How to specify degenerate dimension of boost multi_array at runtime?

Question

I have a 3D multi_array and I would like to make 2D slices using dimensions specified at runtime. I know the index of degenerate dimension and the index of a slice that I want t

Answer 1

What you're trying to do is move a variable from run time to compile time. This can only be done with a chain of if else statements or a switch statement.

A simplified example

// print a compile time int
template< int I >
void printer( void )
{
   std::cout << I << '\n';
}

// print a run time int
void printer( int i )
{
   // translate a runtime int to a compile time int
   switch( i )
   {
      case 1: printer<1>(); break;
      case 2: printer<2>(); break;
      case 3: printer<3>(); break;
      case 4: printer<4>(); break;
      default: throw std::logic_error( "not implemented" );
   }
}

// compile time ints
enum{ enum_i = 2 };
const int const_i = 3;
constexpr i constexper_i( void ) { return 4; }

// run time ints
extern int func_i( void ); // { return 5; }
extern int global_i; // = 6

int main()
{
   int local_i = 7;
   const int local_const_i = 8;

   printer();
   printer();
   printer();
   //printer();
   //printer();
   //printer();
   printer();

   printer( enum_i );
   printer( const_i );
   printer( constexpr_i() );
   printer( func_i()      ); // throws an exception
   printer( global_i      ); // throws an exception
   printer( local_i       ); // throws an exception
   printer( local_const_i ); // throws an exception
}