如何使整数模板参数作为模板化类成员可访问？

Question

A common pattern with templated classes is that template argument is typedef'ed inside the class to be easily accessible:

#include <type_traits>
template<class T> struct Foo{
    typedef T type;
};
static_assert(std::is_same<Foo<int>::type,int>::value,"");

How can I do the same with non-type template argument? I only had the following idea, but there gotta be something more elegant?

template<int I> struct Bar{
   constexpr static int getI(){ return I; }
};
static_assert(Bar<5>::getI()==5,"error");

Answer 1

I might use an enum , but the utility of this seems somewhat limited to me...

#include <iostream>
using namespace std;

template<int N> struct Foo
{
    enum {value_ = N};
};

int main(int argc, char* argv[])
{            
    Foo<42> foo;

    cout << foo.value_;

    return 0;
}

Edit to include that this kind of thing is frequently done in template metaprogramming.

Answer 2

You can just use a static const variable:

template<int I> struct Bar{
    static const int i = I;
};

static_assert(Bar<5>::i==5,"error");