RE: [boost] When to use which ETI workaround?

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2003-03-30 20:21:47

Andreas Huber wrote:
> Hi Aleksey,

Hi Andreas,

Sorry for the late reply, got too many things on my plate. 

> I've stumbled over ETI again. Browsing through MPL I have found 
> different ways to circumvent it. In my case the following workaround
> seems to be sufficient:
> 
> template< class State >
> struct get_context_ptr_type
> {
>   typedef typename State::context_ptr_type type;
> };
> 
> template<>
> struct get_context_ptr_type< int >
> {
>   typedef int type;
> };
> 
> I.e. by simply specializing a given metafunction with int. 

Yep, in most cases that's enough. It's _always_ enough with MSVC 6.0, but
7.0 sometimes uses another, unknown, type for the instantiation's
placeholder argument, so you cannot simply specialize the template anymore,
but have to use 'is_msvc_eti_arg' predicate instead (which is based on the
knowledge that the type is still int-convertible; most probably it's an
enum), e.g.:

    template< class State >
    struct get_context_ptr_type_impl
    {
        typedef typename State::context_ptr_type type;
    };

    template< class State >
    struct get_context_ptr_type
        : if_c<
              is_msvc_eti_arg<State>::value
            , identity<int>
            , get_context_ptr_type_impl<State>
            >
    {
    };

> How do you decide when to use which workaround? Have you established 
> rules or do you simply work your way through them until one works?

The VC 7.0-specific case is rare enough to always try the 'int'
specialization first (which is obviously less painful than the other), and
then switch to the 'is_msvc_eti_arg'-based workaround if that doesn't help.
VC 7.0 is not our production environment here at work, so I didn't have
enough experience with it to be able to figure out when exactly these cases
occur.

HTH,
Aleksey