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Question #339 Difficulty: 2

According to the C++23 standard, what is the output of this program? 

    #include <future>
#include <iostream>

int main()
{
    std::promise<int> p;
    std::future<int> f = p.get_future();
    p.set_value(1);
    std::cout << f.get();
    std::cout << f.get();
}

Correct!

You can only get the value from a future once. It is undefined behaviour to call get multiple times, but implementations are encouraged to throw an exception, and most (such as GCC, Clang and MSVC) also do so at the same of writing.

More details:

First, we call get for the first time. §[futures.unique.future]¶16:

R future::get();
R& future<R&>::get();
void future<void>::get();

Effects:

— wait()s until the shared state is ready, then retrieves the value stored in the shared state;

— releases any shared state (§[futures.state]).

So the first call to get gets the value, and releases the shared state. What does it mean to release the shared state? §[futures.state]¶5:

When an asynchronous return object [the future in this case] (...) is said to release its shared state, it means:

— (...)

— the return object (...) gives up its reference to its shared state; (...)

Then we try to call get again, but §[futures.unique.future]¶3 says:

The effect of calling any member function other than the destructor, the move-assignment operator, share, or valid on a future object for which valid() == false is undefined.

(...)

Recommended practice: Implementations should detect this case and throw an object of type future_error with an error condition of future_errc::no_state.

So if valid() == false, calling get a second time is UB (but an exception is usually thrown). What does valid() do? §[futures.unique.future]¶20:

bool valid() const noexcept;

Returns: true only if *this refers to a shared state.

The first time we called get, we gave up our reference to the shared state, and from then on *this no longer refers to a shared state, valid() now returns false, and calling get again is UB.

You can explore this question further on C++ Insights or Compiler Explorer!

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