将re * _cast`从T *到T(*)[N]是不确定的行为吗? [英] Is it undefined behavior to `reinterpret_cast` a `T*` to `T(*)[N]`?

问题描述

请考虑以下情形:

std::array<int, 8> a;
auto p = reinterpret_cast<int(*)[8]>(a.data());
(*p)[0] = 42;

这是未定义的行为吗?我认为是.

• a.data()返回一个int*，它与int(*)[8]

不同

• cppreference上的类型别名规则 似乎表明reinterpret_cast无效

• 作为一名程序员，我知道a.data()指向的内存位置是8 int对象的数组

我缺少使该reinterpret_cast有效的任何规则吗?

解决方案

数组对象及其第一个元素不是指针可互换的 ^* ，因此reinterpret_cast的结果是一个指针键入指向8 int的数组的指针"，其值为指向a[0]的指针" ¹ .换句话说，尽管是类型，它实际上并不指向任何数组对象.

然后，代码将数组到指针的转换应用于取消引用此类指针(作为索引表达式(*p)[0]的一部分) ² 所产生的左值.仅当左值实际引用数组对象 ³ 时，才指定该转换的行为.由于在这种情况下不是左值，因此行为由omiss ⁴ 定义.

---

_{^* 如果问题是为什么数组对象及其第一个元素不能进行指针可互转换?"，则已经被询问:> expr.reinterpret.cast]/7 ， [conv.ptr] /2 ， [expr.static.cast]/13 和 [basic.compound]/4 .}

_{² 请参见 [基本. lval]/6 ， [expr.sub] 和 [expr.add] .}

_{³ [conv.array] :结果是指向数组的第一个元素的指针."}

_{⁴ [defns.undefined] :未定义的行为是本文档不对此要求强加的行为"，包括当本文档省略了行为的任何明确定义时."}

Consider the following scenario:

std::array<int, 8> a;
auto p = reinterpret_cast<int(*)[8]>(a.data());
(*p)[0] = 42;

Is this undefined behavior? I think it is.

• a.data() returns a int*, which is not the same as int(*)[8]

• The type aliasing rules on cppreference seem to suggest that the reinterpret_cast is not valid

• As a programmer, I know that the memory location pointed by a.data() is an array of 8 int objects

Is there any rule I am missing that makes this reinterpret_cast valid?

解决方案

An array object and its first element are not pointer-interconvertible^*, so the result of the reinterpret_cast is a pointer of type "pointer to array of 8 int" whose value is "pointer to a[0]"¹.In other words, despite the type, it does not actually point to any array object.

The code then applies the array-to-pointer conversion to the lvalue that resulted from dereferencing such a pointer (as a part of the indexing expression (*p)[0])². That conversion's behavior is only specified when the lvalue actually refers to an array object³. Since the lvalue in this case does not, the behavior is undefined by omission⁴.

---

_{^*If the question is "why is an array object and its first element not pointer-interconvertible?", it has already been asked: Pointer interconvertibility vs having the same address.}

_{¹See [expr.reinterpret.cast]/7, [conv.ptr]/2, [expr.static.cast]/13 and [basic.compound]/4.}

_{²See [basic.lval]/6, [expr.sub] and [expr.add].}

_{³[conv.array]: "The result is a pointer to the first element of the array."}

_{⁴[defns.undefined]: undefined behavior is "behavior for which this document imposes no requirements", including "when this document omits any explicit definition of behavior".}

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