Template specialization for type concretized by template parameter

Question

First of all I'm very sorry for the question title, but it's very hard to describe. What of those two below is valid syntax if I want to specialized Resolve for all instantiation of A?
1)

template<uint32_t I> struct A {};

template<typename> struct Resolve;

template<uint32_t I>
struct Resolve<A<I>>
{
    void f() { printf("im here!\n"); }  
};

2)

template<uint32_t I> struct A {};

template<typename> struct Resolve;

template<>
template<uint32_t I>
struct Resolve<A<I>>
{
    void f() { printf("im here!\n"); }  
};

Or is template<> optional? There's two different answers on SO: here and here.
Also please provide quotation of the standard if possible.

Option 2) doesn't compile on MSVC, but does compile at least on some versions of GCC.

The accepted answer of the second post clearly states that the `template <>` bit is not valid. — R Sahu, Aug 23 '18 at 17:35
I am not sure what you are looking for. If there isn't anything in the standard that says it is valid, then it is invalid. — R Sahu, Aug 23 '18 at 17:39
Well, maybe standard says somewhere about this sort of cases. If not, then I'll go with the answet that the first one is not valid — Criss, Aug 23 '18 at 17:42

score 1 · Accepted Answer · answered Aug 23 '18 at 17:40

This is correct:

template <uint32_t I>
struct Resolve<A<I>>
{ };

The syntax template <> is used to introduce an explicit specialization (of a class template, function template, whatever) (see [temp.spec]/3 and [temp.expl.spec]/1). But we're trying to do a partial specialization. A partial specialization still needs to introduce template parameters, an explicit specialization does not.

On the other hand, if we were trying to specialize a member of an explicit specialization, then we'd use template <>. For instance:

template <class T>
struct A {
    template <class T2> struct B { }; // #1
};

template <>         // for A
template <class T2> // for B
struct A<int>::B<T2> { }; // #2

A<char>::B<int> x; // #1
A<int>::B<char> y; // #2

Template specialization for type concretized by template parameter

1 Answers1