C使用构造函数创建结构数组

Question

I have a C struct:

typedef struct {
  Dataset *datasets;
  int nDatasets;
  char *group_name;      
  enum groupType type;  
} DatasetGroup; 

It has a constructor function like this:

DatasetGroup * new_DatasetGroup(char *group_name, enum groupType type, enum returnCode *ret)
{
    DatasetGroup *dg;
    dg = (DatasetGroup *) malloc(sizeof(DatasetGroup));
    if (dg == NULL)
    {
     *ret = EMEMORY_ERROR;
    }

    // Allocate space for a few datasets
    dg->datasets = malloc(sizeof(Dataset) * INCREMENT);
    if (dg->datasets == NULL)
    {
        *ret = EMEMORY_ERROR;
    }
    dg->group_name= malloc(sizeof(char) * strlen(group_name));
    strcpy(dg->group_name, group_name);
    dg->type = type;
    groupCount++;
    return dg;
  }

I want to dynamically create an array of these structs. Whats the best way to do this?

So far I have something like:

   DatasetGroup * make_array(){

     DatasetGroup *dg_array;
     // Allocate space for a few groups
     dg_array = (DatasetGroup *) malloc(sizeof(DatasetGroup) * INCREMENT);

     return dg_array;

   }

   void add_group_to_array(DatasetGroup *dg_array, ...){
      // Add a datasetgroup
      DatasetGroup *dg = new_DatasetGroup(...);

      // groupCount - 1 as the count is incremented when the group is created, so will always be one ahead of the array index we want to assign to
      dg_array[groupCount - 1] = dg;

     if (groupCount % INCREMENT == 0)
     {
      //Grow the array
      dg_array = realloc(dg_array, sizeof(DatasetGroup) * (groupCount + INCREMENT));
     }
 }

But this doesnt seem right.... any ideas?

Answer 1

A few suggestions:

1. You have groupCount being incremented by the constructor function of the struct. This means you can only have one array of the struct that uses your array function. I would recommend having the array be responsible for managing the count.
2. To that affect if you want to have a managed array I would create a struct for that and have it keep both the pointer to the array,the number of objects and the size of the array (eg the maximum number of structs it can currently hold)
3. If you keep proper track of how many elements you have and the size of the array you can replace groupCount % INCREMENT == 0 with something like groupCount == arraySize which is a lot more intuitive in my opinion.
4. You can avoid the second malloc in the constructor all together by having the array be an array of the elements instead of an array of pointers. The constructor than then just initialize the struct members instead of allocating memory. If you are doing this a lot you will be avoiding a lot of memory fragmentation.
5. Finally, while this depends on your application, I usually recommend when you realloc do not increase by a constant but instead of by a multiple of the current array size. If say you double the array size you only have to do log_2 n number of reallocs with n being the final array size and you waste at most half of memory (memory is generally cheap, like I said it depends on the application). If that is wasting to much memory you can do say 1.5. If you want a more detailed explanation of this I recommend this Joel on Software article, the part about realloc is about 2/3 down.

Update:

A few others things:

dg = (DatasetGroup *) malloc(sizeof(DatasetGroup));
if (dg == NULL)
{
 ret = EMEMORY_ERROR;
}

// Allocate space for a few datasets
dg->datasets = malloc(sizeof(Dataset) * INCREMENT);

As previously pointed out is very bad as you will us dg even if it is NULL. You probably want to exit right after detecting the error.

Furthermore you are setting ret but ret is passed by value so it will not be changed for the caller if the callee changes it. Instead you probably want to pass a pointer and dereference it.

Update 2: Can I give an example, sure, quick not so much ;-D.

Consider the following code (I apologize if there are any mistakes, still half asleep):

#include <stdio.h>
#include <stdlib.h>

#define LESS_MALLOCS

#define MAX_COUNT 100000000

typedef struct _foo_t
{
  int bar1;
  int bar2;
} foo_t;

void foo_init(foo_t *foo, int bar1, int bar2)
{
  foo->bar1 = bar1;
  foo->bar2 = bar2;
}

foo_t* new_foo(int bar1, int bar2)
{
  foo_t *foo = malloc(sizeof(foo_t));
  if(foo == NULL) {
    return NULL;
  }
  foo->bar1 = bar1;
  foo->bar2 = bar2;
  return foo;
}

typedef struct _foo_array_t
{
#ifdef LESS_MALLOCS
  foo_t *array;
#else
  foo_t **array;
#endif
  int count;
  int length;
} foo_array_t;

