为什么允许未定义的行为(而不是不编译/崩溃)? [英] Why is undefined behavior allowed (as opposed to not compiling/crashing)?

问题描述

我了解编译器/解释器语言扩展的原因，但是为什么没有有效定义的行为却被允许静默失败/做一些奇怪的事情而不是引发编译器错误?是因为编译器捕获它们的额外困难(不可能或仅是耗时)?

I understand the reasons for compiler/interpreter language extensions but why is behaviour that has no valid definition allowed to fail silently/do weird things rather then throwing a compiler error? Is it because of the extra difficulty(impossible or simply time consuming) for the compiler to catch them)?

P.S.哪些语言具有未定义的行为，哪些没有?

P.S. what languages have undefined behaviour and which don't?

P.P.S.是否存在未定义行为的实例，这些实例并非不可能/花费太长时间才能被编译捕获，如果是，是否存在一些充分的理由/借口.

P.P.S. Are there instances of undefined behaviour which is not impossible/takes too long to catch in compilation and if so are there any good reasons/excuses for those.

推荐答案

未定义行为的概念在诸如C和C ++之类的语言中是必需的，因为检测导致这种情况的条件将是不可能的，或者是代价高昂的.以下面的代码为例:

The concept of undefined behaviour is required in languages like C and C++, because detecting the conditions that cause it would be impossible or prohibitively expensive. Take for example this code:

int * p = new int(0);
// lots of conditional code, somewhere in which we do
int * q = p;
// lots more conditional code, somewhere in which we do
delete p;
// even more conditional code, somewhere in which we do
delete q;

此处指针已被删除两次，从而导致未定义的行为.对于像C或C ++这样的语言，很难检测到错误.

Here the pointer has been deleted twice, resulting in undefind behaviour. Detecting the error is too hard to do for a language like C or C++.

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