NaN情况下to_string（）可以返回什么 [英] what can to_string() return in NaN cases

问题描述

我正在使用VS 2012，遇到了非常令人讨厌的行为（有时我的浮动对象是NaN）：

Im using VS 2012 and I encountered really irritating behaviour(sometimes my float is NaN):

auto dbgHelp = std::to_string(myFloat);

dbgHelp 最终包含 5008 charachters（您无法发明这种东西）
其中大多数为0，
最终以：
0.＃INF00

dbgHelp ends up containing 5008 charachters ( you cant invent this stuff up) most of them 0, it ends up with : 0.#INF00

那么这是否是一个bug或标准，允许像这样的惯用返回值？

So is this a bug or standard allows idiotic return values like this ?

修复很简单，对我来说，用三元运算符和 isnan（）
，但我想知道标准是否为此特定...

fix is trivial, for me to manualy do the proper thig with ternary operator and isnan() but I wonder if standard is specific regarding this...

推荐答案

std :: to_string 重载在[string.conversions] / 7中指定：

The std::to_string overloads are specified in [string.conversions]/7:

string to_string(int val);
string to_string(unsigned val);
string to_string(long val);
string to_string(unsigned long val);
string to_string(long long val);
string to_string(unsigned long long val);
string to_string(float val);
string to_string(double val);
string to_string(long double val);

⁷ 返回：每个函数返回一个 string 对象，包含通过调用 sprintf（buf，fmt，val）的格式说明符为％d ，％u ， ％ld ，％lu ，％lld ，％llu ，％f ，％f 或％Lf ，其中 buf 表示足够大的内部字符缓冲区。

⁷ Returns: Each function returns a string object holding the character representation of the value of its argument that would be generated by calling sprintf(buf, fmt, val) with a format specifier of "%d", "%u", "%ld", "%lu", "%lld", "%llu", "%f", "%f", or "%Lf", respectively, where buf designates an internal character buffer of sufficient size.

sprintf 在C99中进行了详细说明（实际上是N1256）7.19.6.6/2：

sprintf is detailed in C99 (N1256 actually) 7.19.6.6/2:

sprintf 函数等效于 fprintf ，除了将输出写到
数组（由参数 s 指定）而不是流中。在写入的字符末尾将空字符写入
；它不算作返回值的一部分。如果在重叠的对象之间进行
复制，则行为是不确定的。

The sprintf function is equivalent to fprintf, except that the output is written into an array (specified by the argument s) rather than to a stream. A null character is written at the end of the characters written; it is not counted as part of the returned value. If copying takes place between objects that overlap, the behavior is undefined.

对于<$，跳转到7.19.6.1 c $ c> fprintf ，格式标志在第8段中进行了说明。具体地说，对于 f 格式：

Jumping over to 7.19.6.1 for fprintf, the format flags are described in paragraph 8. Specifically, for the f format:

一个 double 表示浮点数的参数将转换为
十进制表示形式 [−] ddd.ddd ，其中
后小数点字符的位数等于精度规格。如果缺少
精度，则该精度为6；如果精度为零，并且未指定＃标志为
，则不显示小数点字符。如果出现小数点的
字符，则在其前面至少出现一位数字。该值将四舍五入到
的适当位数。

A double argument representing a floating-point number is converted to decimal notation in the style [−]ddd.ddd, where the number of digits after the decimal-point character is equal to the precision specification. If the precision is missing, it is taken as 6; if the precision is zero and the # flag is not specified, no decimal-point character appears. If a decimal-point character appears, at least one digit appears before it. The value is rounded to the appropriate number of digits.

A double 表示无穷大的参数是转换为以下一种样式：
[-] inf 或 [-] infinity —哪种样式实现-定义。表示NaN的
双重参数转换为
[-] nan 或 [-] nan（ n-char-sequence ） —哪种样式以及
的含义 n -char-sequence 是实现定义的。 F 转换说明符
产生 INF ， INFINITY 或 NAN 代替 inf ， infinity 或 nan ，分别是
。

A double argument representing an infinity is converted in one of the styles [-]inf or [-]infinity — which style is implementation-defined. A double argument representing a NaN is converted in one of the styles [-]nan or [-]nan(n-char-sequence) — which style, and the meaning of any n-char-sequence, is implementation-defined. The F conversion specifier produces INF, INFINITY, or NAN instead of inf, infinity, or nan, respectively.

将所有内容放在一起时 float foo 是正无穷大， std :: to_string（foo）应该返回 std :: string（ inf）。

Putting it all together, when float foo is positive infinity, std::to_string(foo) should return std::string("inf").

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