c++: Is the behavior of string::find for empty input string well defined

Question

The following code snippet returns always true on my compiler(visual studio). But is this behavior well defined and portable?

bool return_always_true(std::string const& str)
{
    return str.find("") != std::string::npos;
}

int main(){
     cout << boolapha << return_always_true("") << endl
          << return_always_true("oxylottl") << endl
          << return_always_true("lorem ipsum") << endl;
 //true true true
}

Answer 1

I find cppreference.com easier to read than the standard. Quoting them:

Finds the first substring equal to str ...

Formally, a substring str is said to be found at position xpos if all of the following is true:

1. xpos >= pos
2. xpos + str.size() <= size()
3. for all positions n in str , Traits::eq(at(xpos+n), str.at(n))

An empty string will always match at the start of the target string because

1. 0 >= 0
2. 0+0 <= size()
3. There are no position is str so the match condition is vacuously true.

Answer 2

是的，它是：“当且仅当pos <= size（）”时才在pos处找到一个空子串

Answer 3

According to Cppreference :

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

str.find("") uses the third overload for std::basic_string::find which has a signature of :

size_type find( const CharT* s, size_type pos = 0 ) const;

Which means that pos starts at 0 so pos <= size() is always true.

Yes, the behaviour is well defined and portable.