Hi Simon,
I think it would be worthwhile talking a little more about compile time performance in the documentation.
You're right, I could explain more in the documentation. Unfortunately, the answer is not straightforward. The compilation time (and how far the compiler plays with us) depends on a number of factors: 1) the compiler 2) the compiler version 3) even your hardware (I get very different compile times on 2 different machines with the same processor) 4) the state machine structure and the chosen front-end 1) usually, g++ crashes later than VC and needed fewer workarounds 2) Surprisingly, g++4.2 > g++ 4.4. VC10 > VC9 > VC8 3) you will need RAM, especially with VC8 4) the compile-time is proportional to the number of transitions in the transition table and to the depth of the state machine (meaning submachine of submachine of...). EUML will cost you much more with VC than g++ as it implies a metaprogramming layer above the standard metaprogramming of the standard front-end and because the underlying decltype/typeof seems much better implemented with g++ (if you don't count the generated code size). A small test with VC9 in Release mode (and no other code) gave me: 20 transitions => 11s 30 transitions => 16s 40 transitions => 22s 50 transitions => 31s This is just to give you an idea as it can greatly depend on the factors named above.
Also I'd like to see some suggestions of how to improve compile times (and how factors including the number of entries in a transition table affect the compile time).
There is just one solution: reduce the number of transitions. The best way to do it is using orthogonal regions. For example, if you see in your diagram many transitions to a single state based on the same event, you should factor them all into a second region. Adding submachines could help in some cases. On one hand, the compiler now has to generate code for a second machine, OTOH it reduces the number of transitions in the main one. This will only help if your submachine handles a small number of events, as Msm will add a row in the main table for every event of the submachines. If you can in exchange reduce the number of transitions, it can help.
Is there provision to split a machine across multiple compilation units (I assume not)?
You unfortunately assume correctly, as what costs compile-time really is the heavy metaprogramming.
You include instructions for how to increase the maximum size of an MPL vector, but this only works up to 50 entries. What do we need to do to increase this further?
This is pretty simple, we need to: - add a vector60/70/80....hpp in mpl/vector. - add a map60/70....hpp in mpl/map. - if more than 50 states, add vector60/70....hpp in fusion/vector This is pretty simple. If that helps, I can add them into the Msm sandbox as I already wrote them. We could also request a feature in the MPL. You will however not manage to increase the size of the transition table forever. At some point will almost every compiler crash in a big cloud of dark smoke. To give you an idea, I usually manage: - 60-80 transitions max with VC9. The exact point is variable on a number of different factors which are hard to understand. - probably less with VC8. VC8 will also quite fast need > 2GB RAM. - 80 transitions with g++4.2. It could probably manage more but I ran out of RAM. - 50-60 with g++4.4. Why 4.4 is in this case less good than 4.2 is a mystery to me. - In Debug mode, you'll see the compiler give up faster - Euml will also have an influence.
Euml looks really cool, but I'm getting an ICE when using it with 36 transitions in EumlSimple.cpp (12 and 24 work fine though)
VC8 only has limited support for eUML for 3 reasons: - less than perfect metaprogramming capabilities - bad support for SFINAE - VC problems with decltype/typeof So yes, I would advise eUML for VC8 for only small state machines and without using complicated functors as actions/guards. VC9 would work much better, VC10 better than VC9 too (so Microsoft seems to be working on it).
One of my favourite features of boost::statechart is state local storage. It looks like this would have to be done manually in msm?
Yes, this is not supported yet. However, Msm has always been developed very user-oriented and what users ask, they usually get it. Msm has from the beginning greatly gained from clever people suggesting new features and I will not stop the development after the review, whatever the outcome is. If you'd like a feature, I'd be happy to discuss a solution with you and implement it in the next version, thus making Msm more useful.
The library is looking really powerful and expressive (not to mention fast!) for small state machines.
Thanks. This is the reason for the compile-times ;-) Christophe