Question

I read a book, which gives example of .h file, that equal to interface of queue in java:

void InitQueue(Queue* pQueue, int iSize);
bool Insert(Queue* pQueue, const Item* pItem);
bool Extract(Queue* pQueue, Item** pItem);
bool GetFirst(const Queue* pQueue, Item** pItem);
bool IsFull(const Queue* pQueue);
bool IsEmpty(const Queue* pQueue);

I didn’t understand two things:

1. Why in Extract, GetFirst The second argument is of type Item** and not Item*? What is mean when we wrtoe such a thing?
2. Why in some functions (IsFull, IsEmpty, ..) we get as argument const Queue* and not simply Queue*?

Answer 1

IsFull() and IsEmpty() take const arguments because it implies that they will not change the Queue object; it is constant, and will not be modified.

Extract and GetFirst use ** because of this:

int a;           // Declares an int
a = 2;           // Sets a to 2
int *b = &a;     // Declares a pointer pointing to that int
*b = 4;          // Sets a to 4
int **c = &b;    // Declares a pointer pointing to that pointer to that int
**c = 6;         // Sets a to 6

If I were to pass c into a function:

int global_var;

int main() {
  modify_ptr(c);
}

void modify_ptr(int **ptr)
{
  *ptr = &global_var;
}

The pointer I just passed in was itself modified; **c is now pointing to global_var instead of a.

The reason you pass these pointers into getfirst and extract is because those are “getter” functions – you want them to return a pointer to the data. So they need to be able to return a pointer, and the method these employ is to pass a pointer to a pointer, like in the above example, so they can modify what you pass them to point to the correct element.