为什么 std::future::wait_for 的行为不符合预期？

Question

#include <iostream>
#include <chrono>
#include <future>

using namespace std::literals;

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

    std::cout << std::boolalpha << (
         std::future_status::timeout ==
         fut.wait_for(std::chrono::seconds::max())
    );
}

The code should output nothing, because fut.wait_for will wait for a very very long time. However, it outputs true in no time!

Why does std::future::wait_for not behave as expected?

Answer 1

This is due to overflow with the relative time calculation resulting in Undefined Behavior.

There's nothing I've found in the standard that specifically addresses overflow with wait_for , so we need to rely on the standard handling of overflow, which depends on the underlying types involved.

std::chrono::seconds is typedef for std::duration , using a type that is a signed integral type of at least 35 bits . Adding two duration values involves adding two signed integers. It is Undefined Behavior when signed integer addition overflows .

Therefore, the behavior is undefined when waiting for a really long time.