On declarative programming in C++

Question

Often I face the problem of mapping the parameter space of one API onto the parameter space of another one. Often I see this solved by nested nested nested ... switch statements.

And I was wondering if there would happen to be a library or a technique that allows you to 'declare' the mapping instead of 'program' it.

A trivial example would consist of merging the values of two enumerates into one:

namespace sourceAPI {
  struct A { typedef e { A1, A2, A3 } };
  struct B { typedef e { B1, B2 } };
}

namespace targetAPI {
  struct AB { typedef e { A1B1, A1B2, A2B1, A2B2, A3B1, A3B2 } };
}

In which the mapping is often done like

switch( a ){
  case( A::A1 ): switch( b ) {
     case( B::B1 ): return A1B1;
     case( B::B2 ): return A1B2;
     ....
}

And this mapping still needs a 'reverse' switch, too.

But I would rather like something 'dense' like

declare( source( A::A1, B::B1 ), target( AB::A1B1 ) );
declare( source( A::A1, B::B2 ), target( AB::A1B2 ) );
....

Has anyone seen such a technique or framework or library?

Answer 1

You can use Boost.Bimap, which provides a bidirectional mapping between two types.

It has a bit of runtime overhead (generally, roughly the same amount of overhead you would get by using a pair of std::maps for this purpose, which isn't a whole lot).

It does allow you to define mappings about as densely as your example, though; generally you just add pairs to the map, one pair at a time.