C++ Notes: Arrays as Pointers

Using an array name as a pointer

An array name is really a pointer to the first element of the array. For example, the following is legal.

   int b[100];    // b is an array of 100 ints.
   int* p;        // p is a pointer to an int.
   p = b;         // Assigns the address of first element of b to p.
   p = &b[0];     // Exactly the same assignment as above.

Array name is a const pointer

When you declare an array, the name is a pointer. You cannot alter the value of this pointer. In the previous example, you could never make this assignment.

   b = p;   // ILLEGAL because b is a constant, altho the correct type.

Pointer arithmetic

You can do arithmetic on pointers, if the result can be a meaningful pointer or integer.

Add or subtract integers to/from a pointer. The result is a pointer.
Subtract two pointers to the same type. The result is an int.
Multiplying doesn't make sense, adding pointers doesn't make sense, etc.

Pointer addition and element size

When you add an integer to a pointer, the integer is multiplied by the element size of the type that the pointer points to.

// Assume sizeof(int) is 4.
int b[100];  // b is an array of 100 ints.
int* p;      // p is a a pointer to an int.
p = b;       // Assigns address of first element of b. Ie, &b[0]
p = p + 1;   // Adds 4 to p (4 == 1 * sizeof(int)). Ie, &b[1]

Equivalence of subscription and dereference

Because of the way C/C++ uses pointers and arrays, you can reference an array element either by subscription or * (the unary dereference operator).

int b[100];  // b is an array of 100 ints.
int* p;      // p is a a pointer to an int.
p = b;       // Assigns address of first element of b. Ie, &b[0]
*p = 14;     // Same as b[0] = 14
p = p + 1;   // Adds 4 to p (4 == 1 * sizeof(int)). Ie, &b[1]
*p = 22;     // Same as b[1] = 22;

Understand the following remarkable equivalence (a is an array and i is an integer).

    a[i] == *(a + i) == *(i + a) == i[a]

Altho these are equivalent, never, ever, write i[a] instead of a[i].

Example - Two ways to add numbers in an array

The first uses subscripts, the second pointers. They are completely equivalent.

int a[100];
. . .
int sum = 0;
for (int i=0; i<100; i++) {
   sum += a[i];
}

int a[100];
. . .
int sum = 0;
for (int* p=a; p<a+100; p++) {
   sum += *p;
}