CODEWITHSUNDEEP
c++c++11promisec++14future
Curious
Curious

Reputation: 21510

Who is responsible for the shared state of futures and promises

Who owns the shared state in futures and promises? In particular who is responsible for the construction and deletion of the shared state in these classes? Or is the shared state supposed to be reference counted? I am not able to get an answer by reading the docs for these on cppreference.

The way I was thinking about it the easiest thing to do would be to have the std::promise class be responsible for the creation of the shared state, but then hand it off to the std::future that is fetched from the std::promise for deletion when the future is destroyed. But then this approach can lead to dangling promise objects. So I am not sure how really the state is supposed to be shared between the two.

For example does the code below produce undefined behavior (since the shared state might be destroyed when the future is destroyed)?

auto prom = std::promise<void>{};
{
    auto fut = prom.get_future();
}
prom.set_value();

Further, the docs for std::promise::~promise on cppreference say "if the shared state is ready, releases it" which gets me to think that the shared state is not reference counted.

Upvotes: 8

Views: 1926

Answers (1)

Ap31
Ap31

Reputation: 3314

When the std::future (or std::promise) is destroyed, it releases its shared state.

This rule states that when an asynchronous return object or an asynchronous provider is said to release its shared state, it gives up its reference to the shared state.

If that reference was the last one, the shared state is destroyed.

So yes, shared state seems to be reference counted and no, your code example does not produce UB since prom still holds a reference to the shared state.

Upvotes: 9

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