boost pp repeat to forward declare classes

Question

In short, I would like to accomplish the following:

#define CLASS( C ) class C;
#define CLASSES( ... )

CLASSES( Foo, Bar, Golf )    // Expand: class Foo; class Bar; class Golf;

Is there any way to accomplish this using boost?

Currently, I have tried the following:

#include <boost/preprocessor/repetition/repeat.hpp>

#define CLASS( C ) class C;

#define CLASSES( N, ... ) BOOST_PP_REPEAT( N, CLASS, Name) // problem here

CLASSES( 3, Foo, Bar, Golf ) // Expand: class Name; class Name; class Name;

But I don't know how to incorporate __VA_ARGS__ in order to expand different names.

I am open to other suggestions, not strict on boost, but restricted to C++11.

Answer 1

Here's a BOOST PP example using CLASSES(x, y, z) to expand to class forward declaration statements.

#include <boost/preprocessor/library.hpp> // The whole BOOST_PP library (overkill).

#define CLASSES_SEMI() ;

#define CLASSES_OP(r, data, i, elem) \
    BOOST_PP_IF(i, CLASSES_SEMI, BOOST_PP_EMPTY)() \
    class elem \
    /**/

#define CLASSES_SEQ(seq) \
    BOOST_PP_SEQ_FOR_EACH_I(CLASSES_OP, /*no data*/, seq) \
    /**/

#define CLASSES(...) CLASSES_SEQ(BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__))

CLASSES(Foo, Bar, Golf);

Generates:

class Foo ; class Bar ; class Golf ;

Answer 2

@Eljay 's post is the accepted answer and works perfectly well. I would just like to complement the accepted answer with an alternative method and the modified version I used from Eljay's answer.

Option 1: No boost

#define DEC_CLASS( C ) class C;

#define EACH_0( ELEM )
#define EACH_1( ELEM, X ) ELEM( X ) 
#define EACH_2( ELEM, X, ... ) ELEM( X ) EACH_1( ELEM, __VA_ARGS__ )
#define EACH_3( ELEM, X, ... ) ELEM( X ) EACH_2( ELEM, __VA_ARGS__ )

#define OPTION( _0, _1, _2, _3, NAME, ... ) NAME

#define CLASSES( ... ) OPTION( _, __VA_ARGS__, EACH_3, EACH_2, EACH_1, EACH_0) ( DEC_CLASS, __VA_ARGS__ )

CLASSES( Foo, Bar, Golf )

Option 2: Using boost (modified version from @Eljay)

#include <boost/preprocessor/seq/for_each_i.hpp>
#include <boost/preprocessor/variadic/to_seq.hpp>

#define DEC_CLASS( r, data, i, elem ) class elem;

#define CLASSES_SEQ( SEQ ) BOOST_PP_SEQ_FOR_EACH_I(DEC_CLASS, , SEQ) 

#define CLASSES(...) CLASSES_SEQ(BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__))

CLASSES(Foo, Bar, Golf);