三向比较运算符＜=＞

        三向比较符又称太空飞船操作符，可以用来确定一个值和另外一个值的大小关系，返回枚举类型，定义在<compare>头文件中。

例1：

int n1{ 10 };
int n2{ 20 };
auto result = n1 <=> n2;
if (std::strong_ordering::less == result) //判断n1是否小于n2
{
	cout << "n1 < n2" << endl;
}

if (std::strong_ordering::greater == result) //判断n1是否大于n2
{
	cout << "n1 > n2" << endl;
}

if (std::strong_ordering::equal == result) //判断n1是否等于n2
{
	cout << "n1 = n2" << endl;
}

if (std::strong_ordering::equivalent == result) //判断n1是有与n2等价
{
	cout << "n1 == n2" << endl;
}


//输出结果：n1<n2	

        result的类型为一个名为std::strong_ordering的枚举类型，因为n1 n2均为int，则该比较的结果为强排序，其中equal意为相等，equivalent意为等价，在strong ordering类型下这两个意思相同，以下是这两个值得源码定义：

…
enum class _Compare_eq : _Compare_t { equal = 0, equivalent = equal };
…
inline constexpr strong_ordering strong_ordering::equal{static_cast<_Compare_t>(_Compare_eq::equal)};
inline constexpr strong_ordering strong_ordering::equivalent{static_cast<_Compare_t>(_Compare_eq::equivalent)};
…

可见在源码层次，这两个值是完全一样的。

例2：

如果比较的类型是浮点型，那么返回的结果就是partial ordering

    double n1{ 10 };
	double n2{ 20 };
	auto result = n1 <=> n2;
	if (std::partial_ordering::less == result) //判断n1是否小于n2
	{
		cout << "n1 < n2" << endl;
	}

	if (std::partial_ordering::greater == result) //判断n1是否大于n2
	{
		cout << "n1 > n2" << endl;
	}

	if (std::partial_ordering::equivalent == result) //判断n1是否等于n2
	{
		cout << "n1 = n2" << endl;
	}
	if (std::partial_ordering::unordered == result) //
	{
		cout << "n1 or n2 is unordered" << endl;
	}
	//输出结果：n1<n2

例3：

对于自定义类型重载<=>我们需要弱排序weak_ordering

import <iostream>;
import <string>;
import <compare>;
using namespace std;

class testWeakOrdering
{
public:
	testWeakOrdering(int value):m_value(value)
	{

	}
	std::weak_ordering operator<=> (const testWeakOrdering& other)
	{
		if (m_value < other.m_value)
		{
			return weak_ordering::less;
		}
		else if (m_value == other.m_value)
		{
			return weak_ordering::equivalent;
		}
		else
		{
			return weak_ordering::greater;
		}
	}

private:
	int m_value;
};

int main()
{
	testWeakOrdering n1{ 10 };
	testWeakOrdering n2{ 20 };
	auto result = n1 <=> n2;
	if (std::weak_ordering::less == result) //判断n1是否小于n2
	{
		cout << "n1 < n2" << endl;
	}

	if (std::weak_ordering::greater == result) //判断n1是否大于n2
	{
		cout << "n1 > n2" << endl;
	}

	if (std::weak_ordering::equivalent == result) //判断n1是否等于n2
	{
		cout << "n1 = n2" << endl;
	}

	return 0;
}