codecvt Class

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The latest version of this topic can be found at codecvt Class.

A template class that describes an object that can serve as a locale facet. It is able to control conversions between a sequence of values used to encode characters within the program and a sequence of values used to encode characters outside the program.

Syntax

template <class CharType, class Byte, class StateType>  
class codecvt : public locale::facet, codecvt_base;

Parameters

CharType
The type used within a program to encode characters.

Byte
A type used to encode characters outside a program.

StateType
A type that can be used to represent intermediate states of a conversion between internal and external types of character representations.

Remarks

The template class describes an object that can serve as a locale facet, to control conversions between a sequence of values of type CharType and a sequence of values of type Byte. The class StateType characterizes the transformation -- and an object of class StateType stores any necessary state information during a conversion.

The internal encoding uses a representation with a fixed number of bytes per character, usually either type char or type wchar_t.

As with any locale facet, the static object id has an initial stored value of zero. The first attempt to access its stored value stores a unique positive value in id.

The template versions of do_in and do_out always return codecvt_base::noconv.

The Standard C++ Library defines several explicit specializations:

template<>

codecvt<wchar_t, char, mbstate_t>

converts between wchar_t and char sequences.

template<>

codecvt<char16_t, char, mbstate_t>

converts between char16_t sequences encoded as UTF-16 and char sequences encoded as UTF-8.

template<>

codecvt<char32_t, char, mbstate_t>

converts between char32_t sequences encoded as UTF-32 (UCS-4) and char sequences encoded as UTF-8.

Constructors

codecvt The constructor for objects of class codecvt that serves as a locale facet to handle conversions.

Typedefs

extern_type A character type that is used for external representations.
intern_type A character type that is used for internal representations.
state_type A character type that is used to represent intermediate states during conversions between internal and external representations.

Member Functions

always_noconv Tests whether no conversions need be done.
do_always_noconv A virtual function called to test whether no conversions need be done.
do_encoding A virtual function that tests if the encoding of the Byte stream is state dependent, whether the ratio between the Bytes used and the CharTypes produced is constant, and, if so, determines the value of that ratio.
do_in A virtual function called to convert a sequence of internal Bytes to a sequence of external CharTypes.
do_length A virtual function that determines how many Bytes from a given sequence of external Bytes produce not more than a given number of internal CharTypes and returns that number of Bytes.
do_max_length A virtual function that returns the maximum number of external Bytes necessary to produce one internal CharType.
do_out A virtual function called to convert a sequence of internal CharTypes to a sequence of external Bytes.
do_unshift A virtual function called to provide the Bytes needed in a state-dependent conversion to complete the last character in a sequence of Bytes.
encoding Tests if the encoding of the Byte stream is state dependent, whether the ratio between the Bytes used and the CharTypes produced is constant, and, if so, determines the value of that ratio.
in Converts an external representation of a sequence of Bytes to an internal representation of a sequence of CharTypes.
length Determines how many Bytes from a given sequence of external Bytes produce not more than a given number of internal CharTypes and returns that number of Bytes.
max_length Returns the maximum number of external Bytes necessary to produce one internal CharType.
out Converts a sequence of internal CharTypes to a sequence of external Bytes.
unshift Provides the external Bytes needed in a state-dependent conversion to complete the last character in the sequence of Bytes.

Requirements

Header: <locale>

Namespace: std

codecvt::always_noconv

Tests whether no conversions need be done.

bool always_noconv() const throw();

Return Value

A Boolean value that is true if no conversions need be done; false is at least one needs to be done.

Remarks

The member function returns do_always_noconv.

Example

// codecvt_always_noconv.cpp  
// compile with: /EHsc  
#include <locale>  
#include <iostream>  
using namespace std;  
  
int main( )     
{  
   locale loc ( "German_Germany" );  
   bool result1 = use_facet<codecvt<char, char, mbstate_t> >   
      ( loc ).always_noconv( );  
  
   if ( result1 )  
      cout << "No conversion is needed." << endl;  
   else  
      cout << "At least one conversion is required." << endl;  
  
   bool result2 = use_facet<codecvt<wchar_t, char, mbstate_t> >   
      ( loc ).always_noconv( );  
  
   if ( result2 )  
      cout << "No conversion is needed." << endl;  
   else  
      cout << "At least one conversion is required." << endl;  
}  

No conversion is needed.  
At least one conversion is required.

codecvt::codecvt

The constructor for objects of class codecvt that serves as a locale facet to handle conversions.

explicit codecvt(size_t _Refs = 0);

Parameters

_Refs
Integer value used to specify the type of memory management for the object.

Remarks

The possible values for the _Refs parameter and their significance are:

  • 0: The lifetime of the object is managed by the locales that contain it.

  • 1: The lifetime of the object must be manually managed.

  • > 0: These values are not defined.

