如果没有发生左值到右值转换，是否取消引用无效指针是合法的 [英] Is dereferencing invalid pointers legal if no lvalue-to-rvalue conversion occurs

问题描述

请尝试一下，我所看到的最接近的答案是

Try as I might, the closest answer I've seen is this, with two completely opposing answers(!)

问题很简单，这合法吗?

The question is simple, is this legal?

auto p = reinterpret_cast<int*>(0xbadface);
*p;  // legal?

我对此事

1. 将整数投射到指针:对可以投射什么
2. 间接:仅声明结果为左值.
3. 生命周期:仅说明无法对对象执行的操作，有这里没有对象
4. 表达式语句:*p是一个废弃的值表达式
5. 舍弃的值表达式:不进行左值到右值转换
>
6. 左值的不确定性:也就是严格的别名规则，仅当左值将转换为右值

1. Casting integer to pointer: no restrictions on what may be casted
2. Indirection: only states the result is a lvalue.
3. Lifetimes: only states what can't be done on objects, there is no object here
4. Expression statements: *p is a discarded value expression
5. Discarded value expressions: no lvalue-to-rvalue conversion occurs
6. Undefined-ness of lvalues: aka strict aliasing rule, only if the lvalue is converted to a rvalue

因此，我得出结论，没有明确表示这是未定义的行为.但是我特别记得一些平台在间接指针上捕获了无效的指针.我的推理出了什么问题?

So I conclude there is nothing explicitly saying this is undefined behaviour. Yet I distinctively remember that some platforms trap on indirection for invalid pointers. What went wrong with my reasoning?

推荐答案

[basic.compound] 说:

指针类型的每个值都是以下之一:

Every value of pointer type is one of the following:

• 指向对象或函数的指针(据说该指针指向该对象或函数)，或者
• 超出对象末尾的指针([expr.add])，或
• 该类型的空指针值([conv.ptr])，或
• 无效的指针值.

• a pointer to an object or function (the pointer is said to point to the object or function), or
• a pointer past the end of an object ([expr.add]), or
• the null pointer value ([conv.ptr]) for that type, or
• an invalid pointer value.

通过消除过程，我们可以推断出p是无效的指针值.

By the process of elimination we can deduce that p is an invalid pointer value.

[basic.stc] 说:

通过无效的指针值进行间接并传递无效的值 释放函数的指针值具有未定义的行为.任何 无效指针值的其他用法具有实现定义的 行为.

Indirection through an invalid pointer value and passing an invalid pointer value to a deallocation function have undefined behavior. Any other use of an invalid pointer value has implementation-defined behavior.

据说间接操作符是通过 [expr.]执行间接操作的. unary.op] ，我想说，无论是否使用结果，表达式*p都会导致UB.

As indirection operator is said to perform indirection by [expr.unary.op], I would say, that expression *p causes UB no matter if the result is used or not.

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