std::async function running serially

Question

When using std::async with launch::async in a for loop, my code runs serially in the same thread, as if each async call waits for the previous before launching. In the notes for

Answer 1

The std::future returned by async blocks in the destructor. That means when you reach the } of

for(uint64_t start: chunksToDownload){
    DownloadItem cache = std::make_shared<__DownloadItem__>();
    cache->last_access = time(NULL);
    cache->future =
            std::async(std::launch::async, &FileIO::download, this, api,cache, cacheName, start, start + BLOCK_DOWNLOAD_SIZE - 1);
 }  // <-- When we get here

cache is destroyed which in turn calls the destructor offuture which waits for the thread to finish.

What you need to do is store each future returned from async in a separate persistent future that is declared outside of the for loop.