Data encapsulation in C

Question

I am currently working on an embedded system and I have a component on a board which appears two times. I would like to have one .c and one .h file for the component.

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Answer 1

For the specific case of writing hardware drivers for a microcontroller, which this appears to be, please consider doing like this.

Otherwise, use opaque/incomplete type. You'd be surprised to learn how shockingly few C programmers there are who know how to actually implement 100% private encapsulation of custom types. This is why there's some persistent myth about C lacking the OO feature known as private encapsulation. This myth originates from lack of C knowledge and nothing else.

This is how it goes:

ads1248.h

typedef struct ads1248_options_t ads1248_options_t; // incomplete/opaque type

ads1248_options_t* ads1248_init (parameters); // a "constructor"
void ads1248_destroy (ads1248_options_t* ads); // a "destructor"

ads1248.c

#include "ads1248.h"

struct ads1248_options_t {
    uint32_t pin_reset;
    uint32_t pin_drdy;
    uint32_t pin_start;
    volatile avr32_spi_t *spi_module;
    uint8_t cs_id;  
};

ads1248_options_t* ads1248_init (parameters)
{
  ads1248_options_t* ads = malloc(sizeof(ads1248_options_t));
  // do things with ads based on parameters
  return ads;
}

void ads1248_destroy (ads1248_options_t* ads)
{
  free(ads);
}

main.c

#include "ads1248.h"

int main()
{
  ads1248_options_t* ads = ads1248_init(parameters);
  ...
  ads1248_destroy(ads);
}

Now the code in main cannot access any of the struct members, all members are 100% private. It can only create a pointer to a struct object, not an instance of it. Works exactly like abstract base classes in C++, if you are familiar with that. The only difference is that you'll have to call the init/destroy functions manually, rather than using true constructors/destructors.