std::future in C++

One of the most useful tools are available in the C++ Standard Library for multithreading and asynchronous programming is std:: future. This part is essential to handling asynchronous operations and getting output from jobs that are running concurrently. Included in the C++11 concurrency utilities, it provides a standard method for handling asynchronous computations, allowing programmers to create responsive and effective programs.

What is std::future?

The std::future is a method for getting a value from a concurrent operation, which can be operating asynchronously in a different thread or task. It contains a value that might not be accessible right now and offers a means to retrieve the outcome after the related task is finished. This feature is a part of C++'s larger concurrency architecture, which promotes effective communication between asynchronous tasks.

Basic Usage:

The <future> header file needs to be included in the code to use std::future. Creating a std::future object linked to a specific asynchronous job and then retrieving the result when it becomes available constitute the basic workflow.

Example:

Let us take a look at this straightforward example, in which the function performTask() yields an asynchronous result:

Output:

Result: 42

Explanation:

Function performTask():-

• performTask() is a function that uses std::this_thread::sleep_for(std::chrono::seconds(3)) to simulate a laborious operation.
• It returns the integer 42 after three seconds of waiting.

main() Function:

• A std::future named result is created inside the main()
• An asynchronous job is started using std::async() by using performTask() in a different thread, which is identified by std::launch::async.
• The outcome of the asynchronous task is linked to the std::future object result.

Get the Result:

• The program continues execution after starting the asynchronous job ("Do other work while the performTask is running asynchronously").
• Use the result function when the outcome of an asynchronous action is required.
• The std::future get() method freezes execution until the result is ready.
• The acquired result is saved in the int variable finalResult.
• Finally, the program uses std::cout to print the received result.

std::future in C++:

• std::future: It is a placeholder for a value that may not yet be available. It is used to get the outcome of an asynchronous operation.
• std::async(): It is a function that starts an asynchronous task and returns a std::future with the task's outcome.
• The std::launch::async keyword specifies that the job should be run asynchronously in a different thread.
• std::future::get(): This function is used to retrieve the outcome of an asynchronous operation. If the result is not yet ready, the get() method will block the current thread until it is.
• Asynchronous Operations: The code uses std::async to start a time-consuming action (performTask()) in a separate thread, allowing the main thread to continue processing while the operation is running asynchronously.
• Synchronization: The program uses get() method to synchronise with the asynchronous process and receive the final result when it becomes available, guaranteeing that the programme only continues after the result is retrieved.

Overall, this code shows how to use std::future in C++ to conduct an asynchronous job (performTask()) and efficiently retrieve its result while allowing the main thread to continue its execution.

Managing Exceptions:-

Additionally, std::future offers a mechanism to handle exceptions that may arise while the asynchronous task is being executed. Exceptions are caught using std::future::wait() or std::future::wait_for() and are propagated by utilizing the std::future::get() method.

Timeouts and Wait Functions:-

There are various ways to wait for the outcome to become available or to specify a waiting timeout period with std::future.

The wait(), wait_for(), and wait_until() functions are among them.

wait(): Waits for the result to become available before blocking the current thread.

wait_for(): This method is used to hold out on releasing the result for a predetermined amount of time.

wait_until(): This method is used to hold out on releasing the result until a specified moment.

• These techniques provide flexibility in handling situations when waiting endlessly might not be desired and in managing asynchronous processes.

Shared Futures and Promises:-

• C++ provides std::promise and std::shared_future in addition to std::future to help with interthread communication.
• When setting a value or exception that can be retrieved asynchronously via a std::future, use std::promise.
• The asynchronous outcome of many std::future instances can be shared due to the std::shared_future.

Best Practices and Considerations:

• Avoid unnecessary blocking: Over-blocking a future could cause problems with performance in a multi-threaded setting. Employ non-blocking methods whenever it is practical.
• Exception Handling: When using std::future, always handle exceptions carefully to avoid unexpected program termination.
• Resource management: Exercise caution while determining the ownership and lifespan of resources, particularly when working with shared futures and numerous threads.
• Use std::async wisely: Although std::async is a handy technique to start asynchronous operations, different implementations may behave differently. Before widespread adoption, be aware of its subtleties.

Conclusion:

In conclusion, C++'s std::future module is an effective tool for organizing asynchronous tasks and generating the output of concurrent processes. Because of its features, developers can create asynchronous computations in a standard manner and create responsive and effective apps. Programmers can use std::future's capabilities to create scalable and reliable concurrent programming applications by learning how to use it, handling errors, employing wait functions, and applying best practices.