Iterate through Struct and Class Members [duplicate]

Is it possible in C++ to iterate through a Struct or Class to find all of its members? For example, if I have struct a, and class b:

struct a
{
  int a;
  int b;
  int c;
}

class b
{
  public:
    int a;
    int b;
  private:
    int c;
}

Would it be possible to loop them to say get a print statement saying "Struct a has int named a, b, c" or "Class b has int named a, b, c"

Solution 1:

There are a couple of ways to do this, but you need to use some macros to either define or adapt the struct.

You can use the REFLECTABLE macro given in this answer to define the struct like this:

struct A
{
    REFLECTABLE
    (
        (int) a,
        (int) b,
        (int) c
    )
};

And then you can iterate over the fields and print each value like this:

struct print_visitor
{
    template<class FieldData>
    void operator()(FieldData f)
    {
        std::cout << f.name() << "=" << f.get() << std::endl;
    }
};

template<class T>
void print_fields(T & x)
{
    visit_each(x, print_visitor());
}

A x;
print_fields(x);

Another way is to adapt the struct as a fusion sequence (see the documentation). Here's an example:

struct A
{
    int a;
    int b;
    int c;
};

BOOST_FUSION_ADAPT_STRUCT
(
    A,
    (int, a)
    (int, b)
    (int, c)
)

Then you can print the fields as well using this:

struct print_visitor
{
    template<class Index, class C>
    void operator()(Index, C & c)
    {

        std::cout << boost::fusion::extension::struct_member_name<C, Index::value>::call() 
                  << "=" 
                  << boost:::fusion::at<Index>(c) 
                  << std::endl;
    }
};


template<class C>
void print_fields(C & c)
{
    typedef boost::mpl::range_c<int,0, boost::fusion::result_of::size<C>::type::value> range;
    boost::mpl::for_each<range>(boost::bind<void>(print_visitor(), boost::ref(c), _1));
}

Solution 2:

No, it's not possible, because there is no reflection in C++.

Solution 3:

If you have members of the same type (as you do in your first specific example) that you want to both (a) have names, and (b) be iterable, then you can combine an array with an enum:

enum names { alice, bob, carl };
struct myStruct;
{
  std::array<int, 3> members;
}

Then you can both

myStruct instance;
// iterate through them...
for (auto elem : instance.members)
{
    // work with each element in sequence
} 
// and call them by name, taking away the need to remember which element is the first, etc.
instance.members[bob] = 100;

Clearly not a general solution, but I've found this useful in my own work.