c++ - cout << "\n"[a==N] 是什么意思？做？

Question

在以下示例中:

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

我不知道 [] 选项在 cout 中的作用，但是当 a 的值时它不会打印换行符等于 N。

Answer 1

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

这实际上不是 cout 选项，发生的是 "\n" 是 string literal .字符串文字的类型为 array of n const char，[] 只是一个字符数组的索引，在这种情况下包含:

\n\0

注意 \0 被附加到所有字符串字面量。

== 运算符导致 true 或 false，因此索引将为:

• 0 如果为 false，如果 a 不等于 N 导致 \n
• 1 如果为真，如果 a 等于 N 导致 \0

这是相当神秘的，可以用一个简单的 if 代替。

引用 C++14 标准(Lightness 确认草案与实际标准匹配)，最接近的草案是 N3936在 2.14.5 部分字符串文字 [lex.string] 说(强调我的):

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).

和:

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.

第 4.5 节 [conv.prom] 说:

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

将空字符写入文本流

声称将空字符 (\0) 写入文本流是未定义的行为。

据我所知，这是一个合理的结论，cout 是根据 C 流定义的，从 27.4.2 [ narrow.stream.objects] 说:

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

7.21.2 Streams 节中的 C11 草案标准说:

[...]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;

和打印字符在7.4中有介绍 字符处理:

[...]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.

脚注 199 说:

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).

最后我们可以看到发送空字符的结果没有被指定，我们可以看到这是未定义的行为，来自 4 部分的一致性，它说:

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

我们也可以查看C99 rationale其中说:

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.