template< template <typename> something_else >, what is this?

Question

I'm just starting learning C++11 and I never saw this syntax in the list of new features:

template <template <typename> class F>
struct fun;

what is it and how does it work?

Answer 1

^{Note: What you are looking at is an "old" feature, and has been there since long before c++11.}

---

template <template <typename> class F> struct Obj;

In the above Obj is a template only accepting a template-parameter which is also a template^[1]; this is most often referred to as a template-template parameter ^[2].

^{1) in this specific example it will only accept a template that takes one type-parameter.
2) Link to SO question: Template Template Parameters}

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Imagine that you'd like to have a wrapper around some class template; you don't care which class template this is as long as you can specify a template argument for it.

If so, you can use a template-template parameter as in the below example:

template<template<typename T> class TemplateType>
struct Obj {
  TemplateType<  int> m1;
  TemplateType<float> m2;
};

template<typename T>
struct SomeTemplate { /* ... */  };

Obj<SomeTemplate> foo;

---

In the above, foo will be a Obj<SomeTemplate> having two members:

1. SomeTemplate< int> m1
2. SomeTemplate<float> m2

Answer 2

This should works in C++98 as well. This is a template as an argument from a template. I mean a template class will expected as the argument for F. Maybe this page will help you: http://www.cprogramming.com/tutorial/templates.html