嵌套类中的别名模板

Question

I am trying to get a template alias for T2<B> starting from an instance of C.

template<typename A>
struct T1
{
   template<typename B>
   struct T2{};
};

template<typename A>
class C
{
   T1<A> t;
};

template<typename A>
using U1=decltype(C<A>::t);

template<typename A, typename B>
using U2=typename U1<A>::T2<B>;

I get a compiler failure with gcc 4.8:

gg.cc:18:28: error: expected ‘;’ before ‘<’ token
 using U2=typename U1<A>::T2<B>;

I have used the typename keyword in every sensible location, but can't get the U2 definition to compile.

What is the correct syntax here? It would be even better if I could get a definition of U2 without resorting to U1.

Answer 1

You need to use the template disambiguator to tell the compiler to parse T2 as the name of a template (and the subsequent < and > as delimiters of the corresponding template arguments):

    template<typename A, typename B>
    using U2=typename U1<A>::template T2<B>;
//                           ^^^^^^^^

Here is a compiling live example .

Here you can find some more information on when you should use the template disambiguator (and the typename disambiguator as well, although you seem to be aware of that one).

Answer 2

When the compiler compiles the following:

template<typename A, typename B>
using U2=typename U1<A>::T2<B>;

After it reads the nested name specifier U1<A>:: , it doesn't know which specialization of U1 it is in, because A is unknown. Each U1 specializaiton could be totally different, and depend on what A is. So it has no idea what kind of name T1 is. In particular it doesn't know whether it is a template name or not. (For example U1<int>::T1 could be totally different to what U1<char>::T1 is.)

For this reason you need to explicitly tell the compiler that T1 is going to be a template name by using the template keyword before T1 .