpp'.
2 observations:
1. The above does not even compile (obviously the real code doesnt
have ???)
2. the type parameter is dependant on the derived classes -- each of
them pass in their own stuff. How do I forward declare it?
Forward declare what? You don't need to do anything special, since
you have the template definition (which contains the declaration of
'dosomething').
V
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