Requirements on state type of std::codecvt

Hi,

I have a question about std::codecvt.
The standard says that std::codecvt is defined as follows,


namespace std{

template<class internT, class externT, class stateT>
class codecvt;

}


Then, what are the requirements on `stateT'? I looked at some
implemetations of the standard library, and they seem to require
`stateT' to be DefaultConstructible in order to construct an
initial state. But I would like to know precise descriptions of
the requirements on `stateT'. Could anyone help me?

Thanks in advance.


--
Ai Azuma


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Ai Azuma wrote:
> I have a question about std::codecvt.
> The standard says that std::codecvt is defined as follows,
>
> template<class internT, class externT, class stateT>
> class codecvt;
>
> Then, what are the requirements on `stateT'? I looked at some
> implemetations of the standard library, and they seem to require
> `stateT' to be DefaultConstructible in order to construct an
> initial state. But I would like to know precise descriptions of
> the requirements on `stateT'.

There aren't any, AFAICT. For most codecvt facets (like codepages or the
UTF/UCS encodings), this parameter is in fact not used at all because they
are stateless. There are other encodings like Shift-JIS (if I remember
right, [1] had an example with it) where you can switch between different
character tables with escape sequences. In that case, this additional
context (the current character table) is recorded in this state_type.

For fstreams, this state_type is usually mbstate_t of the C API (or is it
POSIX?), which is part of the C multibyte character handling API. In most
implementations I have seen this was just an integer, only in newer GCC
iostreams this contains a structure which iconv (GNU's swiss chainsaw for
encoding conversion) uses for its state.

BTW: another requirement of state_type is that it is copyable, because the
fileposition type contains the offset into the file and this context.
Overall, I'd treat it like any other data type, but not necessarily a POD.

Uli

[1] "C++ IOStreams and Locales" by Langer and Kreft - the book to read when
you're interested in streams.


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