reinterpret_cast在大多数情况下是没有用的吗? [英] Is reinterpret_cast mostly useless?

问题描述

我以前已经阅读过有关使用reinterpret_cast 的各种问题，而且我还阅读了C ++标准中的相关措辞.从本质上讲，归结为指针到指针reinterpret_cast操作的结果不能安全地用于任何其他操作，除非将其强制转换回原始指针类型.

但是，在实践中，reinterpret_cast的大多数实际用法似乎是基于(错误)的假设，即reinterpret_cast与C样式强制转换相同.例如，我已经看到很多代码，出于字符集转换例程的目的，这些代码使用reinterpret_cast从char*强制转换为unsigned char*.这是完全无害的，但严格来说，它不是可移植的-无法保证当尝试取消引用unsigned char*指针时，从char*到unsigned char*的reinterpret_cast不会使程序崩溃.

根据标准，的唯一实际用途似乎有任何真正的保证，是从指针转换为整数，反之亦然.

但是在许多情况下，我们会想要(并且应该能够)在不同的指针类型之间安全地进行转换.例如:uint16_t*到新的C ++ 0x char16_t*，或者实际上是指向基本数据类型的任何指针，该指针的大小/对齐方式与原始类型相同.但是reinterpret_cast不提供任何保证.

问题:我们如何安全地在指向相同大小/对齐方式的基本数据类型的指针之间进行转换，例如char*-> unsigned char*?由于reinterpret_cast似乎不能保证这确实有效，因此C样式强制转换是这里唯一安全的选择吗?

解决方案

当尝试取消引用未签名的char *指针时，无法保证从char *到unsigned char *的reinterpret_cast不会使程序崩溃.

您无法以其他任何方式进行此类转换，因此必须不得不信任编译器使用此完全合理的转换所做的工作.

由于reinterpret_cast似乎不能保证此方法确实有效，因此C样式强制转换是这里唯一安全的选择吗?

C样式强制转换将仅映射到reinterpret_cast，因此它将完全相同.在某些时候，您必须信任您的编译器.该标准有一个限制，它只能说不.请阅读您的编译器手册".当涉及到交叉转换指针时，这就是这一点.它允许您使用unsigned char左值读取char.不能将char*强制转换为可用的unsigned char*的编译器几乎无法使用，并且由于该原因不存在.

I've read various previous questions about the use of reinterpret_cast, and I've also read the relevant wording in the C++ standard. Essentially, what it comes down to is that the result of a pointer-to-pointer reinterpret_cast operation can't safely be used for anything other than being cast back to the original pointer type.

In practice, however, most real-world uses of reinterpret_cast seem to be based on the (wrong) assumption that a reinterpret_cast is the same as a C-style cast. For example, I've seen lots of code which uses reinterpret_cast to cast from char* to unsigned char* for the purpose of character set conversion routines. This is completely harmless, yet strictly speaking it's not portable - there's no guarantee that a reinterpret_cast from char* to unsigned char* won't crash your program when you try to dereference the unsigned char* pointer.

It's seems the only other real use of reinterpret_cast that has any real guarantees, according to the standard, is converting from pointer to integer, and vice-versa.

And yet there are many cases where we'd want (and should be able to) safely convert between different pointer types. For example: uint16_t* to the new C++0x char16_t*, or really any pointer to a basic data type that is the same size/alignment as the original type. Yet reinterpret_cast provides no guarantees this should work.

Question: How can we safely convert between pointers to basic data-types of the same size/alignment, such as char* --> unsigned char*? Since reinterpret_cast doesn't seem to guarantee this actually works, are C-style casts the only safe option here?

解决方案

there's no guarantee that a reinterpret_cast from char* to unsigned char* won't crash your program when you try to dereference the unsigned char* pointer.

You can't do such a cast in any other way, so you have to have to trust what your compiler does with this completely reasonable cast.

Since reinterpret_cast doesn't seem to guarantee this actually works, are C-style casts the only safe option here?

The C-style cast will just map to reinterpret_cast so it will be exactly the same. At some point you have to trust your compiler. The Standard has a limit on which it simply says "no. read your compiler's manual". When it comes to cross-casting pointers, this is such a point. It allows you to read a char using an unsigned char lvalue. A compiler that cannot cast a char* to a usable unsigned char* to do such is just about unusable and doesn't exist for that reason.

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