c++ - "undefined behaviour"是否扩展到编译时？

Question

我们都听过这样的警告:如果您在 C 或 C++ 中调用未定义的行为，任何事情都可能发生。

这是否仅限于任何运行时行为，还是还包括任何编译时行为？特别是，编译器在遇到调用未定义行为的构造时是否允许拒绝代码(在标准中没有其他要求的情况下)，甚至崩溃？

Answer 1

"You're all ignoring the actual definition and focusing on the note, The standard imposes no requirements." - @R.MartinhoFernandes

^{上面的消息是由给定用户在 Lounge<C++> 中编写的 并提出了一个非常有效的论点；当涉及调用未定义行为的代码时，该标准不会强加任何要求。}

<小时/>

! ! !

未定义行为甚至延伸到编译器解析输入数据(即代码)的最远角落，正如以下来自C++11<的引用所验证的那样/em> 和 C99 标准。

用一句话回答你的问题；

• 未定义的行为不仅限于运行时执行，并且允许在编译期间崩溃“以环境特征的记录方式” ¹

<小时/>

^{“以环境特征的记录方式”是一种奇怪的陈述，您几乎可以编写一个编译器来记录它可能会在任何给定代码上崩溃(这是无效的)以赋予它崩溃的可能性。}

^{1.引用C++11/C99标准}

<小时/>

### c++11

###1.3.24 [defns.undefined]

Undefined behavior; behavior for which this International Standard imposes no requirements

[ Note:

Undefined behavior may be expected when this International Standard omits any explicit definition of behavior or when a program uses an erroneous construct or erroneous data.

Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).

Many erroneous program constructs do not engender undefined behavior; they are required to be diagnosed.

— end note ]

<小时/>

### c99

3.4.3 - Undefined Behavior

1. behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this >International Standard imposes no requirements

2. NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).