Understanding std::shared_future

and Its Usage

In modern C++ programming, std::shared_future plays a vital role in managing asynchronous operations, often considered a powerful tool for dealing with futures and promises. The question of whether it is legal or suitable to pass std::shared_future as a reference to functions arises from considerations of efficiency, safety, and design. Let us delve into the nuances of this topic.

What is std::shared_future

?

std::shared_future is part of the Standard Library’s Concurrency support, designed to hold a shared state as a shared ownership mechanism that can safely be accessed from multiple threads.

Key characteristics of std::shared_future :

• Shared Ownership: Multiple copies of a std::shared_future can co-exist, and all share ownership of the shared state.
• Immutable Handling: Once it holds a valid future, the value cannot be altered, but can be accessed concurrently.
• Methodology: A copy of std::shared_future can be freely passed between threads.

Passing std::shared_future

to Functions

When considering passing std::shared_future to a function, we must understand the implications regarding its immutability and the efficiency of copying.

The Legality

Yes, it is legal to pass std::shared_future as a reference to functions. Here is why:

• Legal Reference: std::shared_future is a class template capable of being passed by reference, like any C++ object.
• Copy Validity: Copying std::shared_future is safe and does not invalidate the original instance.
• Constant Access: The function receiving the reference can call constant member functions to access the contained value safely.

Example

• Copy Overhead: std::shared_future is lightweight when copied because it only shares ownership metadata.
• Reference Efficiency: Passing by reference can be marginally more efficient since it avoids the copy operation.
• Immutability Needs: When passing to functions that should not alter the access semantics of the shared state.
• Performance: When aiming to optimize for minimal copying overhead, especially under scenarios involving heavy multithreading.
• Convenience: When threading patterns distribute ownership without concern for minimal copying costs.
• Semantic Clarity: When the function naturally uses and retains std::shared_future in copying contexts.
• Early Deletion: Ensure that the originating std::promise or future source outlives the std::shared_future .
• get Method Wait: Remember that get in constant context returns immediately and shares previously obtained values.