有人可以告诉我为什么会收到此错误消息吗？ 内联 constexpr 变量

Question

I follow a tutorial from here https://www.learncpp.com/cpp-tutorial/global-constants-and-inline-variables/

main.cpp

#include <iostream>
#include "constants.h"
/* run this program using the console pauser or add your own getch, system("pause") or input loop */

int main() {
    std::cout << "Enter a radius: ";
    int radius{};
    std::cin >> radius;

    std::cout << "The circumference is: " << 2 * radius * constants::pi;
    return 0;
}

constants.h

#ifndef CONSTANTS_H
#define CONSTANTS_H

// define your own namespace to hold constants
namespace constants
{
    inline constexpr double pi { 3.14159 }; // note: now inline constexpr
    inline constexpr double avogadro { 6.0221413e23 };
    inline constexpr double my_gravity { 9.2 }; // m/s^2 -- gravity is light on this planet
    // ... other related constants
}
#endif

error message g++11:

error: 'constants::pi' declared as an 'inline' variable

Answer 1

Inline variables are allowed starting from C++17.

You need to specify the -std=c++17 option on the compiler command line.

( https://en.cppreference.com/w/cpp/language/inline )