C++ standard paragraph about member template specialization of unspecialized template

3
votes

In the current draft of the C++ standard, the paragraph [temp.expl.spec]p.18 says:

A specialization of a member function template, member class template, or static data member template of a non-specialized class template is itself a template.

However, the above paragraph temp.expl.spec]p.17 states (my emphasis):

In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well. [...]

Therefore, it should be impossible to explicitly specialize a member template without specializing its enclosing class template, so [temp.expl.spec]p.18 is wrong. I highlighted the word explicitly because [temp.expl.spec]p.18 says "A specialization [...]". Specialization can have a lot of meanings, one of which could be explicit specialization.

I checked and [temp.expl.spec]p.18 appeared in the standard before [temp.expl.spec]p.17. For example, in the 1996 version of the standard [temp.expl.spec]p.18 is present while [temp.expl.spec]p.17 is not.

What is the original intended meaning of [temp.expl.spec]p.18?

Thank you.

1
c++templateslanguage-lawyer
Probably just that it still counts as a template - specialization a member function template doesn't just turn it into a non-template function. – Barry
But what type of specialization is the paragraph referring to? Explicit specialization? Or the specialization which comes from implicit instantiation? Because if the answer is the former, I don't think the paragraph is correct in the current version of the standard. – user42768
Any specialization. The two paragraphs do not strike me as related. 17 provides some restrictions on specializations, 18 just says that specializations of member templates are still templates. – Barry
Paragraph 17 says that you cannot explicitly specialize a member template without specializing its enclosing class template. Therefore there cannot be any explicit "specialization of a member [...] of a non-specialized class template [...]". – user42768
Note: these points are 16 and 17 in the C++17 standard (N4659); the latest draft changed the numbering – M.M

1 Answers

2
votes

There can be partial specialization of a member class template of a non-specialized class template.

Your point 17 prohibits full specialization of such a member; and point 18 clarifies that the partial specialization in this case is still a template.

The terminology "explicitly specialize" means fully specialize, and excludes partial specialization.

template<typename T>
struct A
{
     template<typename U, typename V>
     struct B
     { 
         void f() {}
     };
};

// OK: Partially specialize A<T>::B. This is a template
template<typename T>
template<typename U>
struct A<T>::B<U, int>   
{
    void g() {}
};