C：我为什么要声明一个指针？

Question

It seems there are many questions of the form "should I declare X?" but not this specific one. I hope it is ok to ask this.

The title says it all: why should I declare a pointer? Even better: there are risks if I do not declare the pointer? Consider the following examples:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <math.h>
#include <time.h>
#include <string.h>

void func(int *ptr);

int main (int argc, char **argv)
{

  int a;
  int *PTRa;

  a = -1;
  PTRa = &a;


  func(PTRa);
  printf("%d\n", a);

  return 0;

}

void func(int *ptr)
{
  *ptr = 1;
  return;
}

I get a=1 . In this case I would say the pointer is declared (and assigned as well): I have the line int *PTRa; (declaration) and the line PTRa = &a; (assignment). The results is correct. I don't get any warning.

Imagine now to replace the main with the following:

int main (int argc, char **argv)
{

  int a;

  a = -1;

  func(&a);
  printf("%d\n", a);

  return 0;

}

Here I do not declare the pointer but just give the address of a to func . The result is correct and I don't get warnings.

My understanding is that the two approaches are identical: func always gets the same input, the address of a . I would even dare to say that I feel the second approach to be better, as I feel it to be clearer and I feel the variable PTRa to be useless and somewhat redundant. However, I always see codes where the first approach is used and I have the feeling I will be told to do so. Why?

Answer 1

In this case there's no benefit in creating a separate pointer variable.

It might be necessary in more complex cases, just like it's sometimes necessary to create variables of any other type.

Answer 2

You are correct: there's no point in declaring a pointer in your example. A pointer is just a variable that holds an address. The cleaner approach is to pass directly the address of the variable: func(&a) instead of doing one extra step and declaring PTRa .

Note that not all cases are this simple. For example, if you want to have an array of int s, but you want to be able to grow that array dynamically because you don't know how big it should be you have to declare a pointer:

int count = ...; // get the count from the user, from a file, etc
int *list_of_ints = malloc(sizeof(int) * count);
if (list_of_ints == NULL)
{
    // malloc failed.
    printf("Not enough memory!\n");
    exit(1);
}

// Now `list_of_ints` has enough space to store exactly `count` `int`s

EDIT: as @paulsm4 pointed out in a comment, the question Why use pointers? is a great source of information related to this topic.

EDIT 2: one good reason to want a pointer to the address of a variable might be that you want a pointer inside a structure or array:

struct foo
{
    int x;
};

struct bar
{
    int y;
    struct foo f;
};

struct bar b;
struct foo *ptr_foo = &b.f;

You can now work more easily with bf because you're just working with a struct foo .

Answer 3

From the title, I thought you're talking about pointer type, but actually, you are asking if declaring a variable is needed.

• Variable is a piece of memory, storing some numbers(bytes), and the type of the variable, indicating how you and your program interpret those bytes: integer? float? character? etc.
• Pointer is the memory address, it could be of a variable, or a function, or something else.
• A variable of pointer is a small area in the memory, storing the address of other(or even same) memory.
• You decide if you need an extra variable to store the pointer. It's the same to the decision that if you want a variable to store an integer:

int v = -1;
abs(v); // use variable
abs(-1); // use constant