取消引用null是否不总是UB? [英] Dereference null is not always UB?

问题描述

我一直以标准要求取消引用null的事实为UB.但是，

I've always known for a fact that the Standard mandates dereferencing null is UB. However,

(链接1)说

p = 0; * p;并不是天生的错误.

p = 0; *p; is not inherently an error.

并提供指向的链接

(链接2)说

* p在p为null时不是错误，除非将左值转换为左值

*p is not an error when p is null unless the lvalue is converted to an lvalue

(我相信这是一个错字，应该将 lvalue转换为 rvalue )

(I believe it's a typo and probably should read lvalue is converted to an rvalue)

链接1也说

char * p = 0; char * q =& *(p)

char* p = 0; char *q = &*(p)

是未定义的"，我只能读为定义明确的或至少是实现定义的

is "not undefined", which I could only read as well-defined or at least implementation-defined

语言律师可以对发生的事情提供权威的解释吗?

Can a language lawyer provide an authoritative explanation of what's going on?

推荐答案

我在

I explored the topic of indirection through null pointers in this answer. In short, it is indeed well-defined per se, just as elaborated in your cited Core issues. The committee used the notion of an empty lvalue as proposed many years ago (but never adopted); *p is supposed to be such an empty lvalue, and unless we attempt to access the (non-existent) memory location behind that lvalue (for example, by performing an lvalue-to-rvalue conversion), all other operations behave as expected. E.g. &*p is equivalent to p, unless p is invalid. (This also works for pointers past-the-end of an array, which is necessary for the common idiom &arr[n]).

我还开始起草用于空左值的纸张(这是WIP，还没有完成针对N4640的重新部署)，因此我们有机会在稍后的阶段中看到更多内容.

I also started drafting a paper for empty lvalues (this is WIP, and the rebase against N4640 isn't complete yet), so there's a chance we'll see more of this at a later stage.

这篇关于取消引用null是否不总是UB?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！