Which io_context does std::boost::asio::post / dispatch use?

Question

While using boost::asio 1.66 I read in the documentation that boost::asio::io_context::post is deprecated for boost::asio::post, same for boost::asio::io_context::dispatch. Because before they where member functions of the io_context before, and of course the handler needs to be executed in the context of some io_context i.e. executor my question is:

How does boost::asio::io_context::post simplest overload know which io_context i.e. executor to use?

The documentation of template< typename CompletionToken> DEDUCED post(CompletionToken && token); states that

Obtains the handler's associated executor object ex by performing get_associated_executor(handler).

But the documentation of get_associated_executor does not make it clear to me either. My guess is due to the Template argument deduction it can grab it somehow in the currently executed handler, but I want to make sure and also, this wouldn't suffice if I call post outside of a boost::asio handler.

Answer 1

The core of the documentation is found with the associated_executor trait:

• get()

  If T has a nested type executor_type, returns t.get_executor(). Otherwise returns ex.

• executor_type

  If T has a nested type executor_type, T::executor_type. Otherwise Executor.

If your handler type¹ has a nested executor_type type, then it is assumed that calling token.get() will return the correct executor to use.

If you pass a vanilla calleable to post without specifying an executor/execution context you will get a default-constructed instance of an execution context: boost::asio::system_executor.

The purpose of this is for the implementation to DoTheRightThing with custom handler types. E.g. if you post something on a strand, the handler will be wrapped in a type specific to the strand implementation. The associated_executor trait and ditto get_executor() member function will then coordinate to direct to the executor for that strand.

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¹ or any token, in case your call model is different, like a yield context