Character array initialization and trailing null

Question

When I declare a char array of size 2 and assign data to it when is there a trailing '\0' (null character)?

I know that:

char data[2] = {'a', 'b'}; // array holds 'a', 'b'

I also know that

char data[] = "ab"; // array holds 'a', 'b', '\0'

However, I'm wondering what this does?

char data[2] = "ab"; // Is there a trailing '\0'?

I always thought that this was an error however looking at cppreference it says:

If the size of the array is known, it may be one less than the size of the string literal, in which case the terminating null character is ignored:

char str[3] = "abc"; // str has type char[3] and holds 'a', 'b', 'c'

So what does "may" mean? Is it implementation dependent?

Answer 1

No, in C++ you must always make room for the NUL terminator:

char str[3] = "abc";

is required to issue a diagnostic.

In C, you can substitute "it is allowed to be" for may, and indeed

char str[3] = "abc";

is permitted, with the NUL terminator not being copied.

This is one of a number of important differences between C and C++.