Re: Understanding MPL

31 Mar 2003

      "Edward Diener"  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 ;-)

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

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com