c++11 - 浮点除以零不是 constexpr

Question

编译时:

constexpr double x {123.0};
constexpr double y = x / 0.0;
std::cout << x << " / 0 = " << y << "\n";

编译器(gcc 4.9.2，-std=c++11 或 c++14)失败，给出错误:

(1.23e+2 / 0.0)' is not a constant expression
  constexpr double y = x / 0.0;

在决定是否 y 时，结果 (Inf) 有何相关性可以是 constexpr 还是不是？

作为引用，这似乎是这样做的方法:

static constexpr double z = std::numeric_limits<double>::quiet_NaN();
static constexpr double w = std::numeric_limits<double>::infinity();

Answer 1

Infinity 是实现定义的结果，标准不要求 IEEE floating point被零除的形式是 undefined behavior和 constant expression have an exclusion for undefined behavior .

来自 C++ 标准草案部分 5.6 [expr.mul]:

The binary / operator yields the quotient, and the binary % operator yields the remainder from the division of the first expression by the second. If the second operand of / or % is zero the behavior is undefined.