When is null character being used and what does it do?

Question

The title almost says it all but: 1. I am reading a tutorial about character sequences and encountered a section that I do not really understand. They initialize an array of characters with a null character at the end like so:

char myword[] = { 'H', 'e', 'l', 'l', 'o', '\0' };

However, they do not really explain anything further about the null character(I have also tried to use google but sadly I did not find the answer I want). The tutorial says

"By convention, the end of strings represented in character sequences is signaled by a special character: the null character"

which to me means "Most of the times but not every time, a null character is used." So my first question is - When is the null character being used and when do you know if you have to input it explicitly?

2.. I have this code:

char foo[22] = { 'H', 'e', 'l', 'l', 'o' };
cout << foo << endl;

Where the output is: Hello

But this is without me adding the null character and in the tutorial, they initialize the the array with a null character at the end as I showed previously even though that the output of both examples is the exact same?? :/

So my second question is - What is it actually that the null character do?

Thanks, your time is much appreciated!

Answer 1

'\0' is the null termination character. Without this you won't be able to know when the strings end. In your example

char foo[22] = { 'H', 'e', 'l', 'l', 'o' };

Is the same as

char foo[22] = { 'H', 'e', 'l', 'l', 'o', '\0', '\0', ..., '\0' };

With 17 times '\0'. When you Output the string you could have something like:

auto i = 0
do
{
    // Do smoething with foo[i++]
}
while('\0' != foo[i]);

Answer 2

When is the null character being used

Whenever a null terminated character string is needed. When do I need a null terminated character string, you might ask. Well, whenever you call a function that requires a null terminated character string. Whether a function requires a null terminated character string or not can be found from the documentation of the function. Examples of such functions are: std::strlen(const char*), operator<<(std::ostream&, const char*) and countless others.

when do you know if you have to input it explicitly

Whenever you need a null terminated character string and you either know that your character array would not otherwise be null terminated or you don't know whether it would be.

even though that the output of both examples is the exact same??

If you were to insert into an output stream a pointer to a character array that is not null terminated, the behaviour of the program would be undefined.

However, your character array contains nulls after the characters that you initialized explicitly. This is because value initialization sets characters without an explicit initializer to null.

What is it actually that the null character do?

It designates the end of a null terminated character string.

---

aren't all character sequences a null terminated character sequence then, since they all need that null character to indicate the end

If a character sequence ends in a null terminating character, then it is null terminated by definition. You can use such sequence as an argument to standard functions like std::strlen.

But no: A character sequence doesn't necessarily contain a null character. A character sequence that doesn't contain a null character is not null terminated by definition, and must not be used as an argument to functions that expect a null terminated string. An example:

char non_terminated[] = {'h', 'i', '!'};
auto len = std::strlen(non_terminated); // oops, UB!