问题
I am uncertain if it is a bug in Clang 3.2 or a violation of C++03, but it appears that explicit instantiation of templated constructors for template classes fails, but explicit instantiation of templated member functions of template classes succeeds.
For instance, the following compiles without a problem with both clang++ and g++:
template<typename T>
class Foo
{
public:
template<typename S>
void Bar( const Foo<S>& foo )
{ }
};
template class Foo<int>;
template class Foo<float>;
template void Foo<int>::Bar( const Foo<int>& foo );
template void Foo<int>::Bar( const Foo<float>& foo );
template void Foo<float>::Bar( const Foo<int>& foo );
template void Foo<float>::Bar( const Foo<float>& foo );
whereas the following compiles without warning with g++ but fails with clang++:
template<typename T>
class Foo
{
public:
template<typename S>
Foo( const Foo<S>& foo )
{ }
};
template class Foo<int>;
template class Foo<float>;
template Foo<int>::Foo( const Foo<int>& foo );
template Foo<int>::Foo( const Foo<float>& foo );
template Foo<float>::Foo( const Foo<int>& foo );
template Foo<float>::Foo( const Foo<float>& foo );
In particular, I see two error messages of the form:
TemplateMember.cpp:12:20: error: explicit instantiation refers to member
function 'Foo<int>::Foo' that is not an instantiation
template Foo<int>::Foo( const Foo<int>& foo );
^
TemplateMember.cpp:9:16: note: explicit instantiation refers here
template class Foo<int>;
^
Is this a violation of the standard or a bug in clang++?
回答1:
It looks like you've found a GCC bug. These both name the implicitly-declared copy constructor:
template Foo<int>::Foo( const Foo<int>& foo );
template Foo<float>::Foo( const Foo<float>& foo );
Per [temp.explicit]p4,
If the declaration of the explicit instantiation names an implicitly-declared special member function (Clause 12), the program is ill-formed.
Therefore Clang is correct to reject this code.
来源:https://stackoverflow.com/questions/15045879/explicit-instantiation-of-templated-constructor-for-template-class