我这么大个io_service去哪里了

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最近稍微闲了下来，开始debug Boost.Asio 析构时崩溃的问题。在目前代码中，使用到了deadline_timer作定时器。

由于早期仅用简单的定时器，也就没多了解io_service的用法，简单地在每个短周期业务实例中各开了一组thread和io_service来挂deadline_timer。后来出现了一个问题，运行时发现在析构业务实例时delete io_service偶尔会崩溃（具体定位过程就不在这边展开了），就去看了源代码和reference。

想着这么大的库api应该不会有太大的变化，点开boost最新版（1.71）reference，咦 O__O 我这么大个io_service去哪里了？

接下来从1.71版本往回翻，在1.66更新信息里发现一句话：

Implemented interface changes to reflect the Networking TS ( N4656).

• See the list of new interfaces and, where applicable, the corresponding old interfaces that have been superseded.
• The service template parameters, and the corresponding classes, are disabled by default. For example, instead of basic_socket we now have simply basic_socket. The old interface can be enabled by defining the BOOST_ASIO_ENABLE_OLD_SERVICES macro.

为了迎合C++ Networking TS的标准将接口改了，但是旧的接口依然还在支持。但一些service模板和相应的类默认不支持老版本，不过可以打开BOOST_ASIO_ENABLE_OLD_SERVICES宏来支持老版本service。

以下仅列了一些我感兴趣的修改，完整版可以看上文的list链接。

New interface	Old interface	Notes
io_context	io_service	The name io_service is retained as a typedef.
dispatch	io_service::dispatch	The dispatch free function can be used to submit functions to any Executor or ExecutionContext.
post	io_service::post	The dispatch free function can be used to submit functions to any Executor or ExecutionContext.
defer	io_service::post when the asio_handler_is_continuation hook returns true	The defer free function can be used to submit functions to any Executor or ExecutionContext.
io_context::poll	io_service::poll overload that takes error_code&	The error_code overload is not required.
io_context::poll_one	io_service::poll_one overload that takes error_code&	The error_code overload is not required.
io_context::run	io_service::run overload that takes error_code&	The error_code overload is not required.
io_context::run_one	io_service::run_one overload that takes error_code&	The error_code overload is not required.
io_context::run_for, io_context::run_until, io_context::run_one_for, and io_context::run_one_until		These functions add the ability to run an io_context for a limited time.
io_context::restart	io_service::reset
execution_context, execution_context::service, and execution_context::id	io_service, io_service::service, and io_service::id	The service-related functionality has been moved to the execution_context base class. This may also be used as a base for creating custom execution contexts.
make_service	add_service
strand	io_service::strand	This template works with any valid executor, and is itself a valid executor.
executor_work_guard and make_work_guard	io_service::work	Work tracking is now covered by the Executor requirements. These templates work with any valid executor.

在新接口表中我们可以看到`io_service`已经换成了`io_context`，且在1.66的[io_service](https://www.boost.org/doc/libs/1_66_0/doc/html/boost_asio/reference/io_service.html) reference中可以发现最上面就写了