C++ async lambda "recursive" call

Question

Can't figure out why this, seemingly recursive, invocation of readKey doesn't result in a growing call stack:

#include <future>
#include <iostream>

void readKey()
{
    std::async(std::launch::async, [](){ 
        if (getchar() != 113) // 'q' to quit
            readKey();
    });
}

int main(int, char**)
{
    readKey();
    return 0;
}

Thank you fort explaining!

:-)

No thread ever calls readKey(); more than once. Each call to std::async creates a new thread with a fresh stack. — François Andrieux
– François Andrieux, Commented Apr 20, 2017 at 14:06
@FrançoisAndrieux The std::future returned by async blocks in the destructor until the thread finishes. — NathanOliver
– NathanOliver, Commented Apr 20, 2017 at 14:11
@NethanOliver: does it also joins the thread? isn't it UB to not join it? — Andriy Tylychko
– Andriy Tylychko, Commented Apr 20, 2017 at 14:15
@AndyT That is what the block is. It waits for the thread to return(join()). — NathanOliver
– NathanOliver, Commented Apr 20, 2017 at 14:50

Andriy Tylychko · Accepted Answer · 2017-04-20 14:09:17Z

2

It's not a recursive call because you call it on a new thread (std::launch::async) which has a separate stack. So when you call readKey in main, it spawns a new thread where readKey will be called, and exits even not waiting for it.

answered Apr 20, 2017 at 14:09

Andriy Tylychko

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3 Comments

Robert Jacobs Over a year ago

I say this is a very inefficient recursion using a thread and a full stack instead of just adding a call to the stack. The function calls itself which technically is the definition of recursion.

Andriy Tylychko Over a year ago

@RobertJacobs: technically speaking, it doesn't call itself, at least directly, it rather schedules that call, I'm sure you know what I meant

Boris Over a year ago

@RobertJacobs: The point of this code is not achieving high efficiency, but executing the same operation in an infinite loop. Kind of like event loop, only implemented as pseudo-recursive lambda. I saw this code in some ASIO server examples and couldn't wrap my head around it... Now, it's getting more clear! :) Thank you to all who took their time to provide explanations!

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