Re: How to you access a nested template class?

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Mon, 19 Nov 2007 20:39:32 -0800

Message-ID:

<fhtoer$l5n$1@murdoch.acc.Virginia.EDU>

Adam Nielsen wrote:

You may want to ponder about the following:

#include <cassert>
#include <map>

template < typename T >
class typemap {

  typedef void (*id) ( void );

  template < typename A >
  static
  void type_identifier ( void ) {}

  std::map< id, T > data;

public:

  template < typename A >
  T const & value ( void ) const {
    return ( data[ &type_identifier<A> ] );
  }

  template < typename A >
  T & value ( void ) {
    return ( data[ &type_identifier<A> ] );
  }

};

int main ( void ) {
  typemap<int> table;
  table.value<char>() = 2;
  table.value<int>() = 1;
  assert( table.value<char>() == 2 );
  assert( table.value<int>() == 1 );
}

That makes sense, thanks for the example! Out of curiosity, is there
any way of doing this without using a runtime lookup? Something like a
'template-variable', if there were such a thing.

You can do this for static data (e.g., you can have such data on a per class
basis):

#include <cassert>

template < typename T >
struct static_typemap {

  template < typename A >
  static
  T & value ( void ) {
    static T data;
    return ( data );
  }

};

int main ( void ) {

  static_typemap<int>::value<char>() = 1;
  static_typemap<int>::value<int>() = 2;

  assert( static_typemap<int>::value<char>() == 1 );
  assert( static_typemap<int>::value<int>() == 2 );

}

_If_ we had templated virtual member functions, we could do such tricks on a
per object basis (and all sorts of other cool stuff).

Best

Kai-Uwe Bux