David B. Held wrote:
> > > > template <class Policy>
> > > > struct get_category
> > > > : mpl::if_<
> > > > mpl::is_placeholder<Policy>
> > > > , mpl::identity<Policy>
> > > > , get_category_impl<Policy>
> > > > >::type
> > > > {
> > > > BOOST_MPL_AUX_LAMBDA_SUPPORT(1,get_category,(Policy))
> > > > };
> > [...]
>
> When is it better to derive from something vs. composing it
> in a typedef?
It's mostly the matter of style. Typedef is inherently more verbose:
template <class Policy>
struct get_category
{
typedef typename mpl::if_<
mpl::is_placeholder<Policy>
, mpl::identity<Policy>
, get_category_impl<Policy>
>::type::type type;
BOOST_MPL_AUX_LAMBDA_SUPPORT(1,get_category,(Policy))
};
There are also cases when inheritance is preferable because it brings along
other base class' members, besides the canonical 'type' typedef:
// enables both 'is_same<...>::value' and 'is_same<...>::type'
// notations
template< typename T1, typename T2 >
struct is_same
: bool_c<false>
{
};
> Also, we're really close on the smart pointer code. I've
> checked in the latest version to the sandbox, applying your changes.
> However, I still get 14 errors that are all in mpl/identity.hpp.
No wonder - you are including MPL headers inside the opened 'boost'
namespace (line 194) ;).
[...]
> BOOST_MPL_AUX_VOID_SPEC(1, identity)
> BOOST_MPL_AUX_VOID_SPEC(1, make_identity)
>
> Now, I notice that I'm supposed to define these for the policies under
> bcc.
That's an oversight. I am planning to fix it shortly so
BOOST_MPL_AUX_LAMBDA_SUPPORT will be self-sufficient.
HTH,
Aleksey
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