Re: message processing using template

Zoltan Juhasz <>
Fri, 18 May 2012 14:06:23 -0700 (PDT)
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As you can see, vast majority of the information is actually only
available at run-time. That means that the part that you can actually
do at compile time, is to
- automatically establish the type selection,
- automatically associate user-defined type-id with a given type
- call the appropriate handler in a user-defined functor, given
   that it provides an overload for all the expected types

The run-time type selection is effectively 'emulates' the switch-case
(actually an if-elseif-else) construct. It will not be pretty for
sure, so unless you'll work with a very volatile set of types that
can appear in the binary file, it is probably not worth it.

Anyway, the basic building blocks that you'll need:

- a run-time, user-defined type-id (e.g. int type in your example)
- a type-list that contains all the types appearing in the file
- a base class that
     - acts as a base-class for user-defined types, that contains the
       type-id member data, and initializes at run-time with the
       appropriate type-id (derived from the type-list position)
     - entry point to process the input (let's call it process())
- a tryProcess functor that establishes the if-elseif-else construct by
     recursively calling itself and iterating through the type-list,
     with a termination case if the type is not present

template < typename Derived, typename TypeList >
struct BinaryInterface
     int const type_id;

     typedef TypeList DefinedTypes;

     // in ctr initialize type_id with the Derived type's position
     // inside the type-list (DefinedTypes)
     // see mpl::vector, mpl::find and boost::is_same

     // static function to read from raw input
     // (e.g. mem-location in this case)
     template< typename FunT >
     static FunT process( void * & input, FunT functor )
         return Internal::tryProcess<
                 mpl::size< DefinedTypes >::value - 1, BinaryInterface
             >()( input, functor );

The tryProcess can be something along these lines:

template < std::size_t N, typename BinaryInterfaceT >
struct tryProcess
     typedef typename BinaryInterfaceT::DefinedTypes DefinedTypes;

     // current type at position N in the type-list
     typedef typename mpl::at< DefinedTypes, mpl::int_< N > >::type

     template< typename FunT >
     FunT operator()( void * & input, FunT functor )
         // is this the type we are looking for?
         if ( static_cast< BinaryInterfaceT * >( input )->type_id == N )
             // get a typed ptr out of input
             DerivedType * derivedInput =
                 static_cast< DerivedType * > ( input );

             // tell the user-defined functor the last read-size
             // and the advance the pointer appropriately
             reinterpret_cast< char * & >( input )
                 += functor.readSize = sizeof( DerivedType );

             // execute the user-defined functor on the last object we read
             // it needs to overload the operator() for all expected types
             functor( derivedInput );
             return functor;
         // nope, keep looking
             return tryProcess< N-1, BinaryInterfaceT >()( input, functor );

// partial-specialization for the termination case
// the 0th position in the type-list is reserved for the error case
// this is not really needed if you are willing to play with indeces
template< typename BinaryInterfaceT >
struct tryProcess< 0, BinaryInterfaceT >
     template< typename FunT >
     FunT operator()( void * & input, FunT functor )
         throw OutOfBound(
             static_cast< BinaryInterfaceT * >( input ) -> type_id
         return functor; // never executed, but to prevent warnings

And finally the type-list is:

typedef boost::mpl::vector<
     boost::mpl::void_ // placeholder, place types after this
     , MyType1
     , MyType2



-- Zoltan

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