带指针和数组的typedef结构

Question

 #include <stdio.h>
 #define DEFAULT_CAPACITY 5

Here's my typedef'ed struct - I am not allowed to change this.

typedef struct Vector
{
   int items[DEFAULT_CAPACITY];
   int size;   
}*VectorP;

*VectorP is already a pointer which confuses me a lot.

Next I'm initializing the vector, calling malloc

void initVector(VectorP vector)
{
      vector = (VectorP)malloc(DEFAULT_CAPACITY * sizeof(VectorP));

      if(vector == NULL)
       {
           fprintf(stderr, "Memory allocation failed!\n");
           exit(1);
       } 

   (*vector).size = 0;        //Does this change the size on the main function?
   vector->items[0] = 1;      // And this one too?
 }

And Here's my main function:

int main()
{
  // Create a new vector ... check size
    struct Vector vector;
    VectorP v1 = &vector;
    initVector(v1);
    fprintf(stderr, "\nThe size is (0) %i\n", v1->size);

    printf("items[0] = %i ", v1->items[0]);        
}

The problem is that function initVector didn't change the size or the item. How would i make it to change the size in the vector that is created in main?

Answer 1

In the function initVector , you allocated memory for vector , this overwrites its previous value(the argument that's passed in), so it doesn't modify vector in `main. This also means there's memory leak here.

To fix it, just remove the memory allocation lines in initVector .

void initVector(VectorP vector)
{
     vector->size = 0;       //the same as (*vector).size
     vector->items[0] = 1;   
}

Answer 2

I think your problem is you are re-allocating the structure after you create it with struct Vector vector; c knows enough to create the simple things in structures like basic ints and chars along with int arrays which have a predefined length like int items[DEFAULT_CAPACITY]; .

So what is happening is you are creating a new structure in your init function after you instantiated it in main. Like Yu Hao suggests just remove the memory allocation in your init function and it should work.

Answer 3

When you define a vector in your main like this:

struct Vector vector;

That mean you are already allocating space in the program stack for this struct. So allocating memory again within the function initvector is not needed at all. Thats what is causing the problem.