{"id":112774,"date":"2026-10-14T07:00:00","date_gmt":"2026-10-14T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=112774"},"modified":"2026-10-15T12:11:34","modified_gmt":"2026-10-15T19:11:34","slug":"20261014-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20261014-00\/?p=112774\/","title":{"rendered":"C++ futures part 3: The mental model for std::async<\/CODE>"},"content":{"rendered":"

We continue our study of C++ futures<\/a> with std::async<\/code>.<\/p>\n

The more complex case is the std::async<\/code>. You give std::async<\/code> a callable and optional parameters to pass to that callable, and you also specify how and when you want the callable to be invoked. You get two axes: Do you want it to be calculated immediately or on demand? Do you want the calculation to happen on the thread that requests the value or on its own thread?<\/p>\n

Now, you’d think you could fill out the table like this:<\/p>\n\n\n\n\n\n
\u00a0<\/th>\nImmediate<\/th>\nOn demand (“deferred”)<\/th>\n<\/tr>\n
Same thread<\/th>\nCall now and save the value<\/td>\nCall when waited<\/td>\n<\/tr>\n
New thread (“async”)<\/th>\nStart the thread now<\/td>\nStart the thread when waited<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

However, that’s not what the C++ standard says.<\/p>\n

The C++ standard disallows asking for an immediate value on the same thread. There’s really no value to doing it that way, since there is no actual “future”; it all happens in the present! “This future could have been a function call.”<\/p>\n

auto f = std::async({}, calculator);\r\n\r\nauto result = f.get();\r\n<\/pre>\n
This would, in theory, mean “Immediately call the calculator and wait for it to produce a value. Store it in the source<\/code> and return a future that fetches the value from that source.” But that’s the same as just<\/p>\n
auto result = calculator();\r\n<\/pre>\n
I’m guessing that the C++ standard disallows it because there’s no point.<\/p>\n
The C++ standard also says that if you combine both std::launch::deferred<\/code> and std::launch::async<\/code>, it doesn’t mean that you get a calculation that lazy-starts on its own thread. Instead, the standard says that the implementation can choose which flag to honor and which to ignore.<\/p>\n
So really, the table is<\/p>\n\n\n\n\n\n
\u00a0<\/th>\nImmediate<\/th>\nOn demand (“deferred”)<\/th>\n<\/tr>\n
Same thread<\/th>\nIllegal<\/td>\nCall when waited<\/td>\n<\/tr>\n
New thread (“async”)<\/th>\nStart the thread now<\/td>\nDealer’s choice<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
In all cases, the result of the callable (either a value or an exception) is stored in the source<\/code>.<\/p>\n
Now, I noted that there’s no point to asking std::async<\/code> for an immediate on-thread calculation. But maybe you want to do it anyway because you are interfacing with some other component that expects to receive a future<\/code>. How could you get the same result?<\/p>\n
We’ll look at that next time.<\/p>\n","protected":false},"excerpt":{"rendered":"
Doing things later in different ways.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-112774","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Doing things later in different ways.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/112774","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=112774"}],"version-history":[{"count":1,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/112774\/revisions"}],"predecessor-version":[{"id":112775,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/112774\/revisions\/112775"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=112774"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=112774"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=112774"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}