reinterpret_cast 在 char* 和 std::byte* 之间

Question

我正在阅读 type aliasing rules 但无法确定此代码中是否包含 UB:

std::vector<std::byte> vec = {std::byte{'a'}, std::byte{'b'}};
auto sv = std::string_view(reinterpret_cast<char*>(vec.data()), vec.size());
std::cout << sv << '\n';

我很确定它不会，但我经常对 C++ 感到惊讶。 reinterpret_cast 在 char*、unsigned char* 和 std::byte* 之间总是允许的吗？

此外，添加 const 是否合法，例如：

std::array<char, 2> arr = {'a', 'b'};
auto* p = reinterpret_cast<const std::byte*>(arr.data());

同样，我怀疑它是合法的，因为它说

AliasedType is the (possibly cv-qualified) signed or unsigned variant of DynamicType

但我想用 reinterpret_cast 确保一劳永逸。

Answer 1

代码没问题

标准允许

char* 和 std::byte* 别名任何指针类型。（小心，因为反过来是不正确的）。

([basic.types]/2):

For any object (other than a base-class subobject) of trivially copyable type T, whether or not the object holds a valid value of type T, the underlying bytes ([intro.memory]) making up the object can be copied into an array of char, unsigned char, or std::byte ([cstddef.syn]).⁴³ If the content of that array is copied back into the object, the object shall subsequently hold its original value.

([basic.lval]/8.8):

If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:

a char, unsigned char, or std::byte type.

是的，您可以添加 const。

reinterpret_cast 在 char* 和 std::byte* 之间

reinterpret_cast between char* and std::byte*

c++

language-lawyer

reinterpret-cast