Re: [Boost-users] Generic indexing by "const char *"

29 Jan 2008

      Joseph Fradley wrote:
...
I have a problem that I would like to solve in the most generic
non-intrusive way possible (such as boost serialization). What I want
is to access class members both via it's initially designed accessors
but also via a "const char *"  key. In addition, I want to pass list
of key's (via a vector of const char * or a single delimited multikey
char *) to get nested member access.
For example:
...
// these should be equivalent
Point p;
int val;
val = p.x;
val = p.getX();
p.getMemberValue("x", val);
// there also
Rectangle r;
int val;
Point pVal;
r.getMemberValue("corner:x", val);
r.getMemberValue("corner", pVal);
Steven Watanabe wrote:
...
You could try something along these lines:
class indexable_base {
public:
 virtual indexable_base* lookup(const char*) = 0;
 virtual const std::type_info& get_type() = 0;
   virtual void* get() = 0;
};
Then you can put macros in the derived class like so:
class Rectangle : public indexable_base {
  DEFINE_LOOKUP() {
      DECLARE_VARIABLE(corner);
  }
  Point Corner;
};
Which would expand to something like
class Rectangle : public indexable_base {
  template<class T>
  void getMemberValue(const char* key, T& out) {
      indexable_base result = lookup(key);
      if(typeid(T) == result->get_type()) {
          out = *(static_cast(result->get()))
      } else {
          throw(std::bad_cast());
      }
  }
  indexable_base* lookup(const char* key) {
      // split key at ":" and lookup call lookup_ recursively until
all the keys are used up.
  }
  static void init() {
      map_.insert(std::make_pair("corner",
&boost::bind(&Rectangle::corner, _1)));
  }
  Point corner;
};
Steven,

Thank you, I'm trying it now. But I'm a confused as to what type of
map to create from your line :
   map_.insert(std::make_pair("corner", &boost::bind(&Rectangle::corner, _1)));

I've just now read up on boost::bind and I can only see documentation
of binding to member functions not member data. Could you elaborate?

Joe