Which templates (if any) in the C++ standard library have one or more template template parameters?
If there are many, then a couple of examples is fine.
If C++ version matters, then the latest draft of C++14/C++1y is preffered.
7
1 Answer
I'm not aware of any templates in the C++ standard library that are specified to take a template template parameter, but there is at least one standard template in C++11 that has a partial specialization with a template template parameter: std::pointer_traits. std::pointer_traits<Ptr>::element_type is specified to be:
Ptr::element_typeif such a type exists; otherwise,TifPtris a class template instantiation of the formSomePointer<T, Args>, whereArgsis zero or more type arguments; otherwise, the specialization is ill-formed.
In order to implement this you need a template template parameter for SomePointer, because it can be an arbitrary class template (as long as it only has type template parameters, to be precise). Here is the libstdc++ helper class partial specialization that does this, for instance:
template<template<typename, typename...> class _SomePtr, typename _Tp, typename... _Args> struct __ptrtr_elt_type<_SomePtr<_Tp, _Args...>, false> { typedef _Tp __type; }; 
1 Comment
Luc Danton
Strictly speaking an implementation must conform to the described behaviour, which in this case doesn’t explicitly mention a partial specialization. They can do so any which way so they could e.g. choose to perform magicks. In either case, the user is not allowed to detect whether such a specialization is present or not.
Containers <-> Iterators <-> Algorithms.template<template<.... I didn't take the time to write up a full and proper RE to scan the entire standard, but I think 40two is perhaps correct, that no standard-lib actual usage exists. I don't have a v14 draft handy; sorry about that.