Using a thread pool in real situations

If I need to improve an existing code by using a thread pool, how can I get data from a SQL DB:

Code:

---

bool CMyClass::GetSQLSelect(std::string sql, void* data)
{
        ....
}

---

and suppose I have another code, higher level:

Code:

---

void CMyClass::DoSomething()
{
        void* data = nullptr;
        if (!GetSQLSelect("SELECT * FROM my_table", data))
                return;

        std::string result = static_cast<std::string>(data);
        // do something with result
}

---

Now, all examples from internet give me some examples of how to add some methods to a thread pool, but those methods are not returning data to the main thread. What if I need data from those posted to thread methods ?

I have found and example right here: https://stackoverflow.com/questions/...pooling-in-c11.

I have seen something very simple using boost:

Code:

---

void my_task() { ... }

int main()
{
        int threadNumbers = thread::hardware_concurrency();
        boost::asio::thread_pool pool(threadNumbers);

        // Submit a function to the pool.
        boost::asio::post(pool, my_task);

        // Submit a lambda object to the pool.
        boost::asio::post(pool, []() {
                ...
        });
}

---

but how can I use it in my case, when I expect data from posted method ? Something like that:

Code:

---

std::string sql = "SELECT * from my_table";
void* data = nullptr;
boost::asio::post(GetSQLSelect, sql, data);

---

but when I retrieve data then ?

Every hint, even small one, I will appreciate it most !