为什么我可以用多个字符串文字构造一个字符串？ [英] Why can I construct a string with multiple string literals?

问题描述

#include <iostream>
#include <string>

int main() {
    std::string str = "hello " "world" "!";
    std::cout << str;
}

以下编译，运行和打印：

The following compiles, runs, and prints:

hello world！

hello world!

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好像字符串文字被串联在一起，但是有趣的是，这不能用 operator + 来完成：

It seems as though the string literals are being concatenated together, but interestingly this can not be done with operator +:

#include <iostream>
#include <string>

int main() {
    std::string str = "hello " + "world";
    std::cout << str;
}

这将无法编译。

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为什么这种行为是该语言的？我的理论是，它允许使用多个 #include 语句构造字符串，因为必须使用 #include 语句自己行。

Why is this behavior in the language? My theory is that it is allows strings to be constructed with multiple #include statements because #include statements are required to be on their own line. Is this behavior simply possible due to the grammar of the language, or is it an exception that was added to help solve a problem?

推荐答案

相邻的字符串文字已连接在一起，我们可以在 C ++标准草稿部分 2.2 翻译阶段第 6 段，其中：

Adjacent string literals are concatenated we can see this in the draft C++ standard section 2.2 Phases of translation paragraph 6 which says:

相邻字符串文字标记已连接

Adjacent string literal tokens are concatenated

在其他情况下，没有定义 operator + 来定义两个* const char **。

In your other case, there is no operator+ defined to take two *const char**.

为什么，这来自 C ，我们可以转到国际标准理论（编程语言）C 并在 6.4.5 字符串文字部分中说：

As to why, this comes from C and we can go to the Rationale for International Standard—Programming Languages—C and it says in section 6.4.5 String literals:

可以使用反斜杠–换行符连续行来跨多行连续一个字符串，但这要求字符串的连续性从下一行的第一个位置开始。为了允许更灵活的布局并解决一些预处理问题（请参见第6.10.3节），C89委员会引入了字符串文字串联。连续将两个字符串文字粘贴在一起，中间没有空字符，以形成一个组合的字符串文字。 C语言的这一新增功能使程序员可以将字符串文字扩展到物理行的末尾，而不必使用反斜杠-换行符机制，从而破坏了程序的缩进方案。没有引入显式的串联运算符，因为串联是一个词法构造，而不是运行时操作。

A string can be continued across multiple lines by using the backslash–newline line continuation, but this requires that the continuation of the string start in the first position of the next line. To permit more flexible layout, and to solve some preprocessing problems (see §6.10.3), the C89 Committee introduced string literal concatenation. Two string literals in a row are pasted together, with no null character in the middle, to make one combined string literal. This addition to the C language allows a programmer to extend a string literal beyond the end of a physical line without having to use the backslash–newline mechanism and thereby destroying the indentation scheme of the program. An explicit concatenation operator was not introduced because the concatenation is a lexical construct rather than a run-time operation.

如果没有此功能，您会必须执行以下操作才能在多行上继续字符串文字：

without this feature you would have to do this to continue a string literal over multiple lines:

   std::string str = "hello \
world\
!";

这很丑。

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