What is the difference between initializing struct in following ways in C?

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I am learning c. I have an struct that looks like below. I see 2 different ways of initializing the struct variable. It would be great, if someone could explain what is the difference between these 2 ways & which one is preferred in which case.

typedef struct {
  int length;
  int* elements;
} Array;

Array create_array_type_one(int size) {
  
  Array result;
  
  result.length = 0;
  result.elements = (int *)malloc(size * sizeof(int));
  
  return result;
  
}

Array* create_array_type_two(int size) {
  
  Array *array = (Array *) malloc(sizeof(Array));
  
  array->length = 0;
  array->elements = (int *) malloc(sizeof(int) * size);
  
  return array;
  
}
malloc struct c
2 Answers

So, here we are talking about STRUCTURES & FUNCTIONS.

Basically there are three methods by which invoker and invoked structure function can communicate to each other.

  1. Passing each member of the structure as an actual argument of the function call, or returning to the each member type.
  2. Sending the copy of the structure to the entire called function or returning to the struct type (call by value).
  3. Using pointers, pass the structure as an argument and indirectly work on the original structure or returning the address of the struct (call by reference).

Your, CASE 1:

Array create_array_type_one(int size) {
  
  Array result;
  
  result.length = 0;
  result.elements = (int *)malloc(size * sizeof(int));
  
  return result;
  
}

In this case you are returning a copy of the entire structure to the calling function. So, in order to use it in your main() function, you should do something like this:

Array new;
new = create_array_type_one(5);

Here, you are assigning the returned structure of type Array to a structure of type Array.

In CASE 2:

Array* create_array_type_two(int size) {
  
  Array *array = (Array *) malloc(sizeof(Array));
  
  array->length = 0;
  array->elements = (int *) malloc(sizeof(int) * size);
  
  return array;
  
}

In this case you are using pointers and returning a pointer to the newly created data structure of type Array. You are not sending a copy of the structure to the calling function instead you are sending its address to the calling function. Therefore, what happening in the main() while calling like this is:

Array *ptr;
ptr = create_array_type_two(5);

Both cases has there own advantages and disadvantages like CASE 1 ensures data safety as all the work done by the called function happens on a copy but uses extra memory. Where CASE 2 is more efficient and faster as compared to the previous method, as it uses same memory location for the operation.

user_121806050

The difference of them is the return type.
create_array_type_one() returns Array type memory location.
create_array_type_two() returns a pointer to Array type memory location.

As Array type has only two elements, int and int *, the size difference between two of them is not big . So you can use anyone for your coding preference in this case. I'd like to recommend you the second one if the size of structure is kind of big...

user_109733750
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