What is <> again after class name in declaration? ... templateclass D{}

Question

I found a strange code about template in SO. It is like declaring template signature twice.

template <typename Tf, Tf F>
class Collection;

template <typename K, typename ... I, HashFunction<K, I...> F>   //<-- #B1
class Collection<HashFunction<K, I...>, F>    //<-- <> again!          #B2
 {    /*.......*/ }

What is the meaning of repeating declaration of the template (#B1 & #B2)?
Which C++ specification allows <> twice?
What is it called? ... I want to dig more about it.
Is it a kind of alias?
How can it be useful, generally?

I don't find such signature in cppreference.
Is it a bleeding-edge C++ syntax?

^ - Mandatory link http://en.cppreference.com/w/cpp/language/partial_specialization — StoryTeller - Unslander Monica, Feb 13 '17 at 09:05
I tried to find a duplicate, without much luck. If you're up for documenting what you learned in an answer to your own question, you can post an answer yourself, and it will benefit the site as a whole. — StoryTeller - Unslander Monica, Feb 13 '17 at 09:08
@StoryTeller If you just post your link (actually just copy your comment to an answer), I will accept it. I think people can easily dig about it more from your link. XD — javaLover, Feb 13 '17 at 09:09
@javaLover - I'm not up for writing a detailed answer at the mo, and a link only answer is as bad as it gets here on SO. I'd rather not pollute your post (and my track record on SO) with a bad answer. — StoryTeller - Unslander Monica, Feb 13 '17 at 09:10

score 7 · Accepted Answer · answered Feb 13 '17 at 09:14

// A
template <typename Tf, Tf F>
class Collection;


// B
template <typename K, typename ... I, HashFunction<K, I...> F>
class Collection<HashFunction<K, I...>, F> 
 {    /*.......*/ }

//A is the primary class-template Collection, while //B is a partial-specialization of the class-template Collection. Basically, you are specializing :

template <typename Tf, Tf F>
class Collection;

on the class-template:

template<typename K, typename ... I>
class HashFunction<K, I...>;

So, if any instantiated type of HashFunction is used as template Argument to Collection, the partial specialization //B gets selected for instantiation.

What is <> again after class name in declaration? ... templateclass D{}

1 Answers1