Re: [boost] Preprocessor directive for a sequence that differs only in datatype

16 Jul 2017

      On 07/11/2017 06:17 AM, Larry Evans via Boost wrote:
...
On 07/11/2017 03:09 AM, Rishabh Arora via Boost wrote:
...
Thank you, Gavin Lambert.
I don't think any of the preprocessor directives will work then? because
type cannot be determined at comile time? Is there any other way to do
that?
What about replacing the switch with an index into a vector
of function pointers?  Something like:
#include <iostream>
template < class T >
void print_column(std::size_t col) {
   T dummy;
   std::cout <<dummy<< std::endl;
   return;
}
using fptr=void (*)(std::size_t);
fptr printers[]=
{ print_column<int>
, print_column<float>
};
int main()
{
   unsigned ncol_=2;
   for(unsigned i=0; i<ncol_; ++i)
   {
     printers[i](0);
   }
   return 0;
}
More specifically:

#ifdef USE_PRINTER_TYPES
   template<typename... Types>
   struct printers
   {
     using fptr = void(data_frame::*)(size_t);
     static constexpr fptr _[sizeof...(Types)]=
     { &data_frame::print_column<Types>...
     };
   };
   using printer_types=printers<COLUMN_DATA_TYPES>;
#endif//USE_PRINTER_TYPES

Then, use printer_types in the data_frame::print function as:

   std::cout << "[" << column_headers_(i) << "]" << ": ";
   auto const itype=base_::operator[](column_headers_(i)).type();
#ifdef USE_PRINTER_TYPES
   printer_types::fptr printer=printer_types::_[itype];
   (this->*printer)(i);
#else
   switch(itype) {
.
.
.
#endif//USE_PRINTER_TYPES

HTH.

-regards,
Larry