P.J. Plauger Dinkumware, Ltd. pjp@dinkumware.com 2006-02-21
Dinkumware has been marketing several character code conversion aids for a number of years, as part of a package of supplemental features we call CoreX. They have now been merged into our latest comprehensive product, the Dinkum Compleat Library. Based on the success of that package, we now feel confident in proposing two template classes from it for inclusion in a future standard C++ library.
We submitted these template classes earlier, as N1683, and met with several criticisms. But after review, we decided to resubmit the proposal without change and better defend the design decisions in committee. The descriptions that follow are taken primarily from our documentation.
Template class wstring_convert performs conversions between a
wide string and a byte string. It lets you specify a code conversion
facet (like template class codecvt) to perform the conversions,
without affecting any streams or locales. Say, for example, you have
a code conversion facet called codecvt_utf8 that you want
to use to output to cout a UTF-8 multibyte sequence corresponding
to a wide string, but you don't want to alter the locale for cout.
You can write something like:
wstring_convert<codecvt_utf8<wchar_t>>
myconv();
std::string mbstring = myconv.to_bytes(L"Hello\n");
cout << mbstring;
Note that the Standard C++ library currently uses code conversion facets
only within template class basic_filebuf, for converting from
multibyte sequences when reading from a file and for converting to
multibyte sequences when writing to a file. Something like template class
wstring_convert is needed to perform similar conversions
between string objects, without involving file I/O.
wstring_convertnamespace Dinkum {
namespace codecvt {
template<class Codecvt,
class Elem = wchar_t>
class wstring_convert
{
typedef std::basic_string<char> byte_string;
typedef std::basic_string<Elem> wide_string;
typedef typename Codecvt::state_type state_type;
typedef typename wide_string::traits_type::int_type int_type;
wstring_convert();
wstring_convert(const byte_string& byte_err);
wstring_convert(const byte_string& byte_err,
const wide_string& wide_err);
wide_string from_bytes(char byte) const;
wide_string from_bytes(const char *ptr) const;
wide_string from_bytes(const byte_string& str) const;
wide_string from_bytes(const char *first, const char *last) const;
byte_string to_bytes(Elem wchar) const;
byte_string to_bytes(const _Elem *wptr) const;
byte_string to_bytes(const wide_string& wstr) const;
byte_string to_bytes(const Elem *first, const Elem *last) const;
// exposition only
private:
byte_string byte_err_string;
wide_string wide_err_string;
};
} // namespace codecvt
} // namespace Dinkum
The template class describes an object that controls conversions
between wide string objects of class std::basic_string<Elem>
and byte string objects of class std::basic_string<char>
(also known as std::string). The template class defines the
types wide_string and byte_string as synonyms for
these two types. Conversion between a sequence
of Elem values (stored in a wide_string object)
and multibyte sequences (stored in a byte_string object)
is performed by an object of class
Codecvt<Elem, char, std::mbstate_t>,
which meets the requirements of the standard code-conversion facet
std::codecvt<Elem, char, std::mbstate_t>.
An object of this template class stores a
wide-error string, called
wide_err_string
here for the sake of exposition. It also stores a
byte-error string, called
byte_err_string
here for the sake of exposition.
wstring_convert::byte_stringtypedef std::basic_string<char> byte_string;
The type is a synonym for std::basic_string<char>.
wstring_convert::from_byteswide_string from_bytes(char byte) const; wide_string from_bytes(const char *ptr) const; wide_string from_bytes(const byte_string& str) const; wide_string from_bytes(const char *first, const char *last) const;
The first member function converts the single-element sequence byte
to a wide string.
The second member function converts the nul-terminated sequence beginning
at ptr to a wide string.
The third member function converts the sequence stored in str
to a wide string.
The fourth member function converts the sequence defined by the range
[first, last) to a wide string.
In all cases:
std::range_error.wstring_convert::int_typetypedef typename wide_string::traits_type::int_type int_type;
The type is a synonym for wide_string::traits_type::int_type.
wstring_convert::state_typetypedef typename Codecvt::state_type state_type;
The type is a synonym for Codecvt::state_type.
wstring_convert::to_bytesbyte_string to_bytes(Elem wchar) const; byte_string to_bytes(const _Elem *wptr) const; byte_string to_bytes(const wide_string& wstr) const; byte_string to_bytes(const Elem *first, const Elem *last) const;
The first member function converts the single-element sequence wchar
to a byte string.
The second member function converts the nul-terminated sequence beginning
at wptr to a byte string.
The third member function converts the sequence stored in wstr
to a byte string.
The fourth member function converts the sequence defined by the range
[first, last) to a byte string.
In all cases:
std::range_error.wstring_convert::wide_stringtypedef std::basic_string<Elem> wide_string;
The type is a synonym for std::basic_string<Elem>.
wstring_convert::wstring_convertwstring_convert();
wstring_convert(const byte_string& byte_err);
wstring_convert(const byte_string& byte_err,
const wide_string& wide_err);
The first constructor constructs a conversion object with no stored
wide-error string or
byte-error string.
The second constructor stores a copy of byte_err in the stored
byte-error string.
The second constructor stores a copy of byte_err in the stored
byte-error string and stores a copy of wide_err in the stored
wide-error string.
Template class wbuffer_convert looks like a wide stream
buffer, but performs all its I/O through an underlying byte stream buffer
that you specify when you construct it. Like template class
wstring_convert, it lets you specify a code conversion
facet to perform the conversions, without affecting any streams or locales.
The previous example can also be written as:
wbuffer_convert<codecvt_utf8<wchar_t> >
mybuf(cout.rdbuf()); // construct wide stream buffer object
std::wofstream mystr(mybuf); // construct wide ostream object
cout << L"Hello";
Something like template class wstring_convert is needed
to perform code conversions when writing to streams other than files.
wbuffer_convertnamespace Dinkum {
namespace codecvt {
template<class Codecvt,
class Elem = wchar_t,
class Tr = std::char_traits<Elem> >
class wbuffer_convert
: public std::basic_streambuf<Elem, Tr>
{
public:
wbuffer_convert(std::streambuf *bytebuf = 0);
std::streambuf *rdbuf();
std::streambuf *rdbuf(std::streambuf *bytebuf);
// exposition only
private:
std::streambuf *bufptr;
};
} // namespace codecvt
} // namespace Dinkum
The template class describes a stream buffer that controls the
transmission of elements of type Elem, whose character traits
are described by the class Tr, to and from a byte stream
buffer of type std::streambuf. Conversion between a sequence
of Elem values and multibyte sequences is performed by an
object of class Codecvt<Elem, char, std::mbstate_t>,
which meets the requirements of the standard code-conversion facet
std::codecvt<Elem, char, std::mbstate_t>.
An object of this template class stores a pointer to its underlying
byte stream buffer, called
bufptr here for the sake of
exposition.
wbuffer_convert::wbuffer_convertwbuffer_convert(std::streambuf *bytebuf = 0);
The constructor constructs a stream buffer object and initializes its
stored byte stream buffer pointer to bytebuf.
wbuffer_convert::rdbufstd::streambuf *rdbuf(); std::streambuf *rdbuf(std::streambuf *bytebuf);
The first member function returns the stored byte stream buffer pointer.
The second member function stores bytebuf in the stored byte
stream buffer pointer.
Copyright © 2002-2006 by Dinkumware, Ltd. All rights reserved.