Small modification for answer of Exercise 3.9 - mention `s` is a empty string

[Danlinasyr]

Exercise information

Exercise 3.9

What does the following program do? Is it valid? If not, why not?

string s;
cout << s[0] << endl;

Question or Bug

Maybe should mention string s is empty string.

Answer was

Try to get the first element of the string. It is invalid, cause this is undefined behavior.

Answer modified

Try to get the first element of the empty string s (default initialization). It is invalid, cause this is undefined behavior.