inline double _SQRT_ (double value) {
const double half = value * double (0.5) ;
union {
static_assert (sizeof (double) == sizeof (__int64) ,"") ;
double a ;
__int64 b ;
} tmp ;
tmp.a = value ;
tmp.b = __int64 (0X5FE6EC85E7DE30DA) - (tmp.b >> 1) ;
value = tmp.a ;
value *= double (1.5) - value * value * half ;
value *= double (1.5) - value * value * half ;
value *= double (1.5) - value * value * half ;
value = 1 / value ;
return value ;
}
本人想要一个constexpr型的_SQRT_用于模板当中，例如
在一个参数为SIZE的模板类中本人需要_SQRT_ (SIZE)的额外缓冲区
应该怎么写？
解决方案
50
#include <limits>
namespace detail {
constexpr double sqrt_newton_raphson(double x, double curr, double prev) {
return curr == prev
? curr
: sqrt_newton_raphson(x, 0.5 * (curr + x / curr), curr);
}
}
constexpr double cesqrt(double x) {
return x >= 0 && x < std::numeric_limits<double>::infinity()
? detail::sqrt_newton_raphson(x, x, 0)
: std::numeric_limits<double>::quiet_NaN();
}
constexpr size_t ceround(double x) {
return size_t(x + 0.5);
}
template<typename T>
auto get_buffer() {
constexpr auto st = cesqrt(static_cast<double>(sizeof(T)));
constexpr auto len = ceround(st);
static char buf[len];
return buf;
}
http://stackoverflow.com/questions/8622256/in-c11-is-sqrt-defined-as-constexpr