Is it C++ standard-compliant to define a variable using `A::template B x;`?

Question

The following code is accepted by gcc, vc++, and clang.

template<class T>
struct A
{
    template<class U>
    struct B
    {};
};

int main()
{
    A<int>::B<int> y; // OK as expected
    A<int>::template B<int> x; // Also OK! Is this standard-compliant?
};

Is it C++ standard-compliant to define a variable using A<int>::template B<int> x;?

B is a template-dependent type template, so the second syntax is perfectly correct. — Quentin, Jul 04 '14 at 14:59
Could you refer me to the page of the standard that specifies this usage? Thanks. — xmllmx, Jul 04 '14 at 15:00
@peppe Not a duplicate, because you do not need to use `template` here. — juanchopanza, Jul 04 '14 at 15:04
@MikeSeymour oh. So it would only be the case with, say, `A::template B` within a scope with a template parameter `W` ? — Quentin, Jul 04 '14 at 15:04
@Quentin: Indeed; it's needed in a dependent context, when the interpretation of the name depends on unknown template parameters. Here, it's not needed, since the template has been specialised and we know exactly what `B` is in that specialiation. (Sorry, I had brief second thoughts and removed my comment saying that it's not dependent here). — Mike Seymour, Jul 04 '14 at 15:07
@MikeSeymour Thank you very much ! I'll sleep wiser tonight :) — Quentin, Jul 04 '14 at 15:09

score 12 · Accepted Answer · answered Jul 04 '14 at 15:07

Even though it is a non-normative note, I think an answer can be given by n3797[temp.names]/6

As is the case with the typename prefix, the template prefix is allowed in cases where it is not strictly necessary; i.e., when the nested-name-specifier or the expression on the left of the -> or . is not dependent on a template-parameter, or the use does not appear in the scope of a template.

In the OP's example, the prefix template is used outside the scope of a template, and the preceding nested-name-specifier is not dependent. Therefore, the prefix template is not required, but allowed here.

[expr.prim.general]/8

qualified-id:
nested-name-specifier template_opt unqualified-id

Plus [temp.names]/5

A name prefixed by the keyword template shall be a template-id or the name shall refer to a class template.

[temp.names]/1 says that B<int> is indeed a (simple-)template-id.

@Constructor In C++03, you could use the keyword `template` even if the *nested-type-name* is not dependent; but you were not allowed to use it outside the scope of templates. — dyp, Jul 04 '14 at 20:33

Is it C++ standard-compliant to define a variable using `A::template B x;`?

1 Answers1