Dynamically Allocated Array of Pointers in Struct

Question

I want to do something like this.

typedef struct Test{
  int value;
  struct Test* parent;
  struct Test** children;
}Test;

So I want a node that points to another parent structure. Then I want a dynamically allocated array that points to child nodes. My question is that I have no idea how this would work syntactically.

For example,

Test* first;
Test* second;
Test* third;
(*third).value = 1;
(*first).parent = second;
(*first).child[0] = third;
printf("%d\n",(*first).(*child[0]).value);

doesn't compile. I'm assuming I need to do something with malloc to allocate space for the array of pointers but I'm not sure. Also I'm not sure how I would access the "value" of the parent and child directories.

std''OrgnlDave · Accepted Answer · 2012-04-22 04:27:01Z

1

EDIT: I've added an ideone link to the end that implements all the concepts for you.

Sorry for the terseness of this answer, I am hoping it will show you how to do it properly.

Test* first = (Test *)malloc(sizeof(Test));  // malloc(sizeof(Test)) allocates enough memory to hold a Test struct
Test* second = (Test *)malloc(sizeof(Test));
first->value = 1; // -> is the proper way to dereference pointers in this situation (sorry wrong term? I am up late) but I suppose your style can work, it just gets a bit confusing IMO
first->*child = (Test *)malloc(intptr_t * number_of_children); // intptr_t will make sure you have the right size of a pointer, you could also use sizeof(Test *) instead. i.e. malloc(sizeof(Test *));
first->child[0] = second; // The array-style subscript is just more readable IMO
printf("%d\n",first->child[0]->value); // child[0]-> will handle the dereferencing in a nice way

But I'm going to show you a bit of a trick to make your life easier

typedef Test* test_array;

// ...later, in the struct...
test_array* child;

// ...later, in the malloc place...

first->child = (test_array *)malloc(sizeof(test_array *) * number_of_children);

Everything else stays the same, you just get much easier to understand syntax IMO. Helps deal with those tricky double stars.

EDIT: here's the link - http://ideone.com/TvSSB

edited Apr 22, 2012 at 4:27

answered Apr 22, 2012 at 3:33

std''OrgnlDave

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5 Comments

Tom Davis Over a year ago

Thanks, that helped. Although here it doesn't compile unless I cast the call to malloc. The error I'm getting in that case is "error: invalid conversion from ‘void*’ to ‘Test* {aka main()::Test*}’ [-fpermissive]"

std''OrgnlDave Over a year ago

Added casts so that malloc is happy

std''OrgnlDave Over a year ago

Also updated the malloc for first->child. Um, it's too late for me to determine if it should be first->child = (Test **) or first->*child = (Test *). I think the way I have it here is right. Double *'s are tricky.

std''OrgnlDave Over a year ago

@TomDavis finally, added a way to manage double *'s easier, I'm out for the night, vote me up and make me an answer if I've answered you :-P or ask more questions and I'll answer in the morning

std''OrgnlDave Over a year ago

@TomDavis grawr, double *'s are my bane, so I implemented it in an easy-to-read manner for you. check the link. really, bed now.

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