javascript - -nan 在 C++ 中是什么意思？为什么 pow(-4, -2.1) 返回 -nan？

Question

我明白为什么 Math.pow(a, b) is NaN when a is a negative number and b is a non-integer在 JavaScript 中。同样发生in other programming languages .

但是，C++ 呢？

在 JavaScript 中 Math.pow(-4, -2.1) 返回 NaN，在 C++ 中 pow (-4, -2.1)返回 -nan。为什么？

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例子:

#include <iostream>
#include <math.h>

using namespace std;

int main () {
    cout << "(4)  ^ (2.1)  = " << pow (4,  -2.1) << endl; // 0.0544094
    cout << "(-4) ^ (-2.1) = " << pow (-4, -2.1) << endl; // -nan
    cout << "(-4) ^ (2.1)  = " << pow (-4,  2.1) << endl; // -nan
    return 0;
}

输出:

(4)  ^ (2.1)  = 0.0544094
(-4) ^ (-2.1) = -nan
(-4) ^ (2.1)  = -nan

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我使用 g++ 编译了代码。

$ g++ --version
g++ (Ubuntu/Linaro 4.8.1-10ubuntu9) 4.8.1
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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尝试使用在线工具编译它，但显示相同的结果:

• http://ideone.com/4xqNU7 (C++11)
• http://ideone.com/4xqNU7 (C++ 4.8.1)
• http://www.compileonline.com/compile_cpp0x_online.php

Answer 1

我能找到的唯一相关引述来自 n1570 .

An implementation may give zero and values that are not floating-point numbers (such as infinities and NaNs) a sign or may leave them unsigned. Wherever such values are unsigned, any requirement in this International Standard to retrieve the sign shall produce an unspecified sign, and any requirement to set the sign shall be ignored.