二维数组 C++ 指针地址

Question

#include <stdio.h>
int main()

    { int arr2D[3][3]; 
    printf("%d\n",((arr2D==*arr2D) && (*arr2D==arr2D[0]))); 
    return o;
    }

How the values stores at *arr2D and arr2d is same whereas arr2D is a constant pointer which will store the address of the first element and arr2D means the value present at the address which is pointed by arr2D?

Answer 1

If we draw out your array on "paper" it will look like this

+-------------+-------------+-------------+-------------+-------------+-------------+-------------+-------------+-------------+
| arr2D[0][0] | arr2D[0][1] | arr2D[0][2] | arr2D[1][0] | arr2D[1][1] | arr2D[1][2] | arr2D[2][0] | arr2D[2][1] | arr2D[2][2] |
+-------------+-------------+-------------+-------------+-------------+-------------+-------------+-------------+-------------+

Then you have to remember that an array naturally decays to a pointer to its first element. That is plain arr2D when a pointer is expected, is the same as &arr2D[0] .

Now if we "redraw" the array, but only for arr2D[0] (which is what is most relevant for your question) with some of the possible pointers:

+-------------+-------------+-------------+-----+
| arr2D[0][0] | arr2D[0][1] | arr2D[0][2] | ... |
+-------------+-------------+-------------+-----+
^
|
&arr2D[0]
|
&arr2D[0][0]

Since we know that arr2D is the same as &arr2D[0] , we can then do that substitution in the expression arr2D == *arr2D . That gets us &arr2D[0] == *&arr2D[0] .

The dereference * and address-of & operators cancel each other out, so we have &arr2D[0] == arr2D[0] .

Now keep up... We know that an array decays to a pointer to its first element, and we know that arr2D[0] is an array; That means it will decay to &arr2D[0][0] , leaving us with the expression &arr2D[0] == &arr2D[0][0] . And as shown those two addresses are the same, which means the comparison will be true.

Important note: While &arr2D[0] and &arr2D[0][0] might both point to the same location, their types are different. The type of &arr2D[0] is int (*)[3] , while &arr2D[0][0] is of type int * .

---

Armed with the above information it should be easy to decipher the other comparison *arr2D == arr2D[0] , especially since all of the parts of that are already mentioned.

Answer 2

This is not valid C code.

arr2D when used in an expression decays to a pointer to the first element, int (*)[3] . Whereas *arr2D gives the first item of the 2D array, an int[3] , which too decays when used in an expression, into an int* .

So the code compares an int (*)[3] with an int* . They are not compatible pointer types and cannot be compared - this is a constraint violation of the standard (C17 6.5.9/2) and the compiler must produce a diagnostic message. Meaning that this is a severe bug, and anyone bothering to try the code in a conforming C compiler would find it.

*arr2D and arr2D[0] are however always equivalent, per definition of the [] operator.

What the code prints is anyone's guess, since code containing constraint violations is to be regarded as having undefined behavior.