Re: Understanding MPL

"Joel de Guzman" writes:

Edward Diener wrote:

...

David Abrahams wrote:

...

"Edward Diener" writes:

...

...

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.

Because it's an implementation detail :-) You need not know the guts of how if_ really works, as long as it works. The same way as you don't need to know how if (c) a; else b; works, (in terms of low level code) as long as it works.

However, perhaps the /*unspecified*/ would better be enhanced by pseudo-code?

template< typename Condition , typename T1 , typename T2 > struct if_ { /* if (Condition) T1 else T2 */ typedef /* implementation detail */ type; };

Thoughts? Dave? Aleksey?

We're not hacking the paper at this point (working on the book instead ;-)) -- Dave Abrahams Boost Consulting www.boost-consulting.com