Visual Studio下的std :: nan/std :: nanf有什么问题? [英] What's wrong with std::nan/std::nanf under Visual Studio?
问题描述
想与<a href="https://en.cppreference.com/w/cpp/numeric/math/nan" rel="nofollow noreferrer"> std :: nan 和 std :: nanf 来创建带有一些自定义有效负载的nan值(非装箱)
Wanted to play with std::nan and std::nanf to create nan values with some custom payload (non boxing).
但是，它确实不能按预期工作:
However, it really does not work as expected:
但是，对于Visual Studio 2015，该功能显然未正确实现. cppreference.com提出的确切样本产生:
However, with Visual Studio 2015, the function apparently is not implemented right. The exact sample proposed by cppreference.com produces:
nan("1") = nan (7ff8000000000000)
nan("2") = nan (7ff8000000000000)
这不是我们期望的. VS实现错误吗?如果不是，那么用于生成7ff8000000000001和7ff8000000000002的正确参数是什么?
Which is not what we expect. Is VS implementation wrong? If not, what would be the right arguments to use to produce 7ff8000000000001 and 7ff8000000000002?
推荐答案
std::nan根据定义具有实现定义的行为，因此VS只要返回某种安静的NaN肯定是没有错的.
std::nan has, by definition, implementation-defined behavior, so VS is certainly not wrong as long as it returns some kind of quiet NaN.
实际上，Microsoft文档有这样的说法:
In fact, the Microsoft docs have this to say:
nan函数返回一个浮点值，该值对应于安静的(非信令)NaN. 输入值将被忽略.有关如何用NaN表示输出的信息，请参见printf，_printf_l，wprintf，_wprintf_l.
The
nanfunctions return a floating-point value that corresponds to a quiet (non-signalling) NaN. The input value is ignored. For information about how a NaN is represented for output, see printf, _printf_l, wprintf, _wprintf_l.
https://docs.microsoft.com/zh-CN/cpp/c-runtime-library/reference/nan-nanf-nanl?view=vs-2017
根据一个小测试，这似乎是事实(毫不奇怪):
This appears to be true (unsurprisingly), based on a small test:
std::nan("1") = 7ff8000000000000
std::nan("2") = 7ff8000000000000
std::nan("NAN(1)") = 7ff8000000000000
std::nan("NAN(2)") = 7ff8000000000000
std::nan("NAN1") = 7ff8000000000000
std::nan("NAN2") = 7ff8000000000000
std::nan("NAN 1") = 7ff8000000000000
std::nan("NAN 2") = 7ff8000000000000
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