My use case is a fast protocol decoder.  I've described the protocol in mpl and generate the instructions I need to decode/encode/store packets.  During decoding I have to be able to take a run time id and map it to the correct compile time generated code.  Traditionally such code was written as a switch/case.

 

The obvious solution is to use metaprogramatic recursion, for example mpl::for_each to check the id against each value.  You would hope the compiler would inline all the recursion, detect the ifs on an id, and do general switch/case optimization.  Unfortunately the code generated by gcc and it's not as good as I hoped.  They actually generate a function for every ~6 levels of recursion and just do a bunch of if/equal/else.

 

boost::variant has this same problem.  They use BOOST_PP_REPEAT to generate an actual switch/case statement(visitation_impl.hpp).  The compiler does a good job with the swich/case.  It removes redundant cases and then optimizes using array lookups and binary searches depending on size.  Good job!

 

1) Could we factor this logic out into a utility like mpl::switch_case? 

 

2) What would the best interface look like?

 

3) What other bits of code could benefit from it?

phoenix::switch_?

Meta State Machine?