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?
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 aint*
, which is not the same asint(*)[8]
The type aliasing rules on cppreference seem to suggest that the
reinterpret_cast
is not validAs a programmer, I know that the memory location pointed by
a.data()
is an array of8
int
objects
Is there any rule I am missing that makes this reinterpret_cast
valid?
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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