Re: Understanding MPL

From: Edward Diener (eddielee_at_[hidden])
Date: 2003-03-31 15:46:31

David Abrahams wrote:
> "Edward Diener" <eddielee_at_[hidden]> writes:
>
>> I am trying to understand how the MPL library works. I first read
>> "The Boost C++ Metaprogramming Library" by Aleksey Gurtovoy and
>> David Abrahams. When I get to Basic Usage I see this:
>>
>> template<
>>       typename Condition
>>     , typename T1
>>     , typename T2
>>     >
>> struct if_
>> {
>>     typedef /*unspecified*/ type;
>> };
>>
>> I am really having trouble understanding the next to last line. How
>> can "typedef type;" be a valid C++ construct and,
>
> It's not.
>
>> if it is, what does it mean ?
>
> There's an unspecified type-expression which goes where the
> /*unspecified*/ comment is.
>
>> I always though that a typedef must specify an actual type but I
>> don't see any in the expression above.
>
> The point is that we're not showing it to you ;-)

I suspected that but I couldn't figure out why you wouldn't show it. However
it made it hard for me to understand how mpl::if_<> really works without
understanding what actually goes there.

>
> If you are coming to the ACCU conference in Oxford, I'll be giving a
> talk which describes how the MPL works in detail.

Much too far away if Oxford means England as I suspect. If it meant
Mississippi I could come<g>.