学习笔记：编译期运算，利用constexpr

#include <iostream>     // std::cout

#include <tuple>        // std::tuple, std::get, std::forward_as_tuple

#include <string>       // std::string

#include <type_traits>

constexpr int isqrt_helper(int sq, int d, int a)

{

    return sq <= a?isqrt_helper(sq+d,d+2,a):d;

}

constexpr int isqrt(int x)

{

    return isqrt_helper(1,3,x)/2-1;

}

struct Point {

    int x,y,z;

    constexpr Point up(int d) { return {x,y,z+d}; }

    constexpr Point move(int dx, int dy) { return {x+dx,y+dy,z}; }

};

constexpr int square(int x){ return x*x; }

constexpr int radial_distance(Point p){

    return isqrt(square(p.x)+square(p.y)+square(p.z));

}

int main()

{

    constexpr Point p1 {10,20,30};                   // the default constructor is constexpr

    constexpr Point p2 (p1.up(20));                   // Point::up() is constexpr

    constexpr int dist = radial_distance(p2);

    return 0;

}