Use C preprocessor to iterate over structure fields - c++

Question

I have several different C++ structs and classes with fields of the same name, that I have to copy between frequently. I would like to do something like: (in bashy pseudocode)
struct S{double a;
double b;
double c;};
class C{public: void set_a(double a);
void set_b(double b);
void set_c(double c); };
S s; C c;
#FOR F in FIELDSOF(S)
c.set_${F}(${F});
#ENDFOR
Whether or not it a good idea, is there a way to abuse either the C++ preprocessor or C++ templates to achieve this? I use g++ and clang++.
I am already aware of templating engines like MAKO, and I'm also aware I could write a program to do code generation. If you must know, one of the things I would like to use this for is filling Google protobufs from C++ structs.

Answer 1

If you already have a Boost dependency, you can use BOOST_FUSION_ADAPT_STRUCT and use Fusion to iterate over the members. This also allows you to work with types, which is not possible in a pure preprocessor approach.
You also will need to map member functions to fusion sequences to make that more automatic.
All in all: Just write a constructor.

Answer 2

See this answer, which shows how to iterate over the members of a class. Then using those macros, the two classes can be reflectable like this:
struct S
{
REFLECTABLE
(
(double) a,
(double) b,
(double) c
)
};
class C
{
private:
REFLECTABLE
(
(double) a,
(double) b,
(double) c
)
public:
void set_a(double a);
void set_b(double b);
void set_c(double c);
};
Then to create a generic assignment, based on the name of the member variable:
struct assign_fields_visitor
{
template<class FieldData1, class FieldData2>
void operator()(FieldData1 fd1, FieldData2 fd2)
{
if (strcmp(fd1.name(), fd2.name()) == 0)
{
fd1.get() = fd2.get();
}
}
};
struct assign_fields
{
template<class X, class FieldData>
void operator()(X & x, FieldData f)
{
visit_each(x, boost::bind(assign_fields_visitor(), f, _1));
}
};
template<class L, class R>
void assign(L & lhs, const R& rhs)
{
visit_each(rhs, boost::bind(assign_fields(), boost::ref(lhs), _1));
}
Finally, it can be called like this:
S s; C c;
assign(c, s);