Are there any implicit memory barriers in C++

In the following code, is using atomics necessary to guarantee race-free semantics on all platforms or does the use the use of a promise.set_value/future.wait imply some kind of implicit memory barrier that would allow me to rely on the flag write having become visible to the outer thread?

std::atomic_bool flag{false}; // <- does this need to be atomic?runInThreadPoolBlocking([&]() {    // do something    flag.store(true);});if (flag.load()) // do something// simplified runInThreadPoolBlocking implementationtemplate <typename Callable>void runInThreadPoolBlocking(Callable func){    std::promise<void> prom;    auto fut = prom.get_future();    enqueueToThreadPool([&]() {        func();        prom.set_value();    });    fut.get();}

In general, are there any "implicit" memory barriers guaranteed by the standard for things like thread.join() or futures?