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  Accessing Class Data Members By Name
  Submitted by



First of all, I apologize for my dubious english, my native language is french.

So here is my first COTD submission. The following code provides a "string" interface for accessing to the data members of an object. For example, instead of doing sphere->SetRadius(12), you'll just have to do sphere->SetProperty("radius",12), given that the object sphere is an instance of a class based on the class I named 'Object'.

This mechanism is pretty useful when you write an interpreter for a description language, say, a scene description language for a raytracer (this is my case).

If you want to compile the following code snip with Visual C++ 6.0 (this comment probably applies to older versions too), make sure you checked the option "Enable Run-Time Type Information (RTTI)" in the Project Settings window, C/C++ tab, C++ Language category. I haven't even tried to compile this with any other compiler, but it should work since the thing is standard (well, at most part). However, you'll have to use a reasonably recent compiler which correctly implements the few features I used.

Enjoy and give me your opinion on the usefulness of such a code, and, especially, tell me if you have a more simple and/or elegant way to do this. Thank to all of you.

Cheers,
Francois

Download Associated File: property.cpp (1,441 bytes)

// Francois Beaune
// January 2001

#include <cstring>
#include <iostream>
#include <map>

using namespace std;

class CStringCmp { public: bool operator()(const char *s1, const char *s2) { return strcmp(s1, s2) < 0; } };

class Object { typedef void (Object::*mptr_double)(double); // pointer to 'Object' method taking double map<const char *,mptr_double,CStringCmp> mm_double; // map string to method taking double public: virtual void RegisterProperty(const char *name, mptr_double method) { mm_double[name] = method; }

virtual bool SetProperty(const char *name, double value) { mptr_double method = mm_double[name];

if(method) (this->*method)(value);

return method; } };

class Sphere : public Object { double radius; public: Sphere(double r) : radius(r) { RegisterProperty("radius",mptr_double(&Sphere::SetRadius)); }

void SetRadius(double r) { radius = r; }

double Radius() const { return radius; } };

int main() { Sphere *s = new Sphere(1);

if(s->SetProperty("radius",10)) cout << "s->radius set to " << s->Radius() << '.' << endl;

if(!s->SetProperty("undefined",1234)) cout << "The property \"undefined\" does not exist for object 's'." << endl;

Object *o = new Sphere(2);

if(o->SetProperty("radius",20)) cout << "o->radius set to " << dynamic_cast<Sphere *>(o)->Radius() << '.' << endl;

return 0; }

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