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

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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<std::future<void>> 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.

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