Calling functions on an array of object pointers

Question

I couldn't find anything similar to this anywhere. I have an array of pointers to objects (a linked list) for a hash table:

LinkList * table[TABLE_SIZE];
for (int i = 0; i < TABLE_SIZE; i++)
    table[i] = NULL;

In one of my functions for my hash table class, there I need to call the one of the functions on the LinkList object in the table array. I'm calling it as such:

void HashMap::add_customer(string first, string last, string phone) {   
    int hash = get_hash(phone);
    if (table[hash] == NULL) {
        table[hash] = new LinkList;
    }
    table[hash]->add_customer(first, last, phone); // I HATE THIS LINE
}

Everything compiles fine, but when I execute the table[hash]->add_customer() line in runtime, I get a Segmentation Fault error. When this line is commented out, I get no errors, but obviously, I can't add any customers to my hash table. Is this not the right syntax?

Are you ensuring that each value in the array is initialized to NULL and also set to NULL when an item is removed? — Ed Swangren
– Ed Swangren, Commented Jun 12, 2012 at 23:38
Do some adds work, and others don't? What happened before the fault? — jxh
– jxh, Commented Jun 12, 2012 at 23:40
They are all initialized to NULL and no removes have happened before this fault happens. This happens everytime HashMap::add_customer is called. — Joshua Dover
– Joshua Dover, Commented Jun 12, 2012 at 23:47
"This happens everytime HashMap::add_customer is called." - That is a different statement than "when I execute the table[hash]->add_customer() line in runtime, I get a Segmentation Fault error". Which is it? Is it that line that always causes the problem? If so then it is the dereference causing the fault. — Ed Swangren
– Ed Swangren, Commented Jun 12, 2012 at 23:48
Also, per the declaration of table that you have shown, it is not initialized to all NULL's, so do you do it later? Why not use LinkList *table[TABLE_SIZE] = {}? — Ed Swangren
– Ed Swangren, Commented Jun 12, 2012 at 23:50

Samy Vilar · Accepted Answer · 2012-06-13 00:03:43Z

2

You have to initialized your array of pointers to NULL, since they'll be allocated on the stack/heap or wherever with junk values ....

LinkList * table[TABLE_SIZE];
memset(table, NULL, sizeof(LinkList *) * TABLE_SIZE);

assuming you've initialized correctly then please check your hashes and assert that indeed they are hash < TABLE_SIZE

try this:

 int hash = get_hash(phone) % TABLE_SIZE;

edited Jun 13, 2012 at 0:03

answered Jun 12, 2012 at 23:45

Samy Vilar

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3 Comments

jxh Over a year ago

Nice catch. Missing modulus operator perhaps?

jxh Over a year ago

Need to modulo on the initialization of the table element too.

Samy Vilar Over a year ago

yeah I shouldn't have being focusing just on that line, I guess highlighting threw me off ... thank you. but I did like // I HATE THIS LINE

The Beruriah Incident · Accepted Answer · 2012-06-12 23:45:26Z

0

This snippet looks right, for the hash map's behavior. I'd check the add_customer method in your LinkList implementation. Try using your linked lists outside of this hash map, just to make sure they work as intended.

answered Jun 12, 2012 at 23:45

The Beruriah Incident

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5 Comments

Ed Swangren Over a year ago

It's not entirely clear, but if the seg fault is occurring on that line then it never even got to the add_customer function, dereferencing the pointer is what caused it.

Joshua Dover Over a year ago

That's right. I have separate implementation that just uses the LinkList and add_customer works fine there. Its just the pointer dereferencing that's giving me issues.

The Beruriah Incident Over a year ago

Yes, but we all we know now is that commenting the line "fixes" it. @Joshua, try using a debugger to follow where the segfault is happening. If you're in an IDE, there are probably nice tools for this already. If you're at a terminal (assuming Unix here), add the -ggdb flag to your compile, then run gdb [executable name here, a.out if you didn't change anything]. It should nicely identify the error location.

The Beruriah Incident Over a year ago

Ok, gotcha, have you run a debugger? (by the way, my explanation there wasn't meant to be condescending, it just sounded like you might possibly be newer to this particular area)

The Beruriah Incident Over a year ago

Also, make sure your pointers are actually null. If your code is unmanaged, that array is just using whatever values were already in that area of member.

David · Accepted Answer · 2012-06-13 00:01:50Z

0

Try this:

LinkList * table[TABLE_SIZE] = {};

That nulls out your pointers to begin with.

Can you confirm get_hash returns values < TABLE_SIZE? Add an assert or something.

edited Jun 13, 2012 at 0:01

answered Jun 12, 2012 at 23:46

David

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