Opaque Pointers

Opaque pointers is a design pattern that can be used to hide implementation details of what is being pointed at. Typically a user for a different place in the code can use as see a pointer to a particular type but cannot see any more details about the type itself

Opaque pointers is commonly used in the C programming language to hide implementation details of a struct (object) from a user, and only operations with the object via a function-based API. It is not as commonly used in C++ because C++ gives you the ability to define private and protected members of your class, removing the need to implement the same behaviour with opaque pointers.

Likely the most significant disadvantage to this pattern is that memory allocation must be used in the constructor to create the object, because the caller cannot create the object since the full size of the type is hidden away from them.

Example

The below example shows how to use the opaque pointer design pattern to implement a simple “Counter” object (you could think of it as a class). The Counter object is defined in Counter.h/Counter.c and created/used in main.c.

In the header Counter.h:

#include <stdint.h>

// Opaque declaration of the counter type.
typedef struct Counter_t Counter_t;

Counter_t *Counter_Init();
void Counter_AddOne(Counter_t *me);
uint8_t Counter_GetCount(Counter_t *me);
void Counter_DeInit(Counter_t *me);

And in the .c file Counter.c:

#include <stdlib.h>

#include "Counter.h"

// Implementation of Counter_t. Hidden from other files!
struct Counter_t {
  uint8_t count;
};

Counter_t *Counter_Init() {
  // NOTE: Memory allocation here! May or not be ok for an embedded
  // project, can be done at initialation time only.
  Counter_t *newCounter = malloc(sizeof(Counter_t));
  newCounter->count = 0;
  return newCounter;
}

void Counter_AddOne(Counter_t *me) { me->count++; }

uint8_t Counter_GetCount(Counter_t *me) { return me->count; }

void Counter_DeInit(Counter_t *me) {
    free(me);
}

We can then use this Counter_t from other source files like main.c:

#include <stdio.h>

#include "Counter.h"

int main(void) {
    printf("Creating counter...\n");
    // From main.c, we can't access any of the members of the
    // opaque Counter_t struct, we only have a pointer and have to use
    // the provided API in Counter.h.
    Counter_t *myCounter = Counter_Init();

    Counter_AddOne(myCounter);
    uint8_t count = Counter_GetCount(myCounter);
    printf("Count: %d\n", count);

    // Finally, destroy counter
    Counter_DeInit(myCounter);
    return 0;
}

Note how in main.c we can only create a pointer to the Counter (Counter_t *), we aren’t given enough information to create a Counter_t directly (if we could, then we would be able to access all the internals — bypassing the public API). If we want to perform operations on it such as adding one or getting the current count, we have to use the API like Counter_AddOne() and Counter_GetCount().