Can I use std::async without waiting for the future limitation?

Question

High level
I want to call some functions with no return value in a async mode without waiting for them to finish. If I use std::async the future object

Answer 1

You can move the future into a global object, so when the local future's destructor runs it doesn't have to wait for the asynchronous thread to complete.

std::vector> pending_futures;

myResonseType processRequest(args...)
{
    //Do some processing and valuate the address and the message...

    //Sending the e-mail async
    auto f = std::async(std::launch::async, sendMail, address, message);

    // transfer the future's shared state to a longer-lived future
    pending_futures.push_back(std::move(f));

    //returning the response ASAP to the client
    return myResponseType;

}

N.B. This is not safe if the asynchronous thread refers to any local variables in the processRequest function.

While using std::async (at least on MSVC) is using an inner thread pool.

That's actually non-conforming, the standard explicitly says tasks run with std::launch::async must run as if in a new thread, so any thread-local variables must not persist from one task to another. It doesn't usually matter though.