Re: explicit template instantiation without all definitions
On 4 Dez., 00:55, Piotr Dobrogost <google...@2008.autoera.pl> wrote:
In the following example should compiler instantiate A<int> without f
function which doesn't have definition or not instantiate A<int> at
all?
I label this as question (A).
What if f and g were templates?
I label this as question (B).
template <typename T>
struct A
{
void f();
void g();
};
template <typename T>
void A<T>::g()
{};
This trailing ; makes the code ill-formed. I assume
this was an oversight.
template struct A<int>;
int main()
{
return 0;
}
Concerning question (A): The current standard C++03 [14882:2003(E)]
requires in 14.7.2 [temp.explicit]/4:
"The definition of a non-exported function template, a non-exported
member function template, or a non-exported member function or
static data member of a class template shall be present in every
translation unit in which it is explicitly instantiated."
and in p.7:
"The explicit instantiation of a class template specialization
implies the instantiation of all of its members not previously
explicitly specialized in the translation unit containing the
explicit instantiation."
So the answer whether your code snippet causes undefined behavior or
not depends on the question, which *kind* of specialization of the
members expressed in p.7 is meant. This question was answered as a
side-effect of the resolution of core issue #470
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#470
which changed the wording of p.7 to say more specifically in p.7+8
(see the recent draft N2798):
"An explicit instantiation that names a class template specialization
is an explicit instantion of the same kind (declaration or
definition)
of each of its members (not including members inherited from base
classes) that has not been previously explicitly specialized in the
translation unit containing the explicit instantiation, except as
described below."
"An explicit instantiation definition that names a class template
specialization explicitly instantiates the class template
specialization and is only an explicit instantiation definition of
members whose definition is visible at the point of instantiation."
For your example we have a so-called "explicit instantiation
definition" according to the new nomenclature (This allows to
distinguish a pure instantiation declaration introduced with
the "extern" keyword from a concrete instantiation). The new
wording makes clear that your example does *not* explicitly
instantiate A<int>::f, but *does* explicitly instantiate
A<int>::g, because no definition of f is in scope of the
instantiation of A. This wording nicely resolves the previous
conflict between para graphs 4 and 7.
It should be noted that there still exists an open issue
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#546
which is related to this situation.
Concerning question (B): I'm not sure how you meant this
question, because assuming that f and g are member
templates, these templates would not be instantiated, if
only the surrounding class template is instantiated. Their
template parameters are independent from that their "parent",
so what would be the type in which their instantiation would
take place? Of course, if you would like to explicitly
instantiate the member templates f and g, either of them
would be required to have their corresponding definition
in scope of the instantiation request, because this falls
in the domain of p. 4 quoted above.
HTH & Greetings from Bremen,
Daniel Kr?gler
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