cpp/string/basic string/npos

This is a special value equal to the maximum value representable by the 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, is an unsigned integer type, and the value of  is the largest positive value it can hold, due to signed-to-unsigned implicit conversion. This is a portable way to specify the largest value of any unsigned type.