gistfile1.cpp

// Here be dragons
template <typename Visitor>
struct VisitorBase {
    template<typename T>
    void visit(T &c) {
        T::template VisitorContinuation__<Visitor,0>::invoke(static_cast<Visitor&>(*this), c);
    }
};

#define STRUCT(name) \
    struct name { \
        typedef name ThisType__; \
        template<typename Visitor, int Tier> \
        struct VisitorContinuation__ { \
            static void invoke(Visitor &visitor, ThisType__ &c) {} \
        }; \
        enum { COUNTER_BASE__ = __COUNTER__ }; // technically this is ODR violation but we can probably get away with it

#define FIELD(name, type) \
    type name; \
    template<typename Visitor> \
    struct VisitorContinuation__<Visitor,(__COUNTER__-COUNTER_BASE__-1)/2> { \
        static void invoke(Visitor &visitor, ThisType__ &c) { \
            visitor.visit_field(# name, c.name); \
            VisitorContinuation__<Visitor,(__COUNTER__-COUNTER_BASE__)/2>::invoke(visitor, c); \
        } \
    };

#define END_STRUCT \
    };
// Here be dragons no longer

#include <cstdio>

struct Printer: VisitorBase<Printer>
{
    void visit_field(const char *name, int val) {
        ::printf("%s %d\n", name, val);
    }
    void visit_field(const char *name, float val) {
        ::printf("%s %.3f\n", name, val);
    }
};

STRUCT(Dummy)
    FIELD(foo, int)
    FIELD(bar, int)
END_STRUCT

STRUCT(Point3D)
    FIELD(x, float)
    FIELD(y, float)
    FIELD(z, float)
END_STRUCT

int main()
{
    Dummy dummy;
    dummy.foo = 42;
    dummy.bar = 0;

    Point3D point;
    point.x = 1.f;
    point.y = -1.f;
    point.z = .5f;

    Printer printer;
    printer.visit(dummy);
    printer.visit(point);

    return 0;
}