Mohammad's Q4 2023 Update

By Mohammad Nejati on

Jan 10, 2024

Over the last few months I have been mainly working on Boost.Beast and Boost.PropertyTree.

Keeping Boost.Beast in Good Form

I’ve recently taken on a more active role in maintaining Boost.Beast. To begin, I reviewed all the open issues to gain a better understanding of the project’s current state. In the course of this process, I successfully addressed several issues that did not necessitate significant refactoring.

Here are a couple of contributions to the project that I find interesting:

Specializing asio::associator for bind_wrapper and bind_front_wrapper

Because bind_wrapper and bind_front_wrapper wrap the user’s handlers, all of the associators with the original handler become invisible to Asio. In order to resolve that, Beast has been specializing each associator individually for bound wrappers. However, this makes Asio consistently assume the presence of an associated executor with the bound handlers.

Fortunately, the fix is easy; we only need to specialize asio::associator for the bound wrappers, and it can query all the associators from the wrapper handler. You can read more in this pull request.

Replacing internal uses of beast::bind_front_handler with asio::prepend

bind_front_handler is a utility in Beast that allows binding a list of arguments to an existing handler and creating a new handler with a different signature. This is especially useful when we want to pass additional arguments to handlers. For example, the following code binds an error code to a handler, making it invokable without needing any argument:

asio::dispatch(ex, beast::bind_front_handler(std::move(handler), ec));

With the introduction of asio::prepend in Boost 1.80, we can replace the previous code with:

asio::dispatch(ex, asio::prepend(std::move(handler), ec));

However, beast::bind_front_handler has a specialized invoke function for member functions, which makes it possible to concisely turn member functions into handlers:

ws_.async_read(
    buffer_,
    beast::bind_front_handler(
        &websocket_session::on_read,
        shared_from_this()));

This isn’t possible with asio::prepend; that’s why I decided to leave uses of beast::bind_front_handler in the examples intact.

Trimming Dead Leaves off Boost.PropertyTree

PropertyTree has served Boost users for almost two decades. However, it experienced a period without an active maintainer, resulting in a backlog of issues and pull requests in its repository.

Given PropertyTree’s shift to maintenance mode, we made the decision to refrain from introducing new features or making breaking changes to the interfaces. Consequently, I reviewed all the issues and pull requests, addressing them unless they involved adding a new feature. With these fixes implemented and an improved CI script, PropertyTree was prepared for the Boost 1.84 release, hopefully in a slightly better state.