Friday, May 23, 2008

Boost.Asio vs Asio

Sometimes I am asked what the difference is between the (non-Boost) Asio and Boost.Asio packages I provide. Here is the definitive word on the subject, presented as a series of questions and answers.

What are the differences in the source code?

— Asio is in a namespace called asio::, whereas Boost.Asio puts everything under boost::asio::.

— The main Asio header file is called asio.hpp. The corresponding header in Boost.Asio is boost/asio.hpp. All other headers are similarly changed.

— Any macros used by or defined in Asio are prefixed with ASIO_. In Boost.Asio they are prefixed with BOOST_ASIO_.

— Asio includes a class for launching threads, asio::thread. Boost.Asio does not include this class, to avoid overlap with the Boost.Thread library

— Boost.Asio uses the Boost.System library to provide support for error codes (boost::system::error_code and boost::system::system_error). Asio includes these under its own namespace (asio::error_code and asio::system_error). The Boost.System version of these classes currently supports better extensibility for user-defined error codes.

— Asio is header-file-only and for most uses does not require linking against any Boost library. Boost.Asio always requires that you link against the Boost.System library, and also against Boost.Thread if you want to launch threads using boost::thread.

Where do I get a release package?

Asio is available for download from sourceforge, in a package named asio-X.Y.Z.tar.gz (or .tar.bz2 or .zip).

Boost.Asio is included in the Boost 1.35 distribution. It is also available as a separate package on sourceforge, named boost_asio_X_Y_Z.tar.gz. The latter is intended to be copied over the top of an existing Boost source code distribution.

Where are the source code repositories?

Asio uses a sourceforge-hosted CVS repository. Details of how to access it may be found here. It may also be browsed via the web.

Boost.Asio is checked into Boost's subversion repository.

How do you maintain both versions?

All development is done in the Asio CVS repository. I periodically convert the source into Boost format using a script called, and merge the changes into the Boost subversion repository.

Will Asio be discontinued now that Boost.Asio is included with Boost?

No. There are projects using Asio and they will continue to be supported. I also prefer to use Asio over Boost.Asio in my own projects, for the convenience of header-file-only and shorter namespaces.

Should I use Asio or Boost.Asio?

It depends. Here are some things to consider:

— If you (like me) prefer the convenience of header-file-only libraries then I'd suggest using Asio over Boost.Asio.

— If you must use a version of Boost older than 1.35 then Boost.Asio is not included. You can use Boost.Asio by copying it over the top of your Boost distribution (see above), but not everyone is comfortable doing this. In that case, I would suggest using Asio over Boost.Asio.

— I will be creating new versions of both the Asio and Boost.Asio packages on a faster release cycle than that followed by Boost. If you want to use the latest features you can still use Boost.Asio as long as you are happy to copy it over the top of your Boost distribution. If you don't want to do this, use Asio rather than Boost.Asio.

Can Asio and Boost.Asio coexist in the same program?

Yes. Since they use different namespaces there should be no conflicts, although obviously the types themselves are not interchangeable. (In case you're wondering why you might want to do this, consider a situation where a program is using third party libraries that are also using Asio internally.)