The constructor initializes its locale::facet base object with locale::facet( _Refs) .

codecvt::do_always_noconv

A virtual function called to test whether no conversions need be done.

virtual bool do_always_noconv() const throw();

Return Value

The protected virtual member function returns true only if every call to do_in or do_out returns noconv.

The template version always returns true.

Example

See the example for always_noconv, which calls do_always_noconv.

codecvt::do_encoding

A virtual function that tests if the encoding of the Byte stream is state dependent, whether the ratio between the Bytes used and the CharTypes produced is constant and, if so, determines the value of that ratio.

virtual int do_encoding() const throw();

Return Value

The protected virtual member function returns:

  • –1, if the encoding of sequences of type extern_type is state dependent.

  • 0, if the encoding involves sequences of varying lengths.

  • N, if the encoding involves only sequences of length N

Example

See the example for encoding, which calls do_encoding.

codecvt::do_in

A virtual function called to convert a sequence of external Bytes to a sequence of internal CharTypes.

virtual result do_in(
    StateType& _State,  
    const Byte* first1,   
    const Byte* last1,   
    const Byte*& next1,  
    CharType* first2,  
    CharType* last2,  
    CharType*& next2,) const;

Parameters

_State
The conversion state that is maintained between calls to the member function.

first1
Pointer to the beginning of the sequence to be converted.

last1
Pointer to the end of the sequence to be converted.

next1
Pointer beyond the end of the converted sequence, to the first unconverted character.

first2
Pointer to the beginning of the converted sequence.

last2
Pointer to the end of the converted sequence.

next2
Pointer to the CharType that comes after the last converted CharType, to the first unaltered character in the destination sequence.

Return Value

A return that indicates the success, partial success, or failure of the operation. The function returns:

  • codecvt_base::error if the source sequence is ill formed.

  • codecvt_base::noconv if the function performs no conversion.

  • codecvt_base::ok if the conversion succeeds.

  • codecvt_base::partial if the source is insufficient or if the destination is not large enough, for the conversion to succeed.

Remarks

_State must represent the initial conversion state at the beginning of a new source sequence. The function alters its stored value as needed to reflect the current state of a successful conversion. Its stored value is otherwise unspecified.

Example

See the example for in, which calls do_in.

codecvt::do_length

A virtual function that determines how many Bytes from a given sequence of external Bytes produce not more than a given number of internal CharTypes and returns that number of Bytes.

virtual int do_length(
    const StateType& _State,  
    const Byte* first1,   
    const Byte* last1,  
    size_t _Len2) const;

Parameters

_State
The conversion state that is maintained between calls to the member function.

first1
Pointer to the beginning of the external sequence.

last1
Pointer to the end of the external sequence.

_Len2
The maximum number of Bytes that can be returned by the member function.

Return Value

An integer that represents a count of the maximum number of conversions, not greater than _Len2, defined by the external source sequence at [ first1, last1).

Remarks

The protected virtual member function effectively calls do_in( _State, first1, last1, next1, _Buf, _Buf + _Len2, next2) for _State (a copy of state), some buffer _Buf, and pointers next1and next2.

It then returns next2 – buf. Thus, it counts the maximum number of conversions, not greater than _Len2, defined by the source sequence at [ first1, last1).

The template version always returns the lesser of last1 – first1 and _Len2.

Example

See the example for length, which calls do_length.

codecvt::do_max_length

A virtual function that returns the maximum number of external Bytes necessary to produce one internal CharType.

virtual int do_max_length() const throw();

Return Value

The maximum number of Bytes necessary to produce one CharType.

Remarks

The protected virtual member function returns the largest permissible value that can be returned by do_length( first1, last1, 1) for arbitrary valid values of first1 and last1.

Example

See the example for max_length, which calls do_max_length.

codecvt::do_out

A virtual function called to convert a sequence of internal CharTypes to a sequence of external Bytes.

virtual result do_out(
    StateType& _State,  
    const CharType* first1,   
    const CharType* last1,  
    const CharType*& next1,  
    Byte* first2,   
    Byte* last2,   
    Byte*& next2) const;

Parameters

_State
The conversion state that is maintained between calls to the member function.

first1
Pointer to the beginning of the sequence to be converted.

last1
Pointer to the end of the sequence to be converted.

next1
Reference to a pointer to the first unconverted CharType, after the last CharType converted.

first2
Pointer to the beginning of the converted sequence.

last2
Pointer to the end of the converted sequence.

next2
Reference to a pointer to the first unconverted Byte, after the last Byte converted.

Return Value

The function returns:

  • codecvt_base::error if the source sequence is ill formed.

  • codecvt_base::noconv if the function performs no conversion.

  • codecvt_base::ok if the conversion succeeds.

  • codecvt_base::partial if the source is insufficient or if the destination is not large enough for the conversion to succeed.

Remarks

_State must represent the initial conversion state at the beginning of a new source sequence. The function alters its stored value as needed to reflect the current state of a successful conversion. Its stored value is otherwise unspecified.

