Re: [Boost-users] Sequence of Types, What For?

From: Terry G (tjgolubi_at_[hidden])
Date: 2006-11-07 23:42:45

>> BTW, on the boost website, there are three versions of the FSM player
>> example.
> Where?

Its in the Boost 1.33.1 download from the website, in 
boost/libs/mpl/example/fsm.
I have the book CD too, of course.  Which is "best"?

>> I write stuff that I think should work, but the compiler issues tons of
>> error messages.
> Did you read the chapter on diagnostics and apply the advice therein?

Read, yes.  Apply, mostly no.  I use my text editor to process the verbiage, 
and I use two compilers.
I've been reluctant to learn another tool, i.e. the filters.
Are any of the Lints especially good with Boost/MPL?

>> Pitfalls like when to use typename.
> Did you read the appendix on the typename and template keywords?

Read, yes.  Understand, sort of.  I can kind of tell when I've omitted the 
typename keyword, because that's what the compilers usually tell me.  I 
never think to use it proactively.  The special use of the template keyword, 
I may have needed once.

> The FSM example shows the basic technique for "generating a switch"
> with TMP... although in this particular case you *might* have been
> better off using the preprocessor library (also covered in the book).

Yeah, sort of tried BOOST_PP too, but it konks out around 130 cases, saying 
something about reaching compiler nesting limits (VC8). It was just plain 
fun to play with the sequences. I used BOOST_PP_... SEQ_ENUM, SEQ_HEAD, 
SEQ_FOR_EACH, and STRINGIZE. It took an entire day to get this to work 
because I tinkered around a lot. I ended up just using a simple macro to 
generate each case in 15 minutes.

BTW, using the DerefTag<TAG> technique from my previous post did net me a 
tiny advantage. Since I know all the message tags right now, from a spec, I 
can create the message printer right now, with a case for each message tag. 
Since the DerefTag<>::type defaults to the Message base class until its 
explicitly specialized, my printer will display a human readable tagname and 
then a hex dump for each  unimplemented message. Then when I get around to 
actually implementing each derived Message, my printer code doesn't need to 
change at all. I just add the appropriate DerefTag specialization and an 
ostream<< operator, right next to the derived message class, and the entire 
message magically appears in human readable format.  Cool.  I didn't see 
that one coming!

> I feel your pain.

Thanks. I needed that.

My son is 6 years old.  He wants things his way, right now, no 
complications.
He is just learning to really read rather than just skimming and looking at 
the pictures.
He reads the same books over and over.
He still has a lot to learn.

terry