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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. 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

Similarly, when two iterators are subtracted (including via std::distance()), both iterators must refer to the same container object or must be obtained via a call to end() (or cend()) on the same container object.

If two unrelated iterators (including pointers) are subtracted, the operation results in undefined behavior [ISO/IEC 14882-2014]. Do not subtract two iterators (including pointers) unless both point into the same container or one past the end of the same container.

Noncompliant Code Example

This noncompliant code example attempts to determine whether the pointer test is within the range [r, r + n]. However, when test does not point within the given range, as in this example, the subtraction produces undefined behavior.

#include <cstddef>
#include <iostream>
 
template <typename Ty>
bool in_range(const Ty *test, const Ty *r, size_t n) {
  return 0 < (test - r) && (test - r) < (std::ptrdiff_t)n;
}
 
void f() {
  double foo[10];
  double *x = &foo[0];
  double bar;
  std::cout << std::boolalpha << in_range(&bar, x, 10);
}

Noncompliant Code Example

In this noncompliant code example, the in_range() function is implemented using a comparison expression instead of subtraction. The C++ Standard, [expr.rel], paragraph 4 [ISO/IEC 14882-2014], states the following:

If two operands p and q compare equal, p<=q and p>=q both yield true and p<q and p>q both yield false. Otherwise, if a pointer p compares greater than a pointer q, p>=q, p>q, q<=p, and q<p all yield true and p<=q, p<q, q>=p, and q>p all yield false. Otherwise, the result of each of the operators is unspecified.

Thus, comparing two pointers that do not point into the same container or one past the end of the container results in unspecified behavior. Although the following example is an improvement over the previous noncompliant code example, it does not result in portable code and may fail when executed on a segmented memory architecture (such as some antiquated x86 variants). Consequently, it is noncompliant.

#include <iostream>
 
template <typename Ty>
bool in_range(const Ty *test, const Ty *r, size_t n) {
  return test >= r && test < (r + n);
}
 
void f() {
  double foo[10];
  double *x = &foo[0];
  double bar;
  std::cout << std::boolalpha << in_range(&bar, x, 10);
}

Noncompliant Code Example

This noncompliant code example is roughly equivalent to the previous example, except that it uses iterators in place of raw pointers. As with the previous example, the in_range_impl() function exhibits unspecified behavior when the iterators do not refer into the same container because the operational semantics of a < b on a random access iterator are b - a > 0, and >= is implemented in terms of <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 contingent 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

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.

#include <iostream>
#include <iterator>
#include <vector>

template <typename RandIter>
bool in_range_impl(RandIter test, RandIter r_begin, RandIter r_end, std::random_access_iterator_tag) {
  return test >= r_begin && test < r_end;
}
 
template <typename Iter>
bool in_range(Iter test, Iter r_begin, Iter r_end) {
  typename std::iterator_traits<Iter>::iterator_category cat;
  return in_range_impl(test, r_begin, r_end, cat);
}
 
void f() {
  std::vector<double> foo(10);
  std::vector<double> bar(1);
  std::cout << std::boolalpha << in_range(bar.begin(), foo.begin(), foo.end());
}

Noncompliant Code Example

In this noncompliant code example, std::less<> is used in place of the < operator. The C++ Standard, [comparisons], paragraph 14 [ISO/IEC 14882-2014], states the following:

For templates greater, less, greater_equal, and less_equal, the specializations for any pointer type yield a total order, even if the built-in operators <, >, <=, >= do not.

Although this approach yields a total ordering, the definition of that total ordering is still unspecified by the implementation. For instance, the following statement could result in the assertion triggering for a given, unrelated pair of pointers, a and b: assert(std::less<T *>()(a, b) == std::greater<T *>()(a, b));. Consequently, this noncompliant code example is still nonportable and, on common implementations of std::less<>, may even result in undefined behavior when the < operator is invoked.

#include <functional>
#include <iostream>
 
template <typename Ty>
bool in_range(const Ty *test, const Ty *r, size_t n) {
  std::less<const Ty *> less;
  return !less(test, r) && less(test, r + n);
}
 
void f() {
  double foo[10];
  double *x = &foo[0];
  double bar;
  std::cout << std::boolalpha << in_range(&bar, x, 10);
}

Compliant Solution

This compliant solution demonstrates a fully portable, but likely inefficient, implementation of in_range() that compares test against each possible address in the range [r, n]. A compliant solution that is both efficient and fully portable is currently unknown.

#include <iostream>
 
template <typename Ty>
bool in_range(const Ty *test, const Ty *r, size_t n) {
  auto *cur = reinterpret_cast<const unsigned char *>(r);
  auto *end = reinterpret_cast<const unsigned char *>(r + n);
  auto *testPtr = reinterpret_cast<const unsigned char *>(test);
 
  for (; cur != end; ++cur) {
    if (cur == testPtr) {
      return true;
    }
  }
  return false;
}
 
void f() {
  double foo[10];
  double *x = &foo[0];
  double bar;
  std::cout << std::boolalpha << in_range(&bar, x, 10);
}

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);

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Image Added Image Added Image Added 9899:1999|AA. C++ References#ISO/IEC 9899-1999]\] Section 6.5.6, "Additive operators" \[[MITRE 07|AA. C++ References#MITRE 07]\] [CWE ID 469|http://cwe.mitre.org/data/definitions/469.html], "Use of Pointer Subtraction to Determine Size"ARR35-CPP. Do not allow loops to iterate beyond the end of an array      06. Arrays (ARR)      ARR37-CPP. Do not add or subtract an integer to a pointer to a non-array object