为什么在C/C ++中交织switch/for/if语句是有效的? [英] Why it is valid to intertwine switch/for/if statements in C/C++?

问题描述

我正在阅读 boost/asio/coroutine.hpp ，无法理解BOOST_ASIO_CORO_REENTER和BOOST_ASIO_CORO_YIELD的实现.

I'm reading boost/asio/coroutine.hpp and cannot understand the implementation of BOOST_ASIO_CORO_REENTER and BOOST_ASIO_CORO_YIELD. The expanded form of

reenter (this) {
  yield ..
  yield ..
}

似乎是交织在一起的switch/if/for语句.我想知道为什么这是有效的C代码?我写了类似的东西(如下所示)，发现它可以使用gcc进行编译.

seems to be intertwined switch/if/for statements. I'm wondering how come this is valid C code? I wrote something similar (shown below) and found it compiles using gcc.

int main() {
  int a = 1;
  switch (a)
  case 0: if (1) a = 2;
  else case 1: for (;;) {
    case 3:
      break;
  }

  return 0;
}

推荐答案

从语法上讲，开关的主体只是一个语句(通常，但不一定是复合语句)

Syntactically, the body of a switch is just a statement (usually, but not necessarily a compound statement)

6.8 :

statement:
               labeled-statement
               compound-statement
               expression-statement
               selection-statement
               iteration-statement
               jump-statement

其中可能带有标签 6.8.1 :

labeled-statement:
                identifier : statement
                case constant-expression : statement
                default : statement

示例:

switch(1) one: case 1: dothis();

如果它是复合语句，则每个子语句都可以递归标记.示例:

If it is a compound statement, then each substatement recursively may also be labeled. Example:

switch(x) {
    if(1) one: case 1: dothis();
    else case 0: orthis();  /*fallthru*/
    three: case 3: three();
}

语法将 case / default -标签和常规标签视为相同，只有语义检查才能验证 case / default-标签在 switch 内.

The syntax treats case/default-labels and regular labels the same, only the semantic check verifies that case/default-labels be inside a switch.

在实施方面，所有内容都将编译为(扁平)汇编.

Implementation-wise, everything compiles into (flat) assembly.

例如

if(test) YesBranch; else ElseBranch;

展平为(伪程序集)

IF_NOT_THEN_GOTO(test, PAST_YES_BRANCH)
YesBranch
goto PAST_NO_BRANCH;
NoBranch
PAST_NO_BRANCH:;

而且没有理由为什么这样的平面代码中的任何内容都无法标记.

and there's no reason why anything in such flat code couldn't be labeled.

case / default 标签也与常规标签一样，除了(通常)在计算的跳转中使用它们之外.

case/default labels are also just like regular labels except they're also used in (most usually) a computed jump.

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