C++ style cast from unsigned char * to const char *

Question

I have:

unsigned char *foo();
std::string str;
str.append(static_cast<const char*>(foo()));

The error: invalid static_cast from type 'unsigned char*' to type 'const char*'

What's the correct way to cast here in C++ style?

Answer 1

char * and const unsigned char * are considered unrelated types. So you want to use reinterpret_cast .

But if you were going from const unsigned char* to a non const type you'd need to use const_cast first. reinterpret_cast cannot cast away a const or volatile qualification.

Answer 2

Try reinterpret_cast

unsigned char *foo();
std::string str;
str.append(reinterpret_cast<const char*>(foo()));

Answer 3

reinterpret_cast

Answer 4

unsigned char* is basically a byte array and should be used to represent raw data rather than a string generally. A unicode string would be represented as wchar_t*

According to the C++ standard a reinterpret_cast between unsigned char* and char* is safe as they are the same size and have the same construction and constraints. I try to avoid reintrepret_cast even more so than const_cast in general.

If static cast fails with what you are doing you may want to reconsider your design because frankly if you are using C++ you may want to take advantage of what the "plus plus" part offers and use string classes and STL (aka std::basic_string might work better for you)

Answer 5

You would need to use a reinterpret_cast<> as the two types you are casting between are unrelated to each other.

Answer 6

Too many comments to make to different answers, so I'll leave another answer here.

You can and should use reinterpret_cast<> , in your case

str.append(reinterpret_cast<const char*>(foo()));

because, while these two are different types, the 2014 standard, chapter 3.9.1 Fundamental types [basic.fundamental] says there is a relationship between them:

Plain char , signed char and unsigned char are three distinct types, collectively called narrow character types. A char , a signed char , and an unsigned char occupy the same amount of storage and have the same alignment requirements (3.11); that is, they have the same object representation.

(selection is mine)

Here's an available link: https://en.cppreference.com/w/cpp/language/types#Character_types

Using wchar_t for Unicode/multibyte strings is outdated: Should I use wchar_t when using UTF-8?

Answer 7

Hope it help. :)

const unsigned attribName = getname();
const unsigned attribVal = getvalue();
const char *attrName=NULL, *attrVal=NULL;
attrName = (const char*) attribName;
attrVal = (const char*) attribVal;