ARR36-CPP. Do not subtract or compare two pointers or iterators that do not refer to the same array or container

When two pointers are subtracted, both must point to elements of the same array object or to one past the last element of the array object; the result is the difference of the subscripts of the two array elements. Likewise, when two iterators are subtracted, both must refer to elements in the same container, or to the value returned by the end() method of the container. This restriction exists because pointer subtraction in C++ produces the number of objects between the two pointers, not the number of bytes.

Similarly, comparing pointers gives the positions of the pointers relative to each other. Subtracting or comparing pointers that do not refer to the same array can result in erroneous behavior.

It is acceptable to subtract or compare two member pointers within a single struct object, suitably cast, because any object can be treated as an array of unsigned char. However, when doing so remember to account for the effects of alignment and padding on the structure.

Noncompliant Code Example (Arrays)

In this noncompliant code example pointer subtraction is used to determine how many free elements are left in the nums array.

int nums[SIZE];
char *strings[SIZE];
int *next_num_ptr = nums;
int free_bytes;

/* increment next_num_ptr as array fills */

free_bytes = strings - (char **)next_num_ptr;

The first incorrect assumption is that nums and strings arrays are necessarily contiguous in memory. The second is that free_bytes is the number of bytes available. The subtraction returns the number of elements between next_num_ptr and strings.

Compliant Solution (Arrays)

In this compliant solution, the number of free elements is kept as a counter and adjusted on every array operation. It is also calculated in terms of free elements instead of bytes. This prevents further mathematical errors.

int nums[SIZE];
char *strings[SIZE];
int *next_num_ptr = nums;
int free_bytes;

/* increment next_num_ptr as array fills */

free_bytes = (&(nums[SIZE]) - next_num_ptr) * sizeof(int);

Noncompliant Code Example (Vectors)

In this noncompliant code example two iterators are subtracted that don't refer to the same vector.

vector<int> nums1(10, 0);
vector<int> nums2(10, 0);
vector<int>::iterator i1 = nums1.begin();
vector<int>::iterator i2 = nums2.begin();

int distance = i2 - i1;

Since C++ vector iterators are usually implemented as pointers to contiguous memory on the heap, much like array pointers, one might expect distance to have a value of 10. However the C++ standard leaves this as undefined. On Linux with G++ 4.3.2, distance receives an unexpected value of 12.

Compliant Solution (Vectors)

In this compliant solution, both iterators point to the same vector, and the distance variable receives the expected value of 10.

vector<int> nums1(10, 0);
vector<int> nums2(10, 0);
vector<int>::iterator i1 = nums1.begin();
vector<int>::iterator i2 = nums1.end();

int distance = i2 - i1;