{"id":105863,"date":"2021-11-01T07:00:00","date_gmt":"2021-11-01T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=105863"},"modified":"2021-11-01T06:53:18","modified_gmt":"2021-11-01T13:53:18","slug":"20211101-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20211101-00\/?p=105863","title":{"rendered":"Why do all this coroutine stuff if you can just use std::future<\/CODE>?"},"content":{"rendered":"

One of my colleagues reacted to my series on creating custom coroutines<\/a> by asking, “Why did you bother with this simple_task<\/code> custom coroutine? Why not just use std::future<\/code> objects produced by std::async<\/code>? You can co_await<\/code> a std::future<\/code>.”<\/p>\n

Okay, first of all, the ability to co_await<\/code> a std::future<\/code> is not a feature of the standard (as of this writing). It’s something that the Microsoft STL provides as a courtesy. And that courtesy has not always been smooth sailing<\/a>.<\/p>\n

For another thing, std::async<\/code> lets you pick how the lambda runs, but your only options are to run it on a separate thread, or to run the lambda only when the value is awaited. The lambda itself can’t do any co_await<\/code>ing because it is not a coroutine. It’s just a plain synchronous lambda.<\/p>\n

But the real kicker is that the Microsoft STL implementation of std::future<\/code> is written in\u2026 PPL, The very library we are trying to get rid of by switching to coroutines in the first place.<\/p>\n

That last bit was the clincher as far as my colleague was concerned. He sent me the astronaut with a gun meme<\/a>, but instead of asking, “Wait, it’s all Ohio?”, the first astronaut asks, “std::future is just PPL?” The reply from the astronaut with the gun remains “Always has been.”<\/p>\n

Meme background<\/b>: Wait it’s all Ohio? Always has been<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"

Escape from PPL.<\/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-105863","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"

Escape from PPL.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/105863","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=105863"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/105863\/revisions"}],"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=105863"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=105863"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=105863"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}