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

29 Jan 2008

      AMDG

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);
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;
};

In Christ,
Steven Watanabe