boost::asio::spawn 被 BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT 破坏了？答案

【问题标题】：boost::asio::spawn is broken with BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT?boost::asio::spawn 被 BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT 破坏了？
【发布时间】：2020-12-02 19:53:47
【问题描述】：

在将 boost 库从 1.71 更新到 1.74 后，我使用 boost::asio::spawn() 的代码无法使用带有预定义宏 BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT 的 MSVC2019（x64 为 19.25.28612）进行编译。 AFAIK，这个宏应该是为了向后兼容，我必须使用这个宏，因为我使用boost::beast::saved_handler，但它使用了未为提议的标准执行器定义的执行器工作守卫。

不仅如此，我的代码还使用 clang-11 成功编译（并定义了 BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT）。

这是一个简单的代码示例，它无法使用 MSVC2019 和 Boost.Asio:1.74 进行编译，但可以使用 Boost.Asio:1.71 进行编译（当然，boost::asio::any_io_executor 应替换为 boost::asio::executor）：

#define BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT
#include <boost/asio.hpp>
#include <boost/asio/spawn.hpp>

int main()
{
  boost::asio::io_context ctx;
  boost::asio::any_io_executor executor = ctx.get_executor();
  boost::asio::spawn(executor, [](auto yield){});
  return 0;
}

输出如下：

C:\local\boost_1_74_0\boost/asio/execution/execute.hpp(192): error C2672: 'boost::asio::strand<Executor>::execute': no matching overloaded function found
        with
        [
            Executor=boost::asio::executor
        ]
C:\local\boost_1_74_0\boost/asio/impl/dispatch.hpp(57): note: see reference to function template instantiation 'void asio_execution_execute_fn::impl::operator ()<boost::asio::strand<Executor>,boost::asio::detail::spawn_helper<handler_type,function_type>&>(T &&,F) noexcept(false) const' being compiled
        with
        [
            Executor=boost::asio::executor,
            T=boost::asio::strand<boost::asio::executor>,
            F=boost::asio::detail::spawn_helper<handler_type,function_type> &
        ]
C:\local\boost_1_74_0\boost/asio/async_result.hpp(151): note: see reference to function template instantiation 'void boost::asio::detail::initiate_dispatch::operator ()<boost::asio::detail::spawn_helper<handler_type,function_type>&>(CompletionHandler,void *) const' being compiled
        with
        [
            CompletionHandler=boost::asio::detail::spawn_helper<handler_type,function_type> &
        ]
C:\local\boost_1_74_0\boost/asio/async_result.hpp(363): note: see reference to function template instantiation 'boost::asio::async_result<boost::asio::detail::spawn_helper<handler_type,function_type>,Signature>::return_type boost::asio::async_result<boost::asio::detail::spawn_helper<handler_type,function_type>,Signature>::initiate<Initiation,boost::asio::detail::spawn_helper<handler_type,function_type>&,>(Initiation &&,RawCompletionToken)' being compiled
        with
        [
            Signature=void (void),
            Initiation=boost::asio::detail::initiate_dispatch,
            RawCompletionToken=boost::asio::detail::spawn_helper<handler_type,function_type> &
        ]
C:\local\boost_1_74_0\boost/asio/async_result.hpp(364): note: see reference to function template instantiation 'boost::asio::async_result<boost::asio::detail::spawn_helper<handler_type,function_type>,Signature>::return_type boost::asio::async_result<boost::asio::detail::spawn_helper<handler_type,function_type>,Signature>::initiate<Initiation,boost::asio::detail::spawn_helper<handler_type,function_type>&,>(Initiation &&,RawCompletionToken)' being compiled
        with
        [
            Signature=void (void),
            Initiation=boost::asio::detail::initiate_dispatch,
            RawCompletionToken=boost::asio::detail::spawn_helper<handler_type,function_type> &
        ]
C:\local\boost_1_74_0\boost/asio/impl/dispatch.hpp(212): note: see reference to function template instantiation 'void boost::asio::async_initiate<CompletionToken,void(void),boost::asio::detail::initiate_dispatch,>(Initiation &&,CompletionToken)' being compiled
        with
        [
            CompletionToken=boost::asio::detail::spawn_helper<handler_type,function_type> &,
            Initiation=boost::asio::detail::initiate_dispatch
        ]
C:\local\boost_1_74_0\boost/asio/impl/spawn.hpp(452): note: see reference to function template instantiation 'void boost::asio::dispatch<boost::asio::detail::spawn_helper<handler_type,function_type>&>(CompletionToken)' being compiled
        with
        [
            CompletionToken=boost::asio::detail::spawn_helper<handler_type,function_type> &
        ]
C:\local\boost_1_74_0\boost/asio/impl/spawn.hpp(493): note: see reference to function template instantiation 'void boost::asio::spawn<boost::asio::executor_binder<void (__cdecl *)(void),boost::asio::strand<Executor>>,Function>(Handler &&,Function &&,const boost::coroutines::attributes &,void *)' being compiled
        with
        [
            Executor=boost::asio::executor,
            Function=main::<lambda_c142a977195366454fba7e579637857b>,
            Handler=boost::asio::executor_binder<void (__cdecl *)(void),boost::asio::strand<boost::asio::executor>>
        ]
C:\local\boost_1_74_0\boost/asio/impl/spawn.hpp(483): note: see reference to function template instantiation 'void boost::asio::spawn<Function,Executor>(const boost::asio::strand<Executor> &,Function &&,const boost::coroutines::attributes &)' being compiled
        with
        [
            Function=main::<lambda_c142a977195366454fba7e579637857b>,
            Executor=boost::asio::executor
        ]
C:\Users\Artem Suleimanov\CLionProjects\test\main.cpp(10): note: see reference to function template instantiation 'void boost::asio::spawn<main::<lambda_c142a977195366454fba7e579637857b>,boost::asio::executor>(const Executor &,Function &&,const boost::coroutines::attributes &,void *)' being compiled
        with
        [
            Executor=boost::asio::executor,
            Function=main::<lambda_c142a977195366454fba7e579637857b>
        ]
C:\local\boost_1_74_0\boost/asio/execution/execute.hpp(192): error C2893: Failed to specialize function template 'enable_if<boost::asio::execution::can_execute<const Executor&,Function>::value,void>::type boost::asio::strand<Executor>::execute(Function &&) const'
        with
        [
            Executor=boost::asio::executor
        ]
C:\local\boost_1_74_0\boost/asio/strand.hpp(276): note: see declaration of 'boost::asio::strand<Executor>::execute'
        with
        [
            Executor=boost::asio::executor
        ]
C:\local\boost_1_74_0\boost/asio/execution/execute.hpp(192): note: With the following template arguments:
C:\local\boost_1_74_0\boost/asio/execution/execute.hpp(192): note: 'Function=boost::asio::detail::spawn_helper<handler_type,function_type> &'

这是编译器/增强错误还是我做错了什么？

【问题讨论】：

我认为这确实是一个特定于平台的问题。我不能说太多，因为在线编译器似乎没有 1.74.0 godbolt.org/z/sPsa8Y 顺便说一句。看起来您使用executor，而不是any_io_executor 作为消息，即使它是1.74。不确定这是否是故意的/重要的
BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT 开启时，executor == any_io_executor，它只是一个 typedef。

标签： c++ boost boost-asio boost-beast

【解决方案1】：

似乎这个问题是fixed。我应用了补丁，代码现在由 MSVC2019 编译。

此修复的发行说明条目：

修复了strand<> Networking TS 执行器在针对旧版本时的适配 C++ 版本或不太符合要求的编译器。

【讨论】：