stdsharedfuture

stdsharedfuture is a C++ standard library component introduced in C++11, part of the concurrency features. It represents a result that may be available in the future, similar to stdfuture, but with the key distinction that stdsharedfuture can be copied and shared among multiple threads or objects. This shared access is crucial in scenarios where multiple consumers need to wait for or retrieve the same asynchronous result.

A stdsharedfuture object holds a shared state, which is a synchronized object that stores the result of an asynchronous operation. When a stdsharedfuture is copied, both the original and the copy refer to the same shared state. This means that when the result becomes available, any stdsharedfuture associated with that shared state can access it. The result can only be retrieved once from the shared state. Subsequent attempts to retrieve the result from any stdsharedfuture referring to that state will throw an exception.

stdsharedfuture is typically obtained from a stdpromise or a stdpackaged_task. A stdpromise is used to set the value or exception in the shared state, and then stdpromise::get_future() can be called to obtain a stdsharedfuture. Similarly, stdpackaged_task wraps a callable entity and provides a stdsharedfuture that will hold the return value of the wrapped callable when it's executed.

The primary use cases for stdsharedfuture involve situations where multiple threads need to access the outcome of a single asynchronous computation. For example, if a web server performs a complex calculation and multiple client requests depend on its result, stdsharedfuture allows each client to wait for and retrieve that single computed result. This contrasts with stdfuture, where each call to get() consumes the result, making it suitable for scenarios with a single consumer.