Initializing an array of pointers to NULL

Question

I got a struct defining a node for a tree data structure:

struct Node {
  int data;
  struct Node *children[10];
}

Given that children is NOT a dynamic array, I would like to initialize each pointer of children to NULL, but what follows doesn't work:

struct Node {
  int data;
  struct Node *children[10]={NULL};
}

Is there any workaround?

Please include your actual code in the question. struct *Node children[10]; is a syntax error. (Struct members cannot have initializers.) — Keith Thompson
– Keith Thompson, Commented May 4, 2015 at 16:12
If by some chance you are working with a C++ compiler, you can add a constructor to your struct which initializes all the member fields. — abelenky
– abelenky, Commented May 4, 2015 at 16:14
Ok my syntax error was actually a typo. Corrected. And no, I cannot use a C++ compiler. — MadHatter
– MadHatter, Commented May 4, 2015 at 16:32
Please accept my apologies, after my last comment I was left without internet connection. All useful answers, I upvoted them all. Probably I should go with designated initializers. — MadHatter
– MadHatter, Commented May 5, 2015 at 22:25

haccks · Accepted Answer · 2015-05-04 16:14:45Z

3

This

 struct *Node children[10];

is wrong. I would not even compile. It should be

 struct Node *children[10];

To initialize all elements of member children to NULL you can use designated initializers.

struct Node {
  int data;
  struct Node *children[10];
} node = {.children = {NULL}};

answered May 4, 2015 at 16:14

haccks

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Community · Accepted Answer · 2017-05-23 11:43:34Z

2

You can't initialize data in the description of the struct because no memory has been allocated yet.

Let's look at the two styles of allocation you'll see:

The Stack

struct Node my_node = {
    0,
    {NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL}
};

Or, because unlisted values will default to 0...

struct Node my_node = {};

The Heap:

Here, are only option is to null out the memory. We could use calloc() to do this since the memory it returns is zeroed out.

struct Node *my_node = calloc(1, sizeof(*my_node));

Or, we can explicitly use memset():

struct Node *my_node = malloc(sizeof(*my_node));
memset(my_node, 0, sizeof(*my_node));

Notes:

I'm generally assuming that NULL == 0. This isn't necessarily true. If you'd like to read more about these (mostly) historical systems: When was the NULL macro not 0?

If you're on one of those systems, or you're concerned about your code working on those platforms, then I would recommend using the first method (and most explicit) method that I described. It will work on all platforms.

edited May 23, 2017 at 11:43

CommunityBot

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answered May 4, 2015 at 16:12

Bill Lynch

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2 Comments

Keith Thompson Over a year ago

memset is not guaranteed to set pointers to NULL; a null pointer can legally have a non-zero representation. It's likely to work on most systems.

ouah Over a year ago

struct Node my_node = {}; is not C.

ouah · Accepted Answer · 2015-05-04 16:15:41Z

2

struct Node {
  int data;
  struct Node *children[10];
} a = {.children = {0}};

a is a struct Node object with all element of children member initialized to a null pointer.

edited May 4, 2015 at 16:15

answered May 4, 2015 at 16:12

ouah

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