non-type template template arguments?

[swa...@gmail.com]

Hi.

So I was wondering why template template argument are only allowed to be "typename" or "class" as since C++14's we have templated global/static variables.

It would allow for things like this.

template <class T>
static constexpr int value = someFunc<T>();

template<template<class> int>
struct Smthg
{
    ...
};

int main()
{
   Smthg<value> t;
}

Or even things like this with use of C++17's auto non-type template parameter.

struct holder
{
     ...
     template <template<class> auto& Func>
     constexpr auto visit() { ... }
     ...
};

template <class T>
void printType() { ... }

template <class T>
static constexpr size_t size_of = []() {
     return sizeof(T);
};

int main()
{
    auto t = returns_holder();
    t.visit<printType>();
    std::cout << t << t.visit<sizeo_of>() << std::endl;
}

[jakob....@gmail.com]

+1, I like this idea.

Jakob

[tko...@google.com]

If you want to allow variable templates as template template parameters, why not also allow function templates?