Why I can't static_cast char* to std::byte*?

Question

I know I can use reinterpret_cast, but it seems weird that I can not go from char to "universal" type like std::byte. Is this just unfortunate mistake/limitation, or there is a reason for this?

Example:

int main(){
    std::string s{"abc"};
    std::byte* ptr  = static_cast<std::byte*>(s.data());
}

Answer 1

Static cast only works between:

1. Numerical types
2. Possibly related class type pointers/referrnces (up and down).
3. Pointers to/fom void pointers.
4. Activating conversion constructors/operators

That is it.

Reinterpreting things as bytes is a reinterpret cast.

Answer 2

In C++20 where std::span was added there are now as_bytes and as_writable_bytes that converts a span<T> to span<byte>. Internally it uses reinterpret_cast, but it makes the code a bit more readable/safe.

https://godbolt.org/z/zY9KPaz9K

#include <string>
#include <span>

void foo(){
    std::string s{"abc"};
    std::span<std::byte> t{std::as_writable_bytes(std::span<char>(s))};
}