The future nobody kept

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

int main() {
    auto start = std::chrono::steady_clock::now();
    std::async(std::launch::async, [] {
        std::this_thread::sleep_for(std::chrono::seconds(1));
    });
    auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(
        std::chrono::steady_clock::now() - start).count();
    std::cout << "reached next line after " << ms << " ms\n";
}

The discarded temporary future's destructor blocks until the task finishes — so the "fire and forget" call waits a full second. GCC also warns about it (ignoring return value of 'std::async', declared with attribute 'nodiscard').

Observed output: reached next line after 1001 ms.

Fix: store the future (auto fut = std::async(...)) and wait on it deliberately.

Lesson: a discarded std::async future is a join, not a launch.

Facts verified 2026-11-04. The code on this page was compiled and run on the box with GCC 16.2.0 at -std=c++23 -Wall -Wextra; the outputs quoted are the observed outputs. MSVC was not tested.

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