Question

In C, you can define structures to hold an assortment of variables;

typedef struct {
    float       sp;
    float       K;                 // interactive form - for display only
    float       Ti;                //  values are based in seconds
    float       Td;
} pid_data_t;

But lets say that K, Ti, and Td should never be set publicly, and should only be used for storing the values after they have been manipulated. So, I want these values not to be updated by;

pid_data_t = pid_data;
pid_data.K = 10;         // no good! changing K should be done via a function

I want them to be set via a function;

int8_t pid_set_pid_params(float new_K_dash, float new_Ti_dash, 
    float new_Td_dash)
{
    …                             // perform lots of things
    pid_data->K  = new_K_dash;
    pid_data->Ti = new_Ti_dash;
    pid_data->Td = new_Td_dash;
}

Any thoughts on this? I know C++ uses like a get/set property, but was wondering what people might do on C.

Answer 1

Your public interface should only offer an opaque pointer (maybe DATA*, or data_handle), as well as handler functions create_data(), set_data_value(), read_data_value(), free_data(), etc., which operate on the opaque pointer.

Much like FILE*.

Just don’t give your clients the internal header files 🙂

// library.h

typedef struct data_t * data_handle;

data_handle create_data();
void        free_data(data_handle);

Private implementation (don’t ship):

#include "library.h"

struct data_t
{
  /* ... */
};

data_handle create_data() { return malloc(sizeof(struct data_t)); }
void        free_data(data_handle h) { free(h); }
/* etc. etc. */