explicit instantiation: function vs. class

explicit instantiation: function vs. class

[Vladimir Prus]

Hello,
I have a problem where explicit instantiation of a class works, but for a
function does not. Consider the following example:

namespace serialization {

    template<class T>
    class helper {};
    
    template<class T, class Seq>
    void
    boost_template_instantiate(T &,Seq &) {}
}

namespace X
{
    class C {};
            
    template
    void
    serialization::boost_template_instantiate<>(X::C &, int &);
    
    template
    class serialization::helper<int>;
}

Using gcc version 3.3.3 20040125 (prerelease) (Debian) I get:

export4.cpp:26: error: declaration of `void
boost_template_instantiate(X::C&,
   int&)' not in a namespace surrounding `serialization'

The questions are:

1. Why the error? I could not find anything in the standard. It say that
explicit specialization should be declared in the same namespace, but I see
no restriction for instantiation. 

2. Why the error happens for function only? The class is instantiated just
fine.

Thanks,
Volodya

Re: explicit instantiation: function vs. class

[Giovanni Bajo]

Vladimir Prus wrote:

> namespace serialization {
>
>     template<class T>
>     class helper {};
>
>     template<class T, class Seq>
>     void
>     boost_template_instantiate(T &,Seq &) {}
> }
>
> namespace X
> {
>     class C {};
>
>     template
>     void
>     serialization::boost_template_instantiate<>(X::C &, int &);
>
>     template
>     class serialization::helper<int>;
> }

> export4.cpp:26: error: declaration of `void
> boost_template_instantiate(X::C&,
>    int&)' not in a namespace surrounding `serialization'
>
> The questions are:
>
> 1. Why the error? I could not find anything in the standard. It say
> that explicit specialization should be declared in the same
> namespace, but I see no restriction for instantiation.

I think the error is bogus. I cannot find evidence in the standard that the
explicit instantiation should be rejected because it happens in a different
scope.

> 2. Why the error happens for function only? The class is instantiated
> just fine.

This is a bug in GCC. Please, file a bug report in Bugzilla.

Giovanni Bajo

Re: explicit instantiation: function vs. class

[Vladimir Prus]

Giovanni Bajo wrote:


>> namespace serialization {
>>
>>     template<class T, class Seq>
>>     void
>>     boost_template_instantiate(T &,Seq &) {}
>> }
>>
>> namespace X
>> {
>>     class C {};
>>
>>     template
>>     void
>>     serialization::boost_template_instantiate<>(X::C &, int &);
>> }
> 
>> export4.cpp:26: error: declaration of `void
>> boost_template_instantiate(X::C&,
>>    int&)' not in a namespace surrounding `serialization'
>>
>> The questions are:
>>
>> 1. Why the error? I could not find anything in the standard. It say
>> that explicit specialization should be declared in the same
>> namespace, but I see no restriction for instantiation.
> 
> I think the error is bogus. I cannot find evidence in the standard that
> the explicit instantiation should be rejected because it happens in a
> different scope.

I've just with online Comeau, and it has objections to the code as well:

"ComeauTest.c", line 25: error: function
          "serialization::boost_template_instantiate(T &, Seq &) [with
T=X::C,
          Seq=int]" cannot be explicitly instantiated in the current scope
      serialization::boost_template_instantiate<>(X::C &, int &);;

Interesting, why such bug occurs in two compilers.

>> 2. Why the error happens for function only? The class is instantiated
>> just fine.
> 
> This is a bug in GCC. Please, file a bug report in Bugzilla.

Sure. I think I'd check with comp.std.c++ first, to make sure it's really
legal code.

- Volodya