LNK2019 unresolved external symbol

#pragma once
#include<future>
 
class threadpool
{
public:
    threadpool(size_t threadCount);
    virtual ~threadpool();
 
    template<class F, class ...Args>
    auto AddTask(F&& f, Args&&... args) ->
        std::future<typename std::result_of<F(Args...)>::type>;
};

#include "threadpool.hpp"
 
threadpool::threadpool(size_t threadCount)
{
}
 
threadpool::~threadpool()
{
}
 
template<class F, class... Args>
auto threadpool::AddTask(F&& f, Args&&... args) 
    -> std::future<typename std::result_of<F(Args...)>::type>
{
    return std::future<typename std::result_of<F(Args...)>::type>();
}

#include <iostream>
#include "threadpool.hpp"
 
int foo(int i1, int i2)
{
    std::cout << i1 + i2 << std::endl;
    return i2 - i1;
}
 
template<class F, class ...Args>
auto add_task(F&& f, Args&&... args) 
    ->std::future<typename std::result_of<F(Args...)>::type>
{
    return std::future<typename std::result_of<F(Args...)>::type>();
}
 
int main()
{
    threadpool th(4);
 
//     Ошибка.
//  auto res = th.AddTask(&foo, 12, 3);
//  std::cout << res.get() << std::endl;
 
//     Работает.
    auto res2 = add_task(&foo, 12, 3);
    std::cout << res2.get() << std::endl;
 
    getc(stdin);
    return 0;
}