cpp, "\0" == NULL returns false? How to figure it is null ptr or not?

Question

I am getting these results. What am I doing wrong?

const char *c = "\0";
cout << (c == NULL); // false
cout << (c == nullptr); //false

Answer 1

All literal strings in C++ are really constant arrays of characters, including the null-terminator character.

So doing e.g.

const char* c = "\0";

is somewhat equivalent to

char const internal_string_array[] = { '\0', '\0' };
const char* c = &internal_string_array[0];

So a pointer to a literal string will never be a null pointer.

Also there's a difference between a null pointer and the string null terminator character. A null pointer is a pointer which have been initialized to point to nowhere. A string null terminator character is the character '\0'. These two are different things.

---

If you want to check to see if a C-style string (using pointers or arrays) is empty, then you first need to make sure it's not a null-pointer (if you're using pointers) and then check to see if the first character is the string null terminator character:

if (c == nullptr)
{
    // No string at all!
}
else if (c[0] == '\0')
{
    // String exists, but is empty
}
else
{
    // String exists, and is not empty
}

For std::string objects (which you should use for almost all your strings) then you don't need the null-pointer check:

if (str.empty())
{
    // String is empty
}
else
{
    // String is not empty
}

Also note that even if a string is not empty, it might contain unprintable characters (like the ASCII control characters), or characters that are "invisible" (like for example space or tab). So if printing the non-empty string it might not show anything.