How do I declare a custom char_traits<> for my own type?

Question

I want to make a custom char_traits class for my own type. I have declared all the functions, but I am having some confusion regarding vague semantics given in the standard.

1. What are fpos_type, off_type and state_type expected to do? Where are they required, if at all?

2. Standard says to keep traits::eof() and valid code points separate and thus asks to use an int_type which is bigger than the char_type. But can I then make my char_type as a signed number where I know valid code points are only in the non-negative range. It leaves me with negative values as possible values of traits::eof(). Or should it be that int_type should be a proper superset of char_type so that no char_type can ever take the value of eof()? Being more to the point, can I make int_type and char_type to be same?

Answer 1

1. std::fpos is a class that encapsulates file positioning semantics for file streams. It uses the current conversion state to identify file positions. off_type and pos_type are types defined in std::char_traits that are types for offset types and position types respectively. pos_type is used as arguments to seek functions and is the type returned from the tell functions. It is a type denoting absolute positions. off_type is used in the overloads of the seek functions for specifying offsets from a provided file position.

   state_type is also defined in std::char_traits. It is used for specifying the conversion state of a multibyte sequence.

   You can look up all the requirements for these types in [char.traits.require].

2. This is the definition of traits::int_type - [char.traits.typedef]

   typedef INT_T int_type
   

   For a certain character container type char_type, a related container type INT_T shall be a type or class which can represent all of the valid characters converted from the corresponding char_type values, as well as an end-of-file value, eof(). The type int_type represents a character container type which can hold end-of-file to be used as a return type of the iostream class member functions.

   This definition carries with it the implication that int_type merely be able to to represent values of type char_type, and also the eof() value. It defines no further requirements for the type other than what types it aliases for certain specializations of std::char_traits