Why string constructor(s, pos) exception is "pos > s.size()" and not "pos >= s.size()"?

Question

Sample below:

string s1 = "abcde";
string s2(s1, s1.size()); // s1.size() = 5.

Notice that s1.size() = 5 and the last allowable index = 4 (for character 'e'). The above runs fine returning empty string. Only when pos = 6 then it fail with exception out-of-range. Why?

According to cppereference site:

Exceptions
3) std::out_of_range if pos > other.size()

Shouldn't the correct exception be "if pos >= other.size()"?

Answer 1

Since C++11, s1[s1.size()] is required to work and will return a reference to the '\0' at the end of the string. Changing the '\0' to something else however leads to undefined behavior. You are however allowed to write '\0' there.

Answer 2

Notice that s1.size() = 5 and the last allowable index = 4 (for character 'e').

This is wrong. Since C++11, the last allowable character index is size(), not size()-1. std::basic_string's data buffer is now required to be null terminated, and accessing index size() (even on an empty string) is required to return a reference to a valid '\0' character.

The above runs fine returning empty string.

As it should be.

Only when pos = 6 then it fail with exception out-of-range.

Correct, because that really is out of bounds.