C Language Structs Pointers to structs
Example
When you have a variable containing a struct, you can access its fields using the dot operator (.). However, if you have a pointer to a struct, this will not work. You have to use the arrow operator (->) to access its fields. Here's an example of a terribly simple (some might say "terrible and simple") implementation of a stack that uses pointers to structs and demonstrates the arrow operator.
#include <stdlib.h>
#include <stdio.h>
/* structs */
struct stack
{
struct node *top;
int size;
};
struct node
{
int data;
struct node *next;
};
/* function declarations */
int push(int, struct stack*);
int pop(struct stack*);
void destroy(struct stack*);
int main(void)
{
int result = EXIT_SUCCESS;
size_t i;
/* allocate memory for a struct stack and record its pointer */
struct stack *stack = malloc(sizeof *stack);
if (NULL == stack)
{
perror("malloc() failed");
return EXIT_FAILURE;
}
/* initialize stack */
stack->top = NULL;
stack->size = 0;
/* push 10 ints */
{
int data = 0;
for(i = 0; i < 10; i++)
{
printf("Pushing: %d\n", data);
if (-1 == push(data, stack))
{
perror("push() failed");
result = EXIT_FAILURE;
break;
}
++data;
}
}
if (EXIT_SUCCESS == result)
{
/* pop 5 ints */
for(i = 0; i < 5; i++)
{
printf("Popped: %i\n", pop(stack));
}
}
/* destroy stack */
destroy(stack);
return result;
}
/* Push a value onto the stack. */
/* Returns 0 on success and -1 on failure. */
int push(int data, struct stack *stack)
{
int result = 0;
/* allocate memory for new node */
struct node *new_node = malloc(sizeof *new_node);
if (NULL == new_node)
{
result = -1;
}
else
{
new_node->data = data;
new_node->next = stack->top;
stack->top = new_node;
stack->size++;
}
return result;
}
/* Pop a value off of the stack. */
/* Returns the value popped off the stack */
int pop(struct stack *stack)
{
struct node *top = stack->top;
int data = top->data;
stack->top = top->next;
stack->size--;
free(top);
return data;
}
/* destroy the stack */
void destroy(struct stack *stack)
{
/* free all pointers */
while(stack->top != NULL)
{
pop(stack);
}
}
