Nested template possibilities

Question

Is the following valid?:

template<typename T> class C { C1<C2<T>> someMember; };

Answer 1

Well, you'd need to do something with the type, either make it a typedef or member, but yes:

template <typename T>
struct C1 {};

template <typename T>
struct C2 {};

template <typename T>
struct C
{
    typedef C1<C2T> > type; // note the space!
};

>> is actually the right shift operator, so you need a space in there for it to work correctly. In C++0x, however, you can just type >>; it is parsed just fine.*

_{*Some lenient compilers will currently accept it.}

Answer 2

Erm, no, at least not in isolation, because you have no definition for C1 and C2. Assuming that they are defined to be template classes with one argument, then no, it's still not valid for the same reason that int; is not valid -- it's not a complete statement, its just a typename.

Now if you're asking about the syntax with the angle-brackets, then ignoring all of the other issues, that is only valid in C++0x. In the current C++ standard, the parse is ambiguous, and you would have to form it as C1<C2<T> >.

Answer 3

Sure, I've used:

vector< pair<int, int> > blah;

before.