Example

See the example for out, which calls do_out.

codecvt::do_unshift

A virtual function called to provide the Bytes needed in a state-dependent conversion to complete the last character in a sequence of Bytes.

virtual result do_unshift(
    StateType& _State,  
    Byte* first2,   
    Byte* last2,   
    Byte*& next2) const;

Parameters

_State
The conversion state that is maintained between calls to the member function.

first2
Pointer to the first position in the destination range.

last2
Pointer to the last position in the destination range.

next2
Pointer to the first unaltered element in the destination sequence.

Return Value

The function returns:

  • codecvt_base::error if _ State represents an invalid state

  • codecvt_base::noconv if the function performs no conversion

  • codecvt_base::ok if the conversion succeeds

  • codecvt_base::partial if the destination is not large enough for the conversion to succeed

Remarks

The protected virtual member function tries to convert the source element CharType(0) to a destination sequence that it stores within [ first2, last2), except for the terminating element Byte(0). It always stores in next2 a pointer to the first unaltered element in the destination sequence.

_ State must represent the initial conversion state at the beginning of a new source sequence. The function alters its stored value as needed to reflect the current state of a successful conversion. Typically, converting the source element CharType(0) leaves the current state in the initial conversion state.

Example

See the example for unshift, which calls do_unshift.

codecvt::encoding

Tests if the encoding of the Byte stream is state dependent, whether the ratio between the Bytes used and the CharTypes produced is constant, and, if so, determines the value of that ratio.

int encoding() const throw();

Return Value

If the return value is positive then that value is the constant number of Byte characters required to produce the CharType character.

The protected virtual member function returns:

  • –1, if the encoding of sequences of type extern_type is state dependent.

  • 0, if the encoding involves sequences of varying lengths.

  • N, if the encoding involves only sequences of length N.

Remarks

The member function returns do_encoding.

Example

// codecvt_encoding.cpp  
// compile with: /EHsc  
#include <locale>  
#include <iostream>  
using namespace std;  
  
int main( )     
{  
   locale loc ( "German_Germany" );  
   int result1 = use_facet<codecvt<char, char, mbstate_t> > ( loc ).encoding ( );  
   cout << result1 << endl;  
   result1 = use_facet<codecvt<wchar_t, char, mbstate_t> > ( loc ).encoding( );  
   cout << result1 << endl;  
   result1 = use_facet<codecvt<char, wchar_t, mbstate_t> > ( loc ).encoding( );  
   cout << result1 << endl;  
}  

1  
1  
1

codecvt::extern_type

A character type that is used for external representations.

typedef Byte extern_type;

Remarks

The type is a synonym for the template parameter Byte.

codecvt::in

Converts an external representation of a sequence of Bytes to an internal representation of a sequence of CharTypes.

result in(
    StateType& _State,  
    const Byte* first1,   
    const Byte* last1,   
    const Byte*& next1,  
    CharType* first2,  
    CharType* last2,  
    CharType*& next2,) const;

Parameters

_State
The conversion state that is maintained between calls to the member function.

first1
Pointer to the beginning of the sequence to be converted.

last1
Pointer to the end of the sequence to be converted.

next1
Pointer beyond the end of the converted sequence to the first unconverted character.

first2
Pointer to the beginning of the converted sequence.

last2
Pointer to the end of the converted sequence.

next2
Pointer to the CharType that comes after the last converted Chartype to the first unaltered character in the destination sequence.

Return Value

A return that indicates the success, partial success or failure of the operation. The function returns:

  • codecvt_base::error if the source sequence is ill formed.

  • codecvt_base::noconv if the function performs no conversion.

  • codecvt_base::ok if the conversion succeeds.

  • codecvt_base::partial if the source is insufficient or if the destination is not large enough for the conversion to succeed.

Remarks

_State must represent the initial conversion state at the beginning of a new source sequence. The function alters its stored value, as needed, to reflect the current state of a successful conversion. After a partial conversion, _State must be set so as to allow the conversion to resume when new characters arrive.

The member function returns do_in( _State, _ First1, last1, next1, First2, _Llast2, next2).

Example

// codecvt_in.cpp  
// compile with: /EHsc  
#define _INTL  
#include <locale>  
#include <iostream>  
using namespace std;  
#define LEN 90  
int main( )     
{  
   char* pszExt = "This is the string to be converted!";  
   wchar_t pwszInt [LEN+1];  
   memset(&pwszInt[0], 0, (sizeof(wchar_t))*(LEN+1));  
   const char* pszNext;  
   wchar_t* pwszNext;  
   mbstate_t state = {0};  
   locale loc("C");//English_Britain");//German_Germany  
   int res = use_facet<codecvt<wchar_t, char, mbstate_t> >  
     ( loc ).in( state,  
          pszExt, &pszExt[strlen(pszExt)], pszNext,  
          pwszInt, &pwszInt[strlen(pszExt)], pwszNext );  