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Question #48 Difficulty: 2

According to the C++23 standard, what is the output of this program? 

    #include <iostream>
#include <string>
#include <future>

int main() {
  std::string x = "x";

  std::async(std::launch::async, [&x]() {
    x = "y";
  });
  std::async(std::launch::async, [&x]() {
    x = "z";
  });

  std::cout << x;
}

Correct!

The destructor of a future returned from async is required to block until the async task has finished (see elaboration below). Since we don't assign the futures that are returned from async() to anything, they are destroyed at the end of the full expression (at the end of the line in this case). §[class.temporary]¶4 in the standard:

Temporary objects are destroyed as the last step in evaluating the full-expression §[intro.execution] that (lexically) contains the point where they were created.

This means that the first async call is guaranteed to finish execution before async() is called the second time, so, while the assignments themselves may happen in different threads, they are synchronized.

Elaboration on synchronization:

According to §[futures.async]¶4 in the standard:

Synchronization: The invocation of async synchronizes with the invocation of f. The completion of the function f is sequenced before the shared state is made ready.

[Note 1: These apply regardless of the provided policy argument (...). — end note]

If the implementation chooses the launch::async policy,

— (...)

— the associated thread completion synchronizes with the return from the first function that successfully detects the ready status of the shared state or with the return from the last function that releases the shared state, whichever happens first.

In this case, the destructor of std::future<> returned by the async() call is "the last function that releases the shared state", therefore it synchronizes with (waits for) the thread completion.

Scott Meyers writes more about this https://scottmeyers.blogspot.com/2013/03/stdfutures-from-stdasync-arent-special.html.

You can explore this question further on C++ Insights or Compiler Explorer!

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