Question

My application is written in C. I have a module that uses some data from a certain given global structure. I now have to extend the module to optionally work against a different given global structure, which basically provides the same fields (as far as my module is concerned), but under different names.

Here’s a car analogy to hopefully make my problem clearer. I’ve got these two global structures I have no control over.

struct {
   unsigned char manufacturer_id;
   unsigned short top_speed;
} Car;

struct {
   RGB_t color;
   unsigned short topSpeed;
   unsigned char mfr;
} Automobile;

Let’s say my Car Manager module uses information from Automobile. E.g.,

const char *car_manager__get_manufacturer_name(car_manager_t *self)
{
    return self->manufacturers[Automobile.mfr];
}

I’d like to extend Car Manager to optionally (perhaps decided by a flag in the car_manager_t instance) use the same information from Car, so the above function would return self->manufacturers[Car.manufacturer_id]. I don’t want to duplicate any logic in the module while adding this functionality.

I assume I’ll have to put an interface on the access to the global structures. Any suggestions on how to do that?

Answer 1

I would define functions for getting the needed values, and pass pointers to the functions. You could even pass a struct which contains the needed function pointers.

struct Interface {
  unsigned char  (*manufacturer)(void);
  unsigned short (*top_speed)(void);
}
struct Interface CarInterface  = {&Car_manufacturer, &Car_top_speed};
struct Interface AutoInterface = {&Auto_manufacturer, &Auto_top_speed};

const char *car_manager__get_manufacturer_name(car_manager_t *self, Interface i)
{
  return self->manufacturers[(*i.manufacturer)()];
}

I haven’t written any C for a long time; please correct my syntax if necessary!