[/
 / Copyright (c) 2003-2023 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:type_erasure Type Erasure, Separate Compilation and Virtual Functions]

The `any_completion_handler<>` class template can be used to type-erase
completion handlers. This template stores a completion handler in a
runtime-polymorphic wrapper, and forwards the function call operator,
associated executor, associated allocator, and associated cancellation slot to
the target handler.

One use case is to enable separate compilation of asynchronous operation
implementations. For example, we can declare our implementation function in a
header file, and provide a thin asynchronous operation wrapper:

  void async_sleep_impl(
      boost::asio::any_completion_handler<void(boost::system::error_code)> handler,
      boost::asio::any_io_executor ex, std::chrono::nanoseconds duration);

  template <typename CompletionToken>
  inline auto async_sleep(boost::asio::any_io_executor ex,
      std::chrono::nanoseconds duration, CompletionToken&& token)
  {
    return boost::asio::async_initiate<
      CompletionToken, void(boost::system::error_code)>(
        async_sleep_impl, token, std::move(ex), duration);
  }

By wrapping a call to `async_initiate`, the `async_sleep` template function
adds full support for completion tokens. The definition of `async_sleep_impl`
is then put into a separately compiled source file:

  void async_sleep_impl(
      boost::asio::any_completion_handler<void(boost::system::error_code)> handler,
      boost::asio::any_io_executor ex, std::chrono::nanoseconds duration)
  {
    auto timer = std::make_shared<boost::asio::steady_timer>(ex, duration);
    timer->async_wait(boost::asio::consign(std::move(handler), timer));
  }

Another use for `any_completion_handler<>` is to vary the implementation behind
an asynchronous operation at runtime, by using virtual functions:

  class line_reader
  {
  public:
    // ...

    template <typename CompletionToken>
    auto async_read_line(std::string prompt, CompletionToken&& token)
    {
      return boost::asio::async_initiate<
        CompletionToken, void(boost::system::error_code, std::string)>(
          [](auto handler, line_reader* self, std::string prompt)
          {
            self->async_read_line_impl(std::move(prompt), std::move(handler));
          }, token, this, prompt);
    }

  private:
    virtual void async_read_line_impl(std::string prompt,
        boost::asio::any_completion_handler<
          void(boost::system::error_code, std::string)> handler) = 0;
  };

A derived class provides the implementation of the operation:

  class stdin_line_reader : public line_reader
  {
  private:
    // ...

    void async_read_line_impl(std::string prompt,
        boost::asio::any_completion_handler<
          void(boost::system::error_code, std::string)> handler) override
    {
      // ...
    }
  };

[heading See Also]

[link boost_asio.reference.any_completion_handler any_completion_handler],
[link boost_asio.examples.cpp11_examples.type_erasure type erasure examples (C++11)],
[link boost_asio.examples.cpp20_examples.type_erasure type erasure examples (C++20)].

[endsect]
