{"id":26524,"date":"2020-09-14T16:30:26","date_gmt":"2020-09-14T16:30:26","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/cppblog\/?p=26524"},"modified":"2020-09-14T18:15:30","modified_gmt":"2020-09-14T18:15:30","slug":"c-coroutines-in-visual-studio-2019-version-16-8","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/cppblog\/c-coroutines-in-visual-studio-2019-version-16-8\/","title":{"rendered":"C++ Coroutines in Visual Studio 2019 Version 16.8"},"content":{"rendered":"

Please see our Visual Studio 2019 version 16.8 Preview 3 release notes<\/a> for more of our latest features.<\/em><\/p>\n

It’s been a long journey for coroutines in C++ and in MSVC. We announced an early preview<\/a> of resumable functions in 2013, followed up by the \/await<\/code> switch and initial C++ standardization proposals<\/a> in 2014, to proposal revisions<\/a> in 2015, and have continued tracking the Coroutines TS (Technical Specification) progress through Visual Studio 2017 and 2019. With the adoption of coroutines into the C++ standard in 2019, we are now pleased to announce feature completion of C++20 coroutines in Visual Studio 2019 version 16.8.<\/p>\n

Standard vs. TS Coroutines<\/h3>\n

The coroutine support that ultimately made it through the standardization process and became part of C++20 is different from the early proposal drafts and from experimental coroutine support we’ve had in MSVC under the \/await<\/code> switch. This led us to two important and contradictory goals in finishing the coroutine language support in 16.8:<\/p>\n

    \n
  1. Provide an implementation of C++20 coroutines that strictly follows the standard, allowing users to write and consume portable code.<\/li>\n
  2. Ensure existing users of experimental coroutines can painlessly upgrade to 16.8 without needing to change their code.<\/li>\n<\/ol>\n

    As the proposal changed, we’ve added new support whenever possible without breaking existing code for early adopters of coroutines. This is of course not standard: It still accepts all the old keywords, names, and signatures, counter to goal 1. There are also a small number of behavior changes from the original versions we implemented under \/await<\/code>, such as how a promise object is constructed. These could cause a program that previously compiled to fail to compile or behave differently at runtime.<\/p>\n

    Standard Mode – \/std:c++latest<\/h4>\n

    Support for C++20 coroutines without legacy TS support is now enabled when using a compiler language version mode newer than C++17. For now, this is \/std:c++latest<\/code> and will continue into numbered version switches after C++17 as these are added. When compiling with such a language switch and without \/await<\/code> you get strict support for C++20 coroutines with library support in the <coroutine><\/code> header and defined in the std<\/code> namespace. This mode will emit errors on non-standard code from earlier proposals, such as a bare await<\/code> keywords or an initial_suspend<\/code> function that returns bool<\/code>, and only supports the standard behaviors when they differ from earlier implementations.<\/p>\n

    Extension Mode – \/await<\/h4>\n

    Early adopters of coroutines can continue to compile their non-standard code with the \/await<\/code> switch and any of the language version switches (including \/std:c++latest<\/code>), and continue to use the experimental headers and namespace. We have added missing standard features and bug fixes in this mode as long as they don’t break compatibility.<\/p>\n

    We recommend existing coroutine users move to standard coroutines as soon as possible, and new users should favor the standard mode over \/await<\/code>.\u00a0 Support for the \/await<\/code> switch will continue for existing users, but the future of coroutines is in the standard mode and new features will be implemented there. Excepting some corner cases migrating a project from \/await<\/code> to C++20 is a straightforward process.<\/p>\n

    What’s New in 16.8<\/h3>\n

    Version 16.8 introduces several new features and improvements in coroutines:<\/p>\n