The C++ standard library(侯捷/孟岩 译) 11--Strings

11. string(p471)

string::npos

It equals to the maximum value representable by the type size_type.
  The exact meaning depends on context, but it is generally used 
    either as end of string indicator by the functions that expect a string index
    or  as the error indicator by the functions that return a string index.

note:
  Although the definition uses -1, size_type is an unsigned integer type,
    and the value of npos is the largest positive value it could hold,
    due to signed-to-unsigned implicit conversion.
  This is a portable way to specify the largest value of any.

11.2 string内部简述(p479)

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