Does an empty string contain an empty string in C+

Question

Just had an interesting argument in the comment to one of my questions. My opponent claims that the statement "" does not contain "" is wrong.

My reasoning is that if "" contained another "", that one would also contain "" and so on.

Who is wrong?

P.S.

I am talking about a std::string

P.S. P.S

I was not talking about substrings, but even if I add to my question " as a substring", it still makes no sense. An empty substring is nonsense. If you allow empty substrings to be contained in strings, that means you have an infinity of empty substrings. What is the point of that?

Edit:

Am I the only one that thinks there's something wrong with the function std::string::find?

C++ reference clearly says

Return Value: The position of the first character of the first match.

Ok, let's assume it makes sense for a minute and run this code:

string empty1 = "";
string empty2 = "";

int postition = empty1.find(empty2);

cout << "found \"\" at index " << position << endl;

The output is: found "" at index 0

Nonsense part: how can there be index 0 in a string of length 0? It is nonsense.

To be able to even have a 0th position, the string must be at least 1 character long.

And C++ is giving a exception in this case, which proves my point:

cout << empty2.at( empty1.find(empty2) ) << endl;

If it really contained an empty string it would had no problem printing it out.

Answer 1

Today I had the same question since I'm currently bound to a lousy STL implementation (dating back to the pre-C++98 era) that differs from C++98 and all following standards:

TEST_ASSERT(std::string().find(std::string()) == string::npos); // WRONG!!! (non-standard)

This is especially bad if you try to write portable code because it's so hard to prove that no feature depends on that behaviour. Sadly in my case that's actually true: it does string processing to shorten phone numbers input depending on a subscriber line spec.

On Cppreference, I see in std::basic_string::find an explicit description about empty strings that I think matches exactly the case in question:

an empty substring is found at pos if and only if pos <= size()

The referred pos defines the position where to start the search, it defaults to 0 (the beginning).

A standard-compliant C++ Standard Library will pass the following tests:

TEST_ASSERT(std::string().find(std::string()) == 0);
TEST_ASSERT(std::string().substr(0, 0).empty());
TEST_ASSERT(std::string().substr().empty());

This interpretation of "contain" answers the question with yes.