Re: [boost] [Boost-bugs] [ boost-Bugs-1617058 ] doc of placeholer expression requirements incomplete

From: David Abrahams (dave_at_[hidden])
Date: 2006-12-18 16:32:08

Larry Evans <cppljevans_at_[hidden]> writes:

> Maybe a caution like:
>
>    If the intended result of applyn<X,a1,...an>::type
>    is X<a1,...an> then to assure this result, there should
>    be, within the body of X, a nested:
>
>       typedef X type;
>
>    otherwise, if X<a1,...,an> has a supertype with such a
>    nested typdef, it will be used instead of the intended
>    result.
>
> within:
>
> http://www.boost.org/libs/mpl/doc/refmanual/placeholder-expression.html
>
> would avoid misunderstandings like mine.

Maybe, but inheriting a nested ::type is such a fundamental pattern of
use with MPL that I'm surprised you could have misunderstood this.

>From the point of view of a client of X,

     struct X { typedef int type; };

and 

     struct Y { typedef int type; };
     struct X : Y {};

are entirely equivalent.

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