C++20三路比较符

三路比较符（C++20）

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When do you actually use <=>?

例子1：

#include <iostream>
#include <compare>

struct Rational_2 {
    int num;
    int den; // > 0
};

constexpr std::weak_ordering operator<=>(Rational_2 lhs, Rational_2 rhs)
{
    return lhs.num * rhs.den <=> rhs.num * lhs.den;
}

void print(std::weak_ordering value)
{
    if (value == 0)
        std::cout << "equal\n";
    else if (value < 0)
        std::cout << "less\n";
    else
        std::cout << "greater\n";
}

int main()
{
    Rational_2 c{6,5};
    Rational_2 d{8,7};
    print(c <=> d);
    print(std::compare_three_way{}(c,d));
}

例子2：

#include <iostream>
#include <compare>
struct Basics {
  int i;
  char c;
  float f;
  double d;
  auto operator<=>(const Basics&) const = default;
};

struct Arrays {
  int ai[1];
  char ac[2];
  float af[3];
  double ad[2][2];
  auto operator<=>(const Arrays&) const = default;
};

struct Bases : Basics, Arrays {
  auto operator<=>(const Bases&) const = default;
};

int main() {
  constexpr Bases a = { { 0, 'c', 1.f, 1. },
                        { { 1 }, { 'a', 'b' }, { 1.f, 2.f, 3.f }, { { 1., 2. }, { 3., 4. } } } };
  constexpr Bases b = { { 0, 'c', 1.f, 1. },
                        { { 1 }, { 'a', 'b' }, { 1.f, 2.f, 3.f }, { { 1., 2. }, { 3., 4. } } } };
  static_assert(a == b);
  static_assert(!(a != b));
  static_assert(!(a < b));
  static_assert(a <= b);
  static_assert(!(a > b));
  static_assert(a >= b);
}