Is it bad to depend on index 0 of an empty std::string?

Question

std::string my_string = \"\";
char test = my_string[0];

I\'ve noticed that this doesn\'t crash, and every time I\'ve tested it, test is 0.

Answer 1

@Bartek Banachewicz's answer explains which circumstances allow you to make your assumption. I would like to add that

this is bad programming.

Why? For several reasons:

1. You have to be a language lawyer just to be sure this isn't a bug. I wouldn't know the answer if not for this page, and frankly - I don't think you should really bother to know either.
2. People without the intuition of a string being a null-terminated sequence of characters will have no idea what you're trying to do until they read the standard or ask their friends.
3. Breaks the principle of least surprise in a bad way.
4. Goes against the principle of "writing what you mean", i.e. having the code express problem-domain concepts.
5. Sort-of-a use of a magic number (it's arguable whether 0 actually constitutes a magic number in this case).

Shall I continue? ... I'm almost certain you have an alternative superior in almost every respect. I'll even venture a guess that you've done something else that's "bad" to manipulate yourself into wanting to do this.

Always remember: Other people, who will not be consulting you, will sooner-or-later need to maintain this code. Think of them, not just of yourself, who can figure it out. Plus, in a decade from now, who's to say you're going to remember your own trick? You might be that confounded maintainer...

Answer 2

C++14

No; you can depend on it.

In 21.4.5.2 (or [string.access]) we can find:

Returns: *(begin() + pos) if pos < size(). Otherwise, returns a reference to an object of type charT with value charT(), where modifying the object leads to undefined behavior.

In other words, when pos == size() (which is true when both are 0), the operator will return a reference to a default-constructed character type which you are forbidden to modify.

It is not special-cased for the empty (or 0-sized) strings and works the same for every length.

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C++03

_{And most certainly C++98 as well.}

It depends.

Here's 21.3.4.1 from the official ISO/IEC 14882:

Returns: If pos < size(), returns data()[pos]. Otherwise, if pos == size(), the const version returns charT(). Otherwise, the behavior is undefined.