C + + <string> inside the Size_type

String::size_type

The Str.size () return value of string is a unsigned that actually returns a value of type Size_type.
The string class and most of the other standard library types define several sets of matching types. These types embody the machine-independent nature of the standard library. String::size_type is an unsigned type, so try to use

Auto len= line.size ();

PS: if n is negative. Line.size () < n the result is almost all true because negative n is converted to a larger unsigned value

C + + <string> inside the Size_type