How to convert string to const unsigned char* without using reinterpret_cast (modern approach)

Question

I have variable input type const std::string&:

const std::string& input

Now I need to convert this to const unsigned char* because this is the input of the function.

Unitl now I have correct code for converting:

reinterpret_cast<const unsigned char*>(input.c_str()) 

This works well, but in clang I got a warning:

do not use reinterpret_cast [cppcoreguidelines-pro-type-reinterpret-cast]

What is the correct way to change a string or const char* to const unsigned char*?

Answer 1

What is the correct way to change a string or const char* to const unsigned char*?

The correct way is to use reinterpret_cast.

If you want to avoid reinterpret_cast, then you must avoid the pointer conversion entirely, which is only possible by solving the XY-problem. Some options:

• You could use std::basic_string<unsigned char> in the first place.
• If you only need an iterator to unsigned char and not necessarily a pointer, then you could use std::ranges::views::transform which uses static cast for each element.
• You could change the function that expects unsigned char* to accept char* instead.

If you cannot change the type of input and do need a unsigned char* and you still must avoid reinterpret cast, then you could create the std::basic_string<unsigned char> from the input using the transform view. But this has potential overhead, so consider whether avoiding reinterpret_cast is worth it.

Answer 2

Edit
Apparently type punning with an union is UB so definitely don't do this.
(Keeping the answer for posterity though!)

---

To strictly answer your question, there's this way:

void foo(const unsigned char* str) {
    std::cout << str << std::endl;
}

int main()
{
    std::string word = "test";
    //foo(word.data()); fails
    union { const char* ccptr; const unsigned char* cucptr; } uword;
    uword.ccptr = word.data();
    foo(uword.cucptr);
}

Is this any better than a reinterpret_cast? Probably not.