What is the need of struct pointer in that case ?

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struct Vec;
/* Allows us to write `Vec` instead of `struct Vec`. */

struct Vec {
  int *data;
  size_t length;
  size_t capacity;
};
typedef struct Vec Vec;

Vec *vec_new() {
  Vec *nstr = malloc(sizeof(struct Vec)); // new_struct = nstr
  if (nstr != NULL) {
    nstr->data = malloc(sizeof(int));
    if (nstr->data == NULL) {
      free(nstr);
      return false;
    }
    nstr->length = 0;
    nstr->capacity = 1;
  } else {
    free(nstr);
  }
  return nstr;
}

Vec *xs = vec_new();

What I have tried:

this function Vec *vec_new() returns a struct pointer...why we need to return struct pointer and not just normal struct?

--
please try to explain it to me in simple way .. i m beginner cs student :)

Posted 25-Jun-21 4:16am

Ahmad Qassym

Updated 25-Jun-21 4:48am

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OriginalGriff · Answer 1 · 2021-06-25T04:46:00

Because C and C++ pass all parameters and return values by value, not by reference.
So if you returned the struct you allocated, what the calling function would get is not the actual item, but a copy:

C

typedef struct MyStruct
   {
   int x;
   int y;
   } MS;
   
   MS* pMS;
MS GetOne(int x, int y)
   {
   MS* p = (MS*) malloc(sizeof(MS));
   p->x = x;
   p->y = y;
   pMS = p;
   return *p;
   }      
int main()
   {
   MS p = GetOne(10, 20);
   pMS->x++;
   printf("%u,%u\n", p.x, p.y);
   printf("%u,%u\n", pMS->x, pMS->y);
   return 0;
   }

gives you:

10,20
11,20

CPallini · Answer 2 · 2021-06-25T04:48:00

struct are usually passed and returned using pointers because it is more efficient. Passing (and returning) a struct by value copies all its fields. Passing it by pointer just copies the pointer value.
Moreover, in your case, the called function dynamically allocates memory, hence it has to pass the address (i.e. the pointer) of the allocated memory to the caller in order to allow the caller itself to release it (via free) when appropriate, in order to avoid memory leaks.

What is the need of struct pointer in that case ?

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What is the need of struct pointer in that case ?

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