NaN到布尔的转换:对还是错? [英] NaN to Bool conversion: True or False?

问题描述

C ++规范或IEEE浮动规范的哪一部分规定NaN值应转换为true而不是false?

What part of the C++ spec, or the IEEE float spec, states that a NaN value should convert to true as opposed to false?

如果我查看C ++标准部分 4.12布尔转换，它会说:

If I look at the C++ standard section 4.12 Boolean Conversions it says:

零值，空指针值或空成员指针值是 转换为false；其他任何值都将转换为true.

A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true.

现在，IEEE浮言说NaN将false与其他任何值进行比较.因此，NaN是正确还是错误取决于您进行比较的方式(如下).因此，我认为必须明确提及.

Now IEEE floats say that NaN compares false to any other value. So whether NaN is true or false depends on how you do your comparision (below). Thus I presume there must be an explicit mention.

value == 0 ? false : true
value != 0 ? true : false

现在，如何转换为整数呢?下面的简短程序显示，将变量NAN转换为整数会生成最小整数，而将常量转换为0(使用GCC).这似乎很奇怪.

Now, what about conversion to an integer. The short program below shows that a variable NAN converted to an integer results in the minimum integer, whereas a constant gets converted to 0 (using GCC). This seems odd.

#include <iostream>
#include <cmath>

void write( double r, int i, bool b )
{
    std::cout << r << " == " <<  i << " == " << (b ? "True" : "False") << std::endl;
}

int main()
{
    double value = NAN;
    write( value, value, value );
    write( NAN, NAN, NAN );
}

输出:

nan == -2147483648 == True
nan == 0 == True

将NaN转换为零，但将布尔值转换为True似乎很麻烦.我也不是说MatLab之类的东西会使用int16之类的函数将NaN转换为0.

The conversion of a NaN to zero but bool conversion as True seems troubling. I also not that something like MatLab will convert a NaN to 0 using a function like int16.

那么，说明NaN如何转换为布尔值和整数值的相关标准的细节是什么?

So, what are the specifics of the relevant standards that state how NaN converts to boolean and integer values?

我也在标记C，因为尽管它可能未定义布尔转换，但它可能定义了整数转换并在条件条件下使用，我怀疑C ++会遵循相同的规则

推荐答案

在C和C ++中，将NAN转换为整数类型(bool以外)时，行为均未定义:

In both C and C++, the behaviour is undefined when converting NAN to an integer type (other than bool):

C99 6.3.1.4/1:当将实数浮点型的有限值转换为_Bool以外的整数类型时，小数部分将被丢弃(即，该值将被截断为零).如果整数部分的值不能用整数类型表示，则行为未定义.

C99 6.3.1.4/1: When a finite value of real floating type is converted to an integer type other than _Bool, the fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part cannot be represented by the integer type, the behavior is undefined.

C ++ 11 4.9/1:浮点类型的prvalue可以转换为整数类型的prvalue.转换被截断；即，小数部分被丢弃.如果无法在目标类型中表示截断的值，则行为未定义. [注意:如果目标类型是bool，请参见4.12. -尾注]

C++11 4.9/1: A prvalue of a floating point type can be converted to a prvalue of an integer type. The conversion truncates; that is, the fractional part is discarded. The behavior is undefined if the truncated value cannot be represented in the destination type. [ Note: If the destination type is bool, see 4.12. —end note ]

在两种语言中，将NAN转换为bool(或_Bool)都会得到true(或1):

In both languages, converting NAN to bool (or _Bool) gives true (or 1):

C99 6.3.1.2/1:将任何标量值转换为_Bool时，如果该值比较等于0，则结果为0；否则，结果为0.否则，结果为1.

C99 6.3.1.2/1: When any scalar value is converted to _Bool, the result is 0 if the value compares equal to 0; otherwise, the result is 1.

C ++ 11 4.12/1:将零值，空指针值或空成员指针值转换为false；其他任何值都将转换为true.

C++11 4.12/1: A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true.

NAN不是零值，并且不等于零.

NAN is not a zero value, and doesn't compare equal to zero.

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