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#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);
}

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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.

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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 <.

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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.

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#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

In this This compliant solution , demonstrates a fully portable, but likely inefficient equality comparing , implementation of in_range() that compares test against each possible address in the range [r, n] is used to implement the in_range() function:. 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);
}

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