Can someone clarify this array/pointer idea to me?

Question

In attempt to explain that arrays are just pointers (in C++) to our class, my professor showed us this:

array[5]      // cout'ing this 
*(array + 5)  // would return the same value as this

I'm having a little trouble completely understanding it. Here's my thinking:

array is the address of the first location and so if we add 5 to that address, we move 5 addresses in memory. The pointer operator pulls the data from the memory location.

Is this the correct idea? The idea still feels foggy with me and just feel like I don't understand it completely. I think hearing someone else explain it might help me understand it more. Thanks in advance!

Answer 1

You've got the right idea.

Arrays implicitly cast to pointers. Interestingly, [] works on pointers, not arrays.

a[b] Subscript operator is defined as *(a + (b)). [] is used as syntactic sugar - it's much more pleasant to write array[5] instead of *(array + 5)

Pointer arithmetic with a pointer and an integer p + i increases the address at p by i * sizeof(*p) bytes.

char* p; 
p + 5; // address increased by 5

int* p;
p + 5; // address increased by 20 as sizeof(*p) is 4

The * operator performs indirection. It will give you what the pointer is pointing to.

int x[2];
int* p = &x[0]; // could also be int* p = x;
*p = 5;         // x[0] is now 5
*(p + 1) = 10;  // x[1] is now 10

Answer 2

Your professor is correct that the two expressions will produce the same results. Your explanation of it leads me to believe you have a good grasp of the mechanics.

It is not quite correct to say an array is the same as a pointer. An array is very easily converted to a pointer, which leads to some confusion.

For example consider this code:

int array[5];
int * pointer = new int[5];
cout << sizeof(array);   // outputs 5*sizeof(int), most probably 20
cout << sizeof(pointer); // outputs sizeof(int*), most probably 4 on a 32 bit OS
array[4] = 906;
pointer[4] = 906;
cout << *(array + 4) << *(pointer + 4); // outputs "906 906"