[cfe-users] Linking problem with implicit instantiation of constructor/destructor

[Richard Smith via cfe-users]

In the latest draft, this is [temp.pre]/10:

"""
A definition of a function template, member function of a class template,
variable template, or static data member of a class template shall be
reachable from the end of every definition domain (6.3) in which it is
implicitly instantiated (13.9.1) unless the corresponding specialization is
explicitly instantiated (13.9.2) in some translation unit; no diagnostic is
required.
"""

... which might be slightly easier to read in the pre-C++20 version
(without the modules-awareness), where it is [temp]/7:

"""
A function template, member function of a class template, variable
template, or static data member of a class template shall be defined in
every translation unit in which it is implicitly instantiated unless the
corresponding specialization is explicitly instantiated in some translation
unit; no diagnostic is required.
"""

On Sun, 19 Apr 2020 at 12:00, David Blaikie <dblaikie at gmail.com> wrote:

> Yeah, can't seem to divine the concrete wording here either - perhaps
> Richard will have a moment to chime in.
>
> On Sun, Apr 19, 2020 at 6:10 AM Jaroslav Zeman via cfe-users <
> cfe-users at lists.llvm.org> wrote:
>
>> >
>> > What happens if you change the order of the .cpp files, putting
>> > template.cpp first; is it stil unresolved?
>> >
>> > clang++ -o test template.cpp main.cpp
>>
>> The order doesn't matter.
>>
>> > I don't believe this code is valid according to C++. I believe it would
>> > require an explicit instantiation of the ctor/dtor somewhere to make
>> that
>> > code valid - though I don't have chapter and verse on the spec at hand
>> just
>> > now to back that up.
>>
>> I tried to read the c++ specs, but didn't find anything, that would
>> clearly
>> state, if this is correct or incorrect. But the specs are too complicated
>> for
>> me to understand.
>>
>> My opinion is that it is bad to rely on implicit instantiation to happen
>> somewhere and don't do the explicit one. So I've already fixed all these
>> problems in our code and now I am just curious, what others think about
>> it.
>>
>> JZ.
>>
>>
>>
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>
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