为什么我需要一个reinterpret_cast将Fred ** const转换为void ** const？ [英] Why do I need a reinterpret_cast to convert Fred ** const to void ** const?
问题描述
我有一个指向 Fred 的指针的const指针，我不明白为什么 static_cast isn足够。
I have a const pointer to a pointer to a Fred and I don't understand why a static_cast isn't sufficient.
typedef struct {
int n;
} Fred;
Fred *pFred;
Fred **const ppFred = &pFred;
void **const ppVoid = static_cast<void ** const>(ppFred);
请有人解释为什么 reinterpret_cast 需要将指向 Fred * 的指针转换为指向 void * 但 static_cast 可以将 Fred 的指针转换为指向 void 的指针。
Please could someone explain why a reinterpret_cast is needed to convert a pointer to Fred*to a pointer to void* but static_cast is fine to convert pointer to Fred to a pointer to void.
推荐答案
没有要求 Fred * 和 void * / code>具有相同的大小
和表示。 （我在他们没有的机器上工作，
虽然是在我的C ++日之前。）当你转换 Fred * 到
void * ，你会得到一个新的指针，可能有不同的大小和
表示，但没有关于对象大小和
表示的信息 void * 指向的。你知道它是
未知，并且使用这个 void * 的唯一方法是将它回复到
Fred * （模数类似cv-qualifiers）。当您将 Fred **
转换为 void ** 时，将指针转换为具体类型（指向 Fred * 的
指针）指向另一个具体类型的指针（指针
到 void * ）。因为不能保证这两个具体的
类型具有相同的大小和表示，所以转换需要
reinterpret_cast 。 void 是一个特殊的，非具体的类型，所以你可以
static_cast 并从指向 void 的指针。
void * 只是另一个具体的指针类型，所以转换和从
指针它遵循通常的规则（并且需要一个
reinterpret_cast ）。
There's no requirement that a Fred* and a void* have the same size
and representation. (I've worked on machines where they didn't,
although that was before my C++ days.) When you convert Fred* to
void*, you get a new pointer, with a possibly different size and
representation, but there is no information about the size and
representation of the object the void* points to. You know that it is
unknown, and the only way to use this void* is to cast it back to a
Fred* (modulo things like cv-qualifiers). When you convert Fred**
to void**, you're converting from a pointer to a concrete type (a
pointer to a Fred*) to a pointer to another concrete type (a pointer
to a void*). And since there's no guarantee that these two concrete
types have the same size and representation, the conversion requires a
reinterpret_cast. void is a special, non-concrete type, so you can
static_cast a pointer to any type to and from a pointer to void.
void* is just another concrete pointer type, so casting to and from
pointers to it follows the usual rules (and requires a
reinterpret_cast).
在许多方面，情况非常类似于 int 和 double ，其中
void * 起 int （say）和 Fred * 角色 double 。
没有问题 static_cast 在 int 和之间，但
在 int * 和之间强制转换 require reinterpret_cast 。
In many ways, the situation is very much like int and double, where
void* plays the role of int (say), and Fred* the role of double.
There's no problem static_casting between int and double, but
casts between int* and double* require reinterpret_cast.
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