指向结构的指针

Question

I have following structure that contains two integer values.

struct data
{
 int v1;
 int v2;
};

/*some functions on struct data*/

struct data* create(int v1,int v2);
void printData(struct data *d);

Now I have the following structure vec wich will kind of act like a list and this structure has size and a pointer to a pointer to a structure of type data.

struct vec
{
 int size;
 struct data **array;
};

/*possible functions for vec structure*/
struct vec* createVec();
void addFront(struct vec *v, struct data* dta);

Since i am a beginner to pointers i would like to clarify and confirm my understanding.

1. struct data **array; I am trying to figure out how this is working so i have drawn a visualization of what it might possibly be and would appreciate if someone can confirm if i am correct or not.

I know this is not the greatest data structure etc this is being done strictly to help me clarify my understanding in pointers.

Answer 1

To help you clarify everything in your mind (and your code), you can do the following:

typedef struct data* data_ptr

Then, the struct vec can be written as

struct vec
{
 int size;
 data_ptr *array;
};

So, array is indeed a pointer to a pointer to a data struct. Array however is a misleading name, since you could be able to get what you want with other data structures like lists etc. If you decide to stick with the array, then you should allocate memory correctly and free it at the end, and the structure looks like what you have drawn.

Therefore, (after the correct initializations), array[i] will give you a pointer to the i-th data struct, and array[i]->v1 should give you the v1 value of the i-th struct.

Answer 2

Your visualization is fine.

If we look at your diagram, we'd expect vec->array[0] (the first element of your array) to be a pointer to struct data , right?

The array indexing operation ( [0] ) will de-reference the first element, which means stripping a pointer from the type.

So, if the type of array is struct data ** , the type of array[0] will be struct data * (notice one fewer * ), which is what we expect.