Question

Is there any way to add a field to a class at runtime ( a field that didn’t exist before ) ? Something like this snippet :

Myobject *ob; // create an object ob->addField('newField',44); // we add the field to the class and we assign an initial value to it printf('%d',ob->newField); // now we can access that field 

I don’t really care how it would be done , I don’t care if it’s an ugly hack or not , I would like to know if it could be done , and a small example , if possible .

Another Example: say I have an XML file describing this class :

<class name='MyClass'>    <member name='field1' />    <member name='field2' /> </class> 

and I want to ‘add’ the fields ‘field1’ and ‘field2’ to the class (assuming the class already exists) . Let’s say this is the code for the class :

class MyClass { }; 

I don’t want to create a class at runtime , I just want to add members/fields to an existing one .

Thank you !

Answer 1

Use a map and a variant.

For example, using boost::variant. See http://www.boost.org/doc/libs/1_36_0/doc/html/variant.html

(But of course, you can create your own, to suit the types of your XML attributes.)

#include <map> #include <boost/variant.hpp>  typedef boost::variant< int, std::string > MyValue ; typedef std::map<std::string, MyValue> MyValueMap ; 

By adding MyValueMap as a member of your class, you can add properties according to their names. Which means the code:

oMyValueMap.insert(std::make_pair("newField", 44)) ; oMyValueMap.insert(std::make_pair("newField2", "Hello World")) ; std::cout << oMyValueMap["newField"] ; std::cout << oMyValueMap["newField2"] ; 

By encapsulating it in a MyObject class, and adding the right overloaded accessors in this MyObject class, the code above becomes somewhat clearer:

oMyObject.addField("newField", 44) ; oMyObject.addField("newField2", "Hello World") ; std::cout << oMyObject["newField"] ; std::cout << oMyObject["newField2"] ; 

But you lose somewhat the type safety of C++ doing so. But for XML, this is unavoidable, I guess.