[boost] Re: intrusive tagging allows omision of unneeded headers

From: Dirk Gerrits (dirk_at_[hidden])
Date: 2003-01-09 13:07:17

Thorsten Ottosen wrote:
> ----- Original Message -----
> From: "Dirk Gerrits" <dirk_at_[hidden]>
> To: "Boost mailing list" <boost_at_[hidden]>
> Sent: Wednesday, January 08, 2003 6:15 PM
> Subject: [boost] Re: intrusive tagging allows omision of unneeded headers
> 
> 
> 
>>Thorsten Ottosen wrote:
>>[snip]
>>
>>>class X
>>>{
>>>public:
>>>    class tag {};
>>>    typedef tag X_tag;
>>
>>       ^^^^^^^^^^^^^^^^^
>>
>>>};
>>>
>>>class bar
>>>{
>>>public:
>>>    class tag {};
>>>    typedef tag bar_tag;
>>
>>       ^^^^^^^^^^^^^^^^^^^
>>
>>>};
>>
>>Why these typedefs? Why would one write X::X_tag instead of X::tag for
>>example?
> 
> 
> how can we specifiy that we want to specialize for a particular class
> otherwise? Only by establising a convetion
> that all (involved) classes have a unique typedef can we destinguish the
> classes. For example
> 
> template< typename C >
> void foo_impl( const C& c, typename C::bar_tag )
> {
>     cout << "bar specialised version" << endl;
> }
> 
> will only be a candidate when C actually has a typedef bar_tag. due to
> SFINAE the instantiation is allowed
> to fail for classes that does not have a bar_tag typdef. And because the
> above foo_impl is more specialized
> than
> 
> template< typename C, typename Tag >
> void foo_impl( const C& c, Tag t )
> {
>     cout << "default version" << endl;
> }
> 
> the first foo_impl will be chosen as a better match.

Ahh, I guess I didn't fully understand everything that was going on. 
This makes sense. Thanks!

Dirk Gerrits