Question:
Could you please explain what the significance of string::npos is in the statement?

someString.find_first_not_of("0123456789")==string::npos

Answer:
string::npos is a constant integer that holds the maximum value of characters that a string can store + 1. This value is used in searching algorithms to indicate that the sought-after value was not found, because no string element can ever have this position. In other words, if the expression

someString.find_first_not_of("0123456789")

equals string::npos, then it means that the find_first_not_of() member function couldn’t locate the sought-after value in the string.