string substr(uint pos, uint size) const/string

Returns a string object with its contents initialized to a substring of the current object.

This substring is the character sequence that starts at character position pos and has a length of size characters.

When the NPOS constant is used as size, all the characters between pos and the end of the string are used as the initialization substring:

const string str = "quick brown fox";
string quick = str.substr(0, 5);
string fox = str.substr(12, NPOS);

If the value of size would make the substring to span past the end of the current string content, only those characters until the end of the string are used.