为什么我们需要 int *ptr = NULL;？

Question

I don't quite understand why in this example we need a part int *ptr = NULL; . I understand what pointers are, but why do we need this * (points at a value attached to an address) here? And why do we need NULL here?

#include <stdio.h>

int main() {
    int a[5] = {22, 33, 44, 55, 66};
    int *ptr = NULL;
    int i;

    ptr = a;
    for (i = 0; i < 5; i++) {
        printf("%d ", *(ptr + i));
    }
}

Answer 1

This intermediate initialization

int *ptr = NULL;

is redundant. You could write at once

int *ptr = a;

Moreover such an initialization is a bad programming style. Each variable should be declared and initialized where it is used. Otherwise the code will confuses users as it confused you.

I would rewrite the program the following way

#include <stdio.h>

int main( void )
{
    int a[] = { 22, 33, 44, 55, 66 };
    const size_t N = sizeof( a ) / sizeof( *a );

    for ( const int *ptr = a; ptr != a + N; ++ptr ) 
    {
        printf( "%d ", *ptr );
    }

    putchar( '\n' );
}

There are cases when the same one pointer can be used in various contexts. In this case it makes sense to declare it and initialize with NULL. But your simple program is not the case.

Answer 2

在你的情况下int *ptr = NULL是完全多余的，因为你可以像 Vlad 已经说过int *ptr = a那样写int *ptr = a ，但更一般地说，初始化指向NULL指针是一个好主意，因为如果由于某种原因你有错误您的代码并且您需要调试它，如果指针以NULL开头，则调试指针发生的情况比未初始化它并且填充了一些垃圾值要容易得多。

Answer 3

I don't quite understand why in this example we need a part int *ptr = NULL; .

[...]

why do we need this * (points at a value attached to an address) here?

You are mistaking the significance of the * . In that context, it is not the unary * operator. Rather, it is part of the syntax for declaring a variable whose type is a pointer type. The declaration declares ptr as a variable of type int * , ie pointer-to-integer. Without the * , the declaration would be designating ptr 's type as int , not as a pointer type.

And why do we need NULL here?

You don't. The = NULL is an initializer for variable ptr , defining its initial value, but you assign a different value to that variable before ever reading it, so the initializer has no practical significance.

Some people are inclined to provide such initializers as a matter of course, on the theory that it avoids accidental usage of wild pointers and / or allows (otherwise-)uninitialized pointers to be detected, but as Emerson wrote, "A foolish consistency is the hobgoblin of little minds." The initializer serves no useful purpose in this particular code. If one insists on providing an initializer then there's no reason not to initialize ptr directly to a :

int *ptr = a;

as your other answers also suggest.