using std::launch::deferred does not defer the function in std::async-CodePudding

Here's the code I'm testing, same as the above godbolt link:

#include <future>
#include <string>
#include <iostream>
#include <chrono>
#include <thread>

using namespace std::chrono_literals;

int main() {
    int v = 0;

    auto a1 = std::async(
        std::launch::async | std::launch::deferred, [&v]() {
            std::cout << "begin 1" << std::endl;
            std::cout << "v = " << v << std::endl;
            std::this_thread::sleep_for(100ms);
            std::cout << "end 1" << std::endl;
    });

    auto a2 = std::async(
        std::launch::async | std::launch::deferred, [&v]() {
            std::cout << "begin 2" << std::endl;
            std::this_thread::sleep_for(200ms);
            v = 123;
            std::cout << "end 2" << std::endl;
    });

    a2.wait();
    // My understanding is a1 does not run until here?
    // thus v should print 123 instead of 0 ?
    a1.wait();

    return 0;
}

If I remove the async policy, it works as expected.

The question is, why do I see a1 running concurrently with a2 and see v printed as 0? Does the async policy affect the behavior of deferred?

CodePudding user response：

std::launch::async | std::launch::deferred means that std::async can choose which of the two policies is selected. It is implementation defined which one is used but it looks like your standard library chooses async rather than deferred.