Re: How to you access a nested template class?

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Sun, 18 Nov 2007 22:05:57 -0800

Message-ID:

<fhr94t$mfn$1@murdoch.acc.Virginia.EDU>

Adam Nielsen wrote:

Hi all,

I'm a bit confused about the syntax used to access a nested template
class. Essentially I have a bunch of class types to represent different
types of records in a database, and I want to store some cache data for
each datatype. The simplified code below demonstrates my issue - I
think I understand why it doesn't work as is (the structure only
declares the storage, I need to instantiate it for each template type)
but I'm not sure how to actually go about doing that.

Any pointers would be appreciated!

Thanks,
Adam.

class RecordTypeA { };
class RecordTypeB { };

class Main {

   private:

     template <typename T>
     struct CacheData {
       int i;
     };

This declares a type.

   public:

     template <typename T>
     void set(T t, int x)
     {
       // Store some type-specific data for record type T
       this->CacheData<T>::i = x;

There is no member variable this->CacheData<T> since CacheData<T> is a type.

     }

};

int main(void)
{
   RecordTypeA A;
   RecordTypeB B;
   Main m;
   m.set(A, 1);

You cannot call set that way. Templates do not make types into valid
arguments for functions.

   m.set(B, 2);

   // At this point, I want this to be the case:
   // m.CacheData<A>::i == 1
   // m.CacheData<B>::i == 2

   return 0;
}

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 );
}

Best

Kai-Uwe Bux