npos is a static member constant value with the greatest possible value for an element of type size_t.
This value, when used as the value for a count parameter n in string's member functions, roughly indicates "as many as possible".
When used in some pos parameters that allow for out-of-range values, npos indicates the end of the string.
As a return value it is usually used to indicate failure.
This constant is actually defined with a value of -1 (for any trait), which because size_t is an unsigned integral type, becomes the largest possible representable value for this type.