C++20 numbers 數學常數

01 C++20 numbers 數學常數

c++20 在 <numbers> 頭文件中增加了一些數學常數。¹

數學常數 (C++20 起)定義於頭文件 ,定義於命名空間 std::numbers。

內聯常函數	常函數定義	數學常數	常數的值
inline constexpr double e	e_v<double>	$e$	2.718288183
inline constexpr double log2e	log2e_v<double>	$log_2{e}$	1.4427
inline constexpr double log10e	log10e_v<double>	$log_{10}{e}$	0.434294
inline constexpr double pi	pi_v<double>	$\pi$	3.14159265
inline constexpr double inv_pi	inv_pi_v<double>	$\dfrac{1}{\pi}$	0.31831
inline constexpr double inv_sqrtpi	inv_sqrtpi_v<double>	$\dfrac{1}{\sqrt{\pi}}$	0.56419
inline constexpr double ln2	ln2_v<double>	$\ln{2}$	0.693147
inline constexpr double ln10	ln10_v<double>	$\ln{10}$	2.30259
inline constexpr double sqrt2	sqrt2_v<double>	$\sqrt{2}$	1.41421
inline constexpr double sqrt3	sqrt3_v<double>	$\sqrt{3}$	1.73205
inline constexpr double inv_sqrt3	inv_sqrt3_v<double>	$\dfrac{1}{\sqrt{3}}$	0.57735
inline constexpr double egamma	egamma_v<double>	歐拉常數 $\gamma$	0.5772156649
inline constexpr double phi	phi_v<double>	黃金比常數 $\Phi = \dfrac{\sqrt{5} + 1}{2}$	0.6180339887

02 測試

在vs2019 的16.5中已經提供了 <numbers>。測試效果如下：
https://github.com/5455945/cpp_demo/blob/master/C%2B%2B20/numbers/numbers.cpp

#include <numbers>
#include <iostream>

void test_numbers01() {
    std::cout << "std::numbers::e: " << std::numbers::e << " " << std::numbers::e_v<double> << std::endl;
    std::cout << "std::numbers::log2e: " << std::numbers::log2e << " " << std::numbers::log2e_v<double> << std::endl;
    std::cout << "std::numbers::log10e: " << std::numbers::log10e << " " << std::numbers::log10e_v<double> << std::endl;
    std::cout << "std::numbers::pi: " << std::numbers::pi << " " << std::numbers::pi_v<double> << std::endl;
    std::cout << "std::numbers::inv_pi: " << std::numbers::inv_pi << " " << std::numbers::inv_pi_v<double> << std::endl;
    std::cout << "std::numbers::inv_sqrtpi: " << std::numbers::inv_sqrtpi << " " << std::numbers::inv_sqrtpi_v<double> << std::endl;
    std::cout << "std::numbers::ln2: " << std::numbers::ln2 << " " << std::numbers::ln2_v<double> << std::endl;
    std::cout << "std::numbers::ln10: " << std::numbers::ln10 << " " << std::numbers::ln10_v<double> << std::endl;
    std::cout << "std::numbers::sqrt2: " << std::numbers::sqrt2 << " " << std::numbers::sqrt2_v<double> << std::endl;
    std::cout << "std::numbers::sqrt3: " << std::numbers::sqrt3 << " " << std::numbers::sqrt3_v<double> << std::endl;
    std::cout << "std::numbers::inv_sqrt3: " << std::numbers::inv_sqrt3 << " " << std::numbers::inv_sqrt3_v<double> << std::endl;
    std::cout << "std::numbers::egamma: " << std::numbers::egamma << " " << std::numbers::egamma_v<double> << std::endl;
    std::cout << "std::numbers::phi: " << std::numbers::phi << " " << std::numbers::phi_v<double> << std::endl;
    std::cout << "std::numbers::e: " << std::numbers::e << " " << std::numbers::e_v<double> << std::endl;
}

int main() {
    test_numbers01();
    return 0;
}

輸出:

std::numbers::e: 2.71828 2.71828
std::numbers::log2e: 1.4427 1.4427
std::numbers::log10e: 0.434294 0.434294
std::numbers::pi: 3.14159 3.14159
std::numbers::inv_pi: 0.31831 0.31831
std::numbers::inv_sqrtpi: 0.56419 0.56419
std::numbers::ln2: 0.693147 0.693147
std::numbers::ln10: 2.30259 2.30259
std::numbers::sqrt2: 1.41421 1.41421
std::numbers::sqrt3: 1.73205 1.73205
std::numbers::inv_sqrt3: 0.57735 0.57735
std::numbers::egamma: 0.577216 0.577216
std::numbers::phi: 1.61803 1.61803
std::numbers::e: 2.71828 2.71828

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1. 數學常數 ↩︎