Boost Statechart vs. Meta State Machine

As there seems to be much interest, please allow me to give my (obviously biased) opinion, which should therefore be taken with a grain of salt:

• MSM is much faster
• MSM requires no RTTI or anything virtual
• MSM has a more complete UML2 support (for example internal transitions, UML-conform orthogonal regions)
• MSM offers a descriptive language (actually several). For example, using the eUML front-end, a transition can be described as Source + Event [Guard] / Action == Target
• MSM will make your compiler suffer for bigger state machines, so you will need a pretty recent compiler (g++ >= 4.x, VC >= 9)

You can make yourself a better opinion by looking for comments posted during the review of MSM. This subject was much discussed on the developer list.

As Christophe has already mentioned, one of the key differences between the two libraries is runtime performance. While MSM probably offers the best you can get here, Statechart consciously trades memory and processor cycles towards better scalability.

With Boost.Statechart you can spread the layout (i.e. states, transitions) of your state machine over multiple translation units (cpp files) in ways you can't with MSM. This allows you to make the implementation of large FSMs more maintainable and get much faster compilation than with MSM.

Whether or not the performance overhead of Statechart compared to MSM will actually be significant for your application is often quite easy to answer when you ask yourself how many events your app will have to process per second.

Assuming a moderately complex FSM implemented with Boost.Statechart, here are a few ballpark numbers:

• Most current PC hardware will easily cope with >100'000 events per second
• Even very resource-constrained hardware will be able to process a few hundred events per second.

Regarding CPU load, if the number of events to process is much lower than these numbers, Boost.Statechart overhead compared to MSM will almost certainly not be noticeable. If the number is much higher, you're definitely better off with MSM.

More in-depth information on the performance/scalability tradeoffs can be found here: http://www.boost.org/doc/libs/1_45_0/libs/statechart/doc/performance.html