Why Does the `std::future` Destructor Block in `std::async`?

Blocking Destructors in std::async Futures: A Technical Explanation

When utilizing std::async to execute tasks asynchronously in C 11, one may encounter an unexpected blocking behavior when destroying the returned std::future object. This article delves into the reasons behind this behavior, its implications, and the ongoing discussions within the C standardization committee.

The destructor of a std::future returned from std::async intentionally blocks to prevent potential inconsistencies and safety concerns. According to the papers published by members of the C standards committee, this blocking behavior is essential to ensure that the associated thread does not outlive the referencing std::future.

Without this blocking behavior, there is a risk of creating "run-away" threads that continue executing after the std::future has been destroyed, leading to potential memory corruption or data loss. This can occur when an unexpected exception is thrown during the execution of the asynchronous task, causing any subsequent get() or wait() operations on the std::future to be bypassed.

To mitigate this risk, the blocking destructor of the std::future provides a safety net by ensuring that the associated thread completes before the std::future object is destroyed. This behavior requires explicit handling by the programmer who must ensure that any necessary get() or wait() operations are performed before the std::future goes out of scope.

Despite the arguments in favor of a non-blocking destructor, the C standards committee has decided to retain the blocking behavior in C 14. However, proposed changes may be introduced in future revisions of the C standard based on ongoing discussions within the community.

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