C++20stladvanced
std::numbers::log2e / std::numbers::log10e (C++20 Addition)
C++20 · The Big Four · Compiler flag: -std=c++20
Explanation
Base-2 and base-10 logarithms of Euler's number e. `log2e` is approximately 1.4427; `log10e` is approximately 0.4343. The reciprocals of `ln2` and `ln10`.
Real-World Scenario
Convert natural logarithm results to base-2 for bit-width calculations in a codec.
Quick Snippet
snippet.cpp
#include <numbers>
#include <cmath>
double log2_value = std::log(x) * std::numbers::log2e;
double log10_value = std::log(x) * std::numbers::log10e;Full Example
math_constants.cpp
// CodeNebula - C++20 Example: Mathematical Constants (std::numbers)
// Compile: g++ -std=c++20 -Wall -Wextra -o example math_constants.cpp
#include <iomanip>
#include <iostream>
#include <numbers>
int main() {
std::cout << std::setprecision(18);
std::cout << "=== std::numbers mathematical constants (double) ===\n\n";
std::cout << "pi = " << std::numbers::pi << "\n";
std::cout << "e = " << std::numbers::e << "\n";
std::cout << "sqrt2 = " << std::numbers::sqrt2 << "\n";
std::cout << "sqrt3 = " << std::numbers::sqrt3 << "\n";
std::cout << "ln2 = " << std::numbers::ln2 << "\n";
std::cout << "ln10 = " << std::numbers::ln10 << "\n";
std::cout << "log2e = " << std::numbers::log2e << "\n";
std::cout << "log10e = " << std::numbers::log10e << "\n";
std::cout << "phi = " << std::numbers::phi << "\n";
std::cout << "inv_pi = " << std::numbers::inv_pi << "\n";
std::cout << "inv_sqrtpi = " << std::numbers::inv_sqrtpi << "\n";
std::cout << "egamma = " << std::numbers::egamma << "\n";
std::cout << "\n=== Float versions (via _v template) ===\n\n";
std::cout << std::setprecision(9);
std::cout << "pi_v<float> = " << std::numbers::pi_v<float> << "\n";
std::cout << "e_v<float> = " << std::numbers::e_v<float> << "\n";
std::cout << "phi_v<float> = " << std::numbers::phi_v<float> << "\n";
std::cout << "\n=== Long double versions ===\n\n";
std::cout << std::setprecision(21);
std::cout << "pi_v<long double> = " << std::numbers::pi_v<long double> << "\n";
std::cout << "e_v<long double> = " << std::numbers::e_v<long double> << "\n";
std::cout << "\n=== Verifying relationships ===\n\n";
std::cout << std::setprecision(15);
std::cout << "ln2 * log2e ~= " << std::numbers::ln2 * std::numbers::log2e
<< " (should be 1)\n";
std::cout << "ln10 * log10e ~= " << std::numbers::ln10 * std::numbers::log10e
<< " (should be 1)\n";
std::cout << "pi * inv_pi ~= " << std::numbers::pi * std::numbers::inv_pi
<< " (should be 1)\n";
std::cout << "sqrt2 * sqrt2 ~= " << std::numbers::sqrt2 * std::numbers::sqrt2
<< " (should be 2)\n";
std::cout << "sqrt3 * sqrt3 ~= " << std::numbers::sqrt3 * std::numbers::sqrt3
<< " (should be 3)\n";
std::cout << "phi = (1+sqrt(5))/2 ~= " << (1.0 + std::sqrt(5.0)) / 2.0
<< " (golden ratio)\n";
return 0;
}
Run command
g++ -std=c++20 -Wall -Wextra -o example math_constants.cpp