void foo_array_init(foo_array_t* foo_array, int size) {
  foo_array->count = 0;
#ifdef LESS_MALLOCS
  foo_array->array = malloc(sizeof(foo_t) * size);
#else
  foo_array->array = malloc(sizeof(foo_t*) * size);
#endif
  foo_array->length = size;
}

int foo_array_add(foo_array_t* foo_array, int bar1, int bar2)
{
  if(foo_array->count == foo_array->length) {
#ifdef LESS_MALLOCS
    size_t new_size = sizeof(foo_t) * foo_array->length * 2;
#else
    size_t new_size = sizeof(foo_t*) * foo_array->length * 2;
#endif
    void* tmp = realloc(foo_array->array, new_size);

    if(tmp == NULL) {
      return -1;
    }
    foo_array->array = tmp;
    foo_array->length *= 2;
  }
#ifdef LESS_MALLOCS
  foo_init(&(foo_array->array[foo_array->count++]), bar1, bar2);
#else
  foo_array->array[foo_array->count] = new_foo(bar1, bar2);
  if(foo_array->array[foo_array->count] == NULL) {
    return -1;
  }
  foo_array->count++;
#endif
  return foo_array->count;
}


int main()
{
  int i;
  foo_array_t foo_array;
  foo_array_init(&foo_array, 20);
  for(i = 0; i < MAX_COUNT; i++) {
    if(foo_array_add(&foo_array, i, i+1) != (i+1)) {
      fprintf(stderr, "Failed to add element %d\n", i);
      return EXIT_FAILURE;
    }
  }

  printf("Added all elements\n");
  return EXIT_SUCCESS;
}

There is a struct ( foo_t ) with two members ( bar1 and bar2 ) and another struct that is an array wrapper ( foo_array_t ). foo_array_t keeps track of the current size of the array and the number of elements in the array. It has an add element function ( foo_array_add ). Note that there is a foo_init and a new_foo , foo_init takes a pointer to a foo_t and new_foo does not and instead returns a pointer. So foo_init assumes the memory has been allocated in some way, heap, stack or whatever doesn't matter, while new_foo will allocate memory from the heap. There is also a preprocess macro called LESS_MALLOCS . This changes the definition of the array member of foo_array_t , the size of the initial array allocation, the size during reallocation and whether foo_init or new_foo is used. The array and its size have to change to reflect whether a pointer or the actually element is in the array. With LESS_MACRO defined the code is following my suggestion for number 4, when not, it is more similar to your code. Finally, main contains a simple micro-benchmark. The results are the following:

[missimer@asus-laptop tmp]$ gcc temp.c # Compile with LESS_MACROS defined
[missimer@asus-laptop tmp]$ time ./a.out 
Added all elements

real    0m1.747s
user    0m1.384s
sys     0m0.357s
[missimer@asus-laptop tmp]$ gcc temp.c #Compile with LESS_MACROS not defined 
[missimer@asus-laptop tmp]$ time ./a.out 
Added all elements

real    0m9.360s
user    0m4.804s
sys     0m1.968s

Not that time is the best way to measure a benchmark but in this case I think the results speak for themselves. Also, when you allocate an array of elements instead of an array of pointers and then allocate the elements separately you reduce the number of places you have to check for errors. Of course everything has trade-offs, if for example the struct was very large and you wanted to move elements around in the array you would be doing a lot of memcpy -ing as opposed to just moving a pointer around in your approach.

Also, I would recommend against this:

dg_array = realloc(dg_array, sizeof(DatasetGroup) * (groupCount + INCREMENT));

As you lose the value of the original pointer if realloc fails and returns NULL . Also like your previous ret, you should pass a pointer instead of the value as you are not changing the value to the caller, just the callee which then exits so it has no real affect. Finally, I noticed you changed your function definition to have a pointer to ret but you need to dereference that pointer when you use it, you should be getting compiler warnings (perhaps even errors) when you do try what you currently have.

Answer 2

You could do two things, either you dynamically create an array of struct pointers, then call your new function to create N datagroups, or you could dynamically request memory for N structures at once, this would mean your N structures would be contiguously allocated.

Datagroup **parry = malloc(sizeof(datagroup *) * N)
for (int i = 0; i < N; i++){
    parry[i] = //yourconstructor
}

Or

//allocate N empty structures
Datagroup *contarr = calloc(N, sizeof(Datagroup))

The second method might need a different initialization routine than your constructor, as the memory is already allocated