RE: [boost] Incorrect MPL example and STATIC_ASSERT trouble

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2002-09-15 20:20:38

Dirk Gerrits wrote:
> I believe the example at 
> http://www.mywikinet.com/mpl/ref/Reference/find.html
> is broken. Intel's C++ Compiler 6.0 complains that iter 
> doesn't have a position member. (The real error message is 
> more cryptic of course. ;)

Fixed in sources. Here's the corrected version:

    typedef vector<char,int,unsigned,long,unsigned long> types;
    typedef find<types,unsigned>::type iter;
    BOOST_STATIC_ASSERT(iter::pos::value == 2);

> If this is true then I think the example should make use of
> distance instead:
> 
> typedef mpl::vector<char,int,unsigned,long,unsigned long> types;
> typedef mpl::find<types,unsigned>::type iter;
> std::size_t const position = 
>   mpl::distance<mpl::begin<types>::type, iter>::type::value;
> BOOST_STATIC_ASSERT(position == 2);

It could be written this way too; may be it's even _should_ be re-written as
you suggest, to keep a reader away from a conclusion that "iter::pos" is a
general-supported iterator notation as opposite to a 'vector'-specific one.

> Additionally, when I change the "== 2" to "== 3" then code 
> still compiles! All I get is a: 
> main.cpp(18): warning #70: incomplete type is not allowed
>   BOOST_STATIC_ASSERT(position == 3);
> But no errors.
> 
> Is this a fault on my part? Or is BOOST_STATIC_ASSERT broken
> for Intel's compiler?

Yes, it's "broken" in the sense that you get a warning instead of an error
(as promised in the docs). Fixed in the CVS.

Aleksey