How can I add reflection to a C++ application?

 

I'd like to be able to introspect a C++ class for its name, contents (ie members and their types) etc. I'm talking native C++ here, not managed C++, which has reflection. I realise C++ supplies some limited information using RTTI. Which additional libraries (or other techniques) could supply this information?

What you need to do is have the preprocessor generate reflection data about the fields. This data can be stored as nested classes.

First, to make it easier and cleaner to write it in the preprocessor we will use typed expression. A typed expression is just an expression that puts the type in parenthesis. So instead of writing int x you will write (int) x . Here are some handy macros to help with typed expressions: 

#define REM(...) __VA_ARGS__
#define EAT(...)

// Retrieve the type
#define TYPEOF(x) DETAIL_TYPEOF(DETAIL_TYPEOF_PROBE x,)
#define DETAIL_TYPEOF(...) DETAIL_TYPEOF_HEAD(__VA_ARGS__)
#define DETAIL_TYPEOF_HEAD(x, ...) REM x
#define DETAIL_TYPEOF_PROBE(...) (__VA_ARGS__),
// Strip off the type
#define STRIP(x) EAT x
// Show the type without parenthesis
#define PAIR(x) REM x

Next, we define a REFLECTABLE macro to generate the data about each field(plus the field itself). This macro will be called like this: 

REFLECTABLE
(
    (const char *) name,
    (int) age
)

So using Boost.PP we iterate over each argument and generate the data like this: 

// A helper metafunction for adding const to a type
template<class M, class T>
struct make_const
{
    typedef T type;
};

template<class M, class T>
struct make_const<const M, T>
{
    typedef typename boost::add_const<T>::type type;
};


#define REFLECTABLE(...) 
static const int fields_n = BOOST_PP_VARIADIC_SIZE(__VA_ARGS__); 
friend struct reflector; 
template<int N, class Self> 
struct field_data {}; 
BOOST_PP_SEQ_FOR_EACH_I(REFLECT_EACH, data, BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__))

#define REFLECT_EACH(r, data, i, x) 
PAIR(x); 
template<class Self> 
struct field_data<i, Self> 
{ 
    Self & self; 
    field_data(Self & self) : self(self) {} 
    
    typename make_const<Self, TYPEOF(x)>::type & get() 
    { 
        return self.STRIP(x); 
    }
    typename boost::add_const<TYPEOF(x)>::type & get() const 
    { 
        return self.STRIP(x); 
    }
    const char * name() const 
    {
        return BOOST_PP_STRINGIZE(STRIP(x)); 
    } 
};

What this does is generate a constant fields_n that is number of reflectable fields in the class. Then it specializes the field_data for each field. It also friends the reflector class, this is so it can access the fields even when they are private: 

struct reflector
{
    //Get field_data at index N
    template<int N, class T>
    static typename T::template field_data<N, T> get_field_data(T& x)
    {
        return typename T::template field_data<N, T>(x);
    }

    // Get the number of fields
    template<class T>
    struct fields
    {
        static const int n = T::fields_n;
    };
};

Now to iterate over the fields we use the visitor pattern. We create an MPL range from 0 to the number of fields, and access the field data at that index. Then it passes the field data on to the user-provided visitor: 

struct field_visitor
{
    template<class C, class Visitor, class I>
    void operator()(C& c, Visitor v, I)
    {
        v(reflector::get_field_data<I::value>(c));
    }
};


template<class C, class Visitor>
void visit_each(C & c, Visitor v)
{
    typedef boost::mpl::range_c<int,0,reflector::fields<C>::n> range;
    boost::mpl::for_each<range>(boost::bind<void>(field_visitor(), boost::ref(c), v, _1));
}

Now for the moment of truth we put it all together. Here is how we can define a Person class that is reflectable: 

struct Person
{
    Person(const char *name, int age)
        :
        name(name),
        age(age)
    {
    }
private:
    REFLECTABLE
    (
        (const char *) name,
        (int) age
    )
};

Here is a generalized print_fields function using the reflection data to iterate over the fields: 

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());
}

An example of using the print_fields with the reflectable Person class: 

int main()
{
    Person p("Tom", 82);
    print_fields(p);
    return 0;
}

Which outputs: 

name=Tom
age=82

And voila, we have just implemented reflection in C++, in under 100 lines of code.

There are two kinds of reflection swimming around.

  • Inspection by iterating over members of a type, enumerating its methods and so on.

    This is not possible with C++.
  • Inspection by checking whether a class-type (class, struct, union) has a method or nested type, is derived from another particular type.

    This kind of thing is possible with C++ using template-tricks . Use boost::type_traits for many things (like checking whether a type is integral). For checking for the existance of a member function, use Is it possible to write a template to check for a function's existence? . For checking whether a certain nested type exists, use plain SFINAE .

    • If you are rather looking for ways to accomplish 1), like looking how many methods a class has, or like getting the string representation of a class id, then i'm afraid there is no Standard C++ way of doing this. You have to use either

  • A Meta Compiler like the Qt Meta Object Compiler which translates your code adding additional meta informations.
  • A Framework constisting of macros that allow you to add the required meta-informations. You would need to tell the framework all methods, the class-names, base-classes and everything it needs.

    • C++ is made with speed in mind. If you want high-level inspection, like C# or Java has, then I'm afraid i have to tell you there is no way without some effort.

    And I would love a pony, but ponies aren't free. :-p

    http://en.wikibooks.org/wiki/C%2B%2B_Programming/RTTI is what you're going to get. Reflection like you're thinking about -- fully descriptive metadata available at runtime -- just doesn't exist for C++ by default.

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