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

[Dirk Gerrits]

Thorsten Ottosen wrote:

----- Original Message -----
From: "Dirk Gerrits" <[EMAIL PROTECTED]>
To: "Boost mailing list" <[EMAIL PROTECTED]>
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


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