Initialize array of pointers to NULL

Question

here's to my first post on this website.

I'm trying to initialize an array of 30 pointers to NULL. The array is stored as a global and I'm doing the initialization in my init() function like so:

static headerT* free_list[30];

....

init() { free_list[30] = {NULL}; }

However, I get the following complier error - "error: expected expression before ‘{’ token".

Any thoughts on what I could be doing wrong?

Thanks in advance.

the element free_list[30] does not exist: the array goes from free_list[0] to free_list[29] — pmg
– pmg, Commented Mar 12, 2014 at 0:19

Ed Swangren · Accepted Answer · 2014-03-12 00:19:16Z

3

objects allocated with static storage duration are value initialized. Your pointers are already initialized to nullptr.

answered Mar 12, 2014 at 0:19

Ed Swangren

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Comments

Benesh · Accepted Answer · 2015-02-24 09:50:24Z

2

If arr is an array of n elements, then the first cell is arr[0] and the last is arr[29]. arr[30] is outside the array and referencing it can cause all sorts of problems.
You can't assign a value to an entire array. Use a for loop to access every cell.

Thus we get:

init() 
{ 
    int i;
    for(i=0; i<30; i++)
        free_list[i] = NULL; 
}

edited Feb 24, 2015 at 9:50

answered Mar 12, 2014 at 0:18

Benesh

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2 Comments

Qaz Over a year ago

To be clear, only pointers with static storage duration are initialized to NULL by default.

david.pfx Over a year ago

You can also initialise a (non-static) array of pointers to NULL (0) by writing T* p[30] = { 0 };, but you can't assign such a value.

DannyK · Accepted Answer · 2014-03-12 00:55:17Z

-2

I prefer memset

memset( free_list, 0x0, 30 * sizeof(headerT*) );

answered Mar 12, 2014 at 0:55

DannyK

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3 Comments

M.M Over a year ago

This will break on a system where the null pointer is not represented by all-bits-zero.

M.M Over a year ago

Also it will break if the size of the list changes, you could at least have used the idiom memset( &x, 0, sizeof x ) to ensure the correct amount of memory is written.

DannyK Over a year ago

Yes I could have but the example that was given was fairly specific and I had no problem with giving a specific answer. The specific version of this function that i coded up for myself is templated but i thought that was overkill for the question.

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