C pointer to array/array of pointers disambiguation?

Question

What is the difference between the following declarations: int* arr1[8]; int (*arr2)[8]; int *(arr3[8]);What is the general rule for understanding more complex declarations?

Answer 1

The main rule is: you have to read from the inside out.

Your first declaration and your third declaration said:

// (1) int* arr1[8];

// (3) int* (arr1[8]);

In case (3), we can throw away the parentheses, because C/C++'s precedence rules make them unnecessary. That means case (3) is the same as case (1).

Ok, then, how to read case (1) ? int* arr1[8];

Read the inside first...

It's an array of 8...

Then look at the data type (int *).

So we have,

int* arr1[8] is an array of 8 ptrs to ints.

What about case (2) ?

int (*arr2)[8];

Read from the inside out:

It's a pointer...

to an array of 8...

ints

int (*arr2)[8] is a pointer to an array of 8 ints.

What is the general rule for understanding more complex declarations?

Let's look at some more examples...

#include <stdio.h>

int main(int argc, const char* argv[]) {

char c = 'A'; // individual char

int x = 5; // individual ints

int y = 10;

int z = 15;

char* pc = &c; // a pointer to a char (say "char star")

int* px = &x; // pointers to ints. (say "int star" -- a ptr to an int)

int* py = &y;

int* pz = &z;

// Why is it better to write it this way?

// Because that's how we decode the more complex declarations

// Note: write pointer declarations on individual lines

int* ps; // good

int* pt; // Now, we always write pointers to int as int* ps.

int *ps, pt; // wrong: pt is an int ! don't declare multiple ptrs on 1 line

int arr_int[] = { x, y, z }; // this is an array of ints

// you wrote it as int* arr1[8]

// this is also equal to: int* (arr1[8]);

// i.e., parentheses could be thrown away

int* arr_pint[3] = { &x, &y, &z }; // an array of int*

int (*p_arrint)[3] = &arr_int; // a ptr to an array of ints

// your example was int (*arr2)[8];

// Let's take it to the next level!

int* (*p_arr_pint)[3] = &arr_pint; // a ptr to an array of ptrs to ints

return 0;

}

Hope that helps...