<numbers>是C++20中新增加的头文件，提供一组常量用于表示数学中的某些特殊值，此头文件是numeric库的一部分。包括：

      1. 圆周率π:std::numbers::pi、std::numbers::pi_v<T>

      2. 圆周率π的倒数：numbers::inv_pi、std::numbers::inv_pi_v<T>

      3. 自然常数e：std::numbers::e、std::numbers::e_v<T>

      4. 以2为底自然常数e的对数:std::numbers::log2e、std::numbers::log2e_v<T>

      5. 以自然常数e为底的2的对数：std::numbers::ln2、std::numbers::ln2_v<T>

      6. 以10为底自然常数e的对数：std::numbers::log10e、std::numbers::log10e_v<T>

      7. 以自然常数e为底的10的对数：std::numbers::ln10、std::numbers::ln10_v<T>

      8. 2的平方根：std::numbers::sqrt2、std::numbers::sqrt2_v<T>

      9. 3的平方根：std::numbers::sqrt3、std::numbers::sqrt3_v<T>

      10. 3的平方根的倒数：std::numbers::inv_sqrt3、std::numbers::inv_sqrt3_v<T>

      注：

      1.不带_v的默认为double类型

      2.T可为float、double、long double

int test_numbers()
{
	std::cout << "pi: " << std::numbers::pi <<"," << std::numbers::pi_v<float> << std::endl; // pi: 3.14159,3.14159
	std::cout << "inv_pi: " << std::numbers::inv_pi << "," << std::numbers::inv_pi_v<float> << std::endl; // inv_pi: 0.31831,0.31831
	std::cout << "e: " << std::numbers::e << "," << std::numbers::e_v<double> << std::endl; // e: 2.71828,2.71828
	std::cout << "log2e: " << std::numbers::log2e << "," << std::numbers::log2e_v<long double> << std::endl; // log2e: 1.4427,1.4427
	std::cout << "ln2: " << std::numbers::ln2 << "," << std::numbers::ln2_v<float> << std::endl; // ln2: 0.693147,0.693147
	std::cout << "log10e: " << std::numbers::log10e << "," << std::numbers::log10e_v<float> << std::endl; // log10e: 0.434294,0.434294
	std::cout << "ln10: " << std::numbers::ln10 << "," << std::numbers::ln10_v<float> << std::endl; // ln10: 2.30259,2.30259
	std::cout << "sqrt2: " << std::numbers::sqrt2 << "," << std::numbers::sqrt2_v<float> << std::endl; // sqrt2: 1.41421,1.41421
	std::cout << "sqrt3: " << std::numbers::sqrt3 << "," << std::numbers::sqrt3_v<float> << std::endl; // sqrt3: 1.73205,1.73205
	std::cout << "inv_sqrt3: " << std::numbers::inv_sqrt3 << "," << std::numbers::inv_sqrt3_v<float> << std::endl; // inv_sqrt3: 0.57735,0.57735

	return 0;
}

      执行结果如下图所示：

      GitHub：https://github.com/fengbingchun/Messy_Test