Equality vs. Equivalence

1#include <cctype>
2#include <compare>
3#include <iostream>
4#include <string>
5
6class CaseInsensitiveString {
7 public:
8  std::string Value;
9
10  std::strong_ordering operator<=>(
11    const CaseInsensitiveString& Other) const {
12    auto it1 = Value.begin();
13    auto it2 = Other.Value.begin();
14
15    while (it1 != Value.end()
16      && it2 != Other.Value.end()
17    ) {
18      char c1 = std::tolower(*it1);
19      char c2 = std::tolower(*it2);
20      if (c1 < c2)
21        return std::strong_ordering::less;
22      if (c1 > c2)
23        return std::strong_ordering::greater;
24      ++it1;
25      ++it2;
26    }
27
28    if (Value.size() < Other.Value.size())
29      return std::strong_ordering::less;
30    if (Value.size() > Other.Value.size())
31      return std::strong_ordering::greater;
32    return std::strong_ordering::equivalent;
33  }
34
35  bool operator==(
36    const CaseInsensitiveString& Other) const {
37    return std::equal(
38      Value.begin(), Value.end(),
39      Other.Value.begin(), Other.Value.end(),
40      [](char a, char b) {
41        return std::tolower(a) == std::tolower(b);
42      });
43  }
44};
45
46int main() {
47  CaseInsensitiveString A{"hello"};
48  CaseInsensitiveString B{"HELLO"};
49
50  if (A == B) {
51    std::cout << "A is equal to B\n";
52  }
53
54  if ((A <=> B) == std::strong_ordering::equivalent) {
55    std::cout << "A is equivalent to B\n";
56  }
57}