public static member constant
static const size_type npos = -1;
Maximum value of size_type
npos is a static member constant value with the greatest possible value for an element of member type size_type.
This value, when used as the value for a len (or sublen) parameter in basic_string's member functions, means "until the end of the string".
As a return value, it is usually used to indicate no matches.
This constant is defined with a value of -1, which because member type size_type is an unsigned integral type, it is the largest possible representable value for this type.