什么是cout&lt; “\\\<br/>”[a == N];做？ [英] what does cout &lt;&lt; &quot;\n&quot;[a==N]; do?

问题描述

在以下示例中：

  cout<<\\\
[a == N] 
  

我不知道 [] 选项在 cout 中，但是当 a 的值等于<$ c时，不打印换行符$ c> N 。

解决方案

[]选项在cout中执行

这实际上不是一个 cout 选项，发生的是\\\
是一个 字符串 。一个字符串字面量具有n const char 类型的数组， [] 只是一个字符数组的索引，在这种情况下包含：

  \\\
\0 
  

注意 \0 附加到所有字符串文字。

code> == 运算符会生成 true 或 false ，因此索引为：

• 0 如果为false，如果 a code> N 导致 \\\



• / code>如果为真，如果 a 等于 N ，则产生 \0



这是相当神秘的，可以用一个简单的 / code>。

为了参考C ++ 14标准（ Lightness确认草稿匹配实际标准）草稿为 2.14.5 中的 N3936 $ c>字符串文字 [lex.string] 说（强调我）：

string literal 具有类型array of n const char，其中n是下面定义的字符串的
大小，并且具有静态存储持续时间
（3.7）。

和：

连接，在翻译阶段7（2.2）中，
'\0'附加到每个字符串文字，以便扫描字符串的程序可以找到它的结尾。

部分 4.5 [conv.prom]


类型bool的prvalue可以转换为int类型的prvalue，
false变为零，true成为一个。

向文本流写入空字符

声称在文本流中写入空字符（ \0 ）是未定义的行为。

因为我可以告诉这是一个合理的结论， cout 是根据C流定义的，我们可以从 27.4.2 [narrow.stream.objects] 其中说：

对象cout控制输出到与对象stdout相关联的流缓冲区，在
（27.9.2）中声明。

部分 p> [...]从文本流读取的数据必然等于之前写入该流的数据
，只有在：数据只包括打印
字符和控件字符水平标签和换行符;

和打印字符< 7.4 字符处理< ctype.h> ：

[... ]术语控制字符
指的是不打印
字符的特定于语言环境的字符集的成员。所有字母和数字都是打印字符。

与脚注 199 说：

在使用7位US ASCII字符集的实现中，打印字符是那些
，其值位于0x20（空格）到0x7E（波形）;控制字符是
值从0（NUL）到0x1F（US）和字符0x7F（DEL）的那些。

，最后我们可以看到没有指定发送空字符的结果，我们可以看到 4 4中的未定义的行为：

[...]未定义的行为否则为
在本国际标准中由未定义的行为或[...]

我们还可以查看 C99 reasone 说：

< blockquote>

需要在文本流I / O中保留的字符集是编写C
程序所需的字符集;意图是标准应该允许C翻译器以最大的
便携式方式编写。控制字符（例如退格键）不需要用于此目的，因此它们在文本流中的
处理不是强制的。

In the following example:

cout<<"\n"[a==N];

I have no clue about what the [] option does in cout, but it does not print a newline when the value of a is equal to N.

解决方案

I have no clue about what the [] option does in cout

This is actually not a cout option, what is happening is that "\n" is a string literal. A string literal has the type array of n const char, the [] is simply an index into an array of characters which in this case contains:

\n\0

note \0 is appended to all string literals.

The == operator results in either true or false, so the index will be:

• 0 if false, if a does not equal N resulting in \n
• 1 if true, if a equals N resulting in \0

This is rather cryptic and could have been replaced with a simple if.

For reference the C++14 standard(Lightness confirmed the draft matches the actual standard) with the closest draft being N3936 in section 2.14.5 String literals [lex.string] says (emphasis mine):

string literal has type "array of n const char", where n is the size of the string as defined below, and has static storage duration (3.7).

and:

After any necessary concatenation, in translation phase 7 (2.2), ’\0’ is appended to every string literal so that programs that scan a string can find its end.

section 4.5 [conv.prom] says:

A prvalue of type bool can be converted to a prvalue of type int, with false becoming zero and true becoming one.

Writing a null character to a text stream

The claim was made that writing a null character(\0) to a text stream is undefined behavior.

As far as I can tell this is a reasonable conclusion, cout is defined in terms of C stream, as we can see from 27.4.2 [narrow.stream.objects] which says:

The object cout controls output to a stream buffer associated with the object stdout, declared in <cstdio> (27.9.2).

and the C11 draft standard in section 7.21.2 Streams says:

[...]Data read in from a text stream will necessarily compare equal to the data that were earlier written out to that stream only if: the data consist only of printing characters and the control characters horizontal tab and new-line;

and printing characters are covered in 7.4 Character handling <ctype.h>:

[...]the term control character refers to a member of a locale-specific set of characters that are not printing characters.199) All letters and digits are printing characters.

with footnote 199 saying:

In an implementation that uses the seven-bit US ASCII character set, the printing characters are those whose values lie from 0x20 (space) through 0x7E (tilde); the control characters are those whose values lie from 0 (NUL) through 0x1F (US), and the character 0x7F (DEL).

and finally we can see that the result of sending a null character is not specified and we can see this is undefined behavior from section 4 Conformance which says:

[...]Undefined behavior is otherwise indicated in this International Standard by the words ‘‘undefined behavior’’ or by the omission of any explicit definition of behavior.[...]

We can also look to the C99 rationale which says:

The set of characters required to be preserved in text stream I/O are those needed for writing C programs; the intent is that the Standard should permit a C translator to be written in a maximally portable fashion. Control characters such as backspace are not required for this purpose, so their handling in text streams is not mandated.

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