[C++14] Variable template linkage

variable template can be looked as syntax sugar for template class type with public static variable.
const/static variable template will also follow the rule, i.e has internal linkage.

main.cpp:

#include <iostream>
#include <atomic>

using namespace std;

template<typename T>
atomic<T> JustForFun{};

// need symbol from other TU.
extern template atomic<int> JustForFun<int>;
//same as below:
template<>
extern atomic<char> JustForFun<char>;


template<typename T>
struct FunType
{
    auto& GiveMeAtomic()
    {
        return JustForFun<T>;
    }
};

int main()
{
    JustForFun<char> = 'V';
    cout << JustForFun<char> << endl;

    FunType<long long> ft;
    cout << ft.GiveMeAtomic() << endl;

    FunType<int> ft_int;
    cout <<< ft_int.GiveMeAtomic() << endl;

    ft_int.GiveMeAtomic() = 24;
    cout << ft_int.GiveMeAtomic() << endl;
}

Library.o

#include <atomic>
using namespace std;

template<typename T>
atomic<T> JustForFun{};

template <typename T>
constexpr T genT()
{
    return T{42};
}

//specialize variable template, will generate/export symbol
template<>
atomic<int> JustForFun<int>{genT<int>()};

// explicit generate variable template symbol for type char.
// Variable template could be seen as template class with static data member.
template atomic<char> JustForFun<char>;

Reference: Variable templates and instantiation -- Is this well formed?