Is promise future needed ?

Question

We don't have callable function passed to thread returns value.
So, Std::ref is the one which helps

Other option is using promise which also help to provide us value out of thread function

I have used both the approach in below code.

Question 1 :
So, Why so much complication of promise and future when we can use std::ref easily

Question 2 :
Is there any other use case of future and promise or is it just unnecessary alternate?

Question 3 :
Also, future promise is complication as we have to join thread also which is blocking call
and future.get is also a blocking call.
https://sololearn.com/compiler-playground/cBMoWNzA2Fph/?ref=app

Answer 1

1. Why use std::promise and std::future over std::ref?
While std::ref can work when you just want to modify a shared variable, it's not thread-safe by default. std::promise and std::future provide a synchronized, safer way to pass results from threads, especially when you don't want to deal with locks.

2. Use cases where promise/future are better:

When the data will be available asynchronously at a later point.

When you want synchronization between threads (e.g., a producer-consumer pattern).

To avoid race conditions in multi-threaded environments.

For complex threading workflows like std::async, task chaining, etc.

3. Blocking concern:
Yes, future.get() is a blocking call, but that’s often what we need—it waits until the thread has finished its job and produced the result. If you don’t want to block, you can use wait_for() or wait_until() to check status.

So, while std::ref works in simpler cases, promise/future give you greater control and safety in complex or asynchronous systems.