specifying object type at compile time

act_x

given a class heirarch where A is the base class and B , C are derived classes . Now I want to implement something like an object factory externally where I want to automatically create an object of the appropriate type . For this I define a struct with the definition ( pardon me for the pseudo code as part of the problem is with usage)

typedef struct 
{ 
  int code ;
  reference_to_type_creator ; 
}OBJ_TYPE ; 

OBJ_TYPE objTable[] = 
{ objA_code  , < reference to create an object of type A > } , 
{ objB_code  , < reference to create an object of type B > } , 
{ objC_code  , < reference to create an object of type C > } , 

{ -1 , NULL }

such that I can use this table in a function that processes random object entries

ProcessObject( int objCode  )
{
for (int i=0;objTable[i].code!=-1;i++)
{
    if( objCode == objTable[i].code)
   {
      A *a = objTable[i].reference_to_type_creator     // Unsure about how this could work 
      a->Foo()
   }
}
}

I was unclear as to how this reference would be specified in the struct array above and its usage. Help is appreciated }

Engineering is the effort !

Chris Losinger

i'd just do it with a switch

ProcessObject( int objCode )
{
A* a = NULL;
switch (objCode)
{
case 0:
a = new whatever0;
break;
case 1:
a = new whatever1;
break;
etc..
}
a->Foo();
}

otherwise, maybe you could add static Create methods to your object types, and put refs to those in your array (off the top of my head):

class whatever0 : public A
{
static public whatever0 * Create()
{
return new whatever0();
}
...
};
class whatever1 : public A
{
static public whatever1 * Create()
{
return new whatever1();
}
...
};

...

OBJ_TYPE objTable[] =
{
{0, whatever0::Create},
{1, whatever1::Create},
};

... where OBJ_TYPE is a struct with an int and a function pointer that returns an A*

you'd need the static Create functions because you can't do function pointers to non-static member classes.

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