std::async

The std::async function template in <future> is a high-level wrapper used to run asynchronous tasks. It manages thread creation internally and returns a std::future containing the result.

The std::async Future Destructor Trap

A common mistake when using std::async is ignoring its return value:

By design, the destructor of a future returned by std::async blocks until the associated task completes. If you do not assign the return value to a variable, the temporary future is destroyed at the end of the statement, forcing the task to run synchronously.

Code Walkthrough

Demonstrates the differences in behavior between the async and deferred launch policies.

#include <iostream>
#include <future>
#include <thread>
int fetchValue() {
    std::this_thread::sleep_for(std::chrono::milliseconds(100));
    return 100;
}
int main() {
    // 1. Force execution on a background thread
    std::future<int> f1 = std::async(std::launch::async, fetchValue);
// 2. Defer execution until get() is called
    std::future<int> f2 = std::async(std::launch::deferred, fetchValue);
std::cout << "Main thread executing concurrently...\n";
std::cout << "Result 1: " << f1.get() << std::endl; // Retrieve result
    std::cout << "Result 2: " << f2.get() << std::endl; // Task runs here
return 0;
}

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