PackagedTask

A packaged_task is a C++ template class defined in the <future> header. It wraps a callable entity, such as a function, lambda, or function object, and allows its execution to be deferred and its result to be retrieved asynchronously. Essentially, it bundles a task and its eventual return value together.

When a packaged_task is constructed, it takes a callable as an argument. This callable represents the work to be performed. The packaged_task itself does not execute the callable immediately. Instead, it stores the callable and provides a future object. This future object acts as a placeholder for the result of the callable when it is eventually executed.

To execute the task, you obtain the future from the packaged_task and then invoke the packaged_task object itself, passing any necessary arguments. The packaged_task then executes the wrapped callable with these arguments and stores the result or exception. The future object can then be used to retrieve this result. If the callable throws an exception, that exception is stored in the future and will be re-thrown when get() is called on the future.

Packaged_tasks are particularly useful for implementing asynchronous operations and for integrating with thread pools. They decouple the creation of a task from its execution and allow for flexible management of results and error handling.