CODEWITHSUNDEEP
coopencapsulationinformation-hidingopaque-pointers
Andy Lin
Andy Lin

Reputation: 447

In this case, how to modularize program as well as achieving information hiding?

I created two classes "DEVICE_s" and "DEVICE_SET_s" as following:

Device_Manager.h

typedef struct DEVICE_s DEVICE_s;
typedef struct DEVICE_SET_s DEVICE_SET_s;

Device_Manager.c

struct DEVICE_s
{
    uint32_t IP;
    TYPE_e Type;
    METHOD_e Method;
    GROUP_RULE_e GroupRule;
    char Name[NAME_SIZE];
};
struct DEVICE_SET_s
{
    uint8_t     Total;
    uint8_t     Used;
    uint8_t     Available;
    DEVICE_s    Set[SET_SIZE];
};
DEVICE_s Instance;
DEVICE_SET_s Objects;

Because I put these two classes within a same file, all functions that manipulate variables "Instance" and "Objects" are put together inside this file.

I think this way is bad considering to modularity, so I want to create another source file to separately manage class "DEVICE_SET_s", just like:

DeviceSet_Manager.h

typedef struct DEVICE_SET_s DEVICE_SET_s;

DeviceSet_Manager.c

#include "Device_Manager.h"
#include "DeviceSet_Manager.h"
struct DEVICE_SET_s
{
    uint8_t     Total;
    uint8_t     Used;
    uint8_t     Available;
    DEVICE_s    Set[SET_SIZE];    //Oops! Incomplete Type Is Not Allowed
};

However, In terms of DeviceSet_Manager.c, class "DEVICE_s" is not visible(not a complete type).

How do I fix this? Thanks

Upvotes: 3

Views: 80

Answers (2)

alk
alk

Reputation: 70951

What you want are opaque types for

  • DEVICE
  • DEVICE_SET

This is straight forward the same way for both:

  • header, defining

    • incomplete type for object structure. A pointer to it is the opaque type to handle an object's instance and to be passed to its interfacing functions
    • interfacing functions' prototypes

  • implementation of

    • complete type
    • interfacing functions

headers

device.h

#ifndef DEVICE_H
#define DEVICE_H

struct device;

struct device * device_new(void);
void device_delete(struct device *);

#endif

device_set.h:

#ifndef DEVICE_H
#define DEVICE_H

#include "device.h"

struct device_set;

struct device_set * device_set_new(size_t);
void device_set_delete(struct device_set *);

int device_set_set_device(struct device_set *, size_t, struct device *);
struct device * device_set_get_device(struct device_set *, size_t); 


#endif

implementations

device.c

#include "device.h"

struct device {
  ...
};

struct device * device_new(void)
{
  struct device * pd = malloc(sizeof * pd);
  if (NULL != pd)
  {
    /* Init members here. */
  }

  return pd;
}

void device_delete(struct device * pd)
{
  if (pd)
  {
    /* de-init (free?) members here. */
  }

  free(pd);
}

device_set.c:

#include "device_set.h"

struct device_set
{
  size_t total;
  size_t used;
  size_t available; /* what is this for? isn't it just total - used? */
  struct device ** pd;
}

struct device_set * device_set_new(size_t nb)
{
  struct device_set pds = malloc(sizeof *pds);
  if (NULL != pds)
  {
    pds->pd = malloc(nb * sizeof *pds->pd);
    if (NULL == pds->pd)
    {
      free(pds);
      pds = NULL;
    }
    else
    {
      for (size_t d = 0; d < nb; ++d)
      {
        pds->pd[d] = NULL;
      }

      pds->total = nb;
      pds->used = 0;
      pds->available = 0;
    }
  }

  return pds;
}

void device_set_delete(struct device_set * pds)
{
  if (pds)
  {
    free(pds->pd);
    free(pds)
  }

  return;
}

int device_set_set_device(struct device_set * pds, size_t d, struct device * pd)
{
  int result = 0;

  if (pds->total <= d)      
  {
    result = ERANGE;
  }
  else
  {
    pds->pd[d] = pd;
  }

  return;
}    

struct device * device_set_get_device(struct device_set * pds, size_t d); 
  int result = 0;
  struct device * pd = NULL;

  if (pds->total <= d)      
  {
    result = ERANGE;
  }
  else
  {
    pd = pds->pd[d];
  }

  return pd;
}

Upvotes: 2

user694733
user694733

Reputation: 16043

Here's what I usually do:

device.h

// insert header guards here

typedef struct DEVICE_s DEVICE_s;
struct DEVICE_s
{
   ...
};

// method declarations here
DEVICE_Init(DEVICE_s * this, ...);
DEVICE_Foo(DEVICE_s * this, ...);

device.c

#include "device.h"
// method implementations here

deviceset.h

//hguards...

#include "device.h"

typedef struct DEVICE_SET_s DEVICE_SET_s;
struct DEVICE_SET_s
{
    uint8_t     Total;
    uint8_t     Used;
    uint8_t     Available;
    DEVICE_s    Set[SET_SIZE];
};

// method declarations here
DEVICE_SET_Init(DEVICE_SET_s * this, ...);
DEVICE_SET_Foo(DEVICE_SET_s * this, ...);

deviceset.c

#include "deviceset.h"
// method implementations here

usercode.c

DEVICE_SET_s myDevices;
void func(void) {
     DEVICE_SET_Init(&myDevices, a, b, c);         
        ...
}

With this approach it's users responsibility to allocate the memory and call init function (=constructor) to initialize object before use.

It does not give really give you encapsulation, but gives maximum freedom for allocation. For encapsualtion to work well it needs support from the language. Because C is so limited language to begin with, I don't recommend adding more limitations just to satisfy programming paradigm.

Upvotes: 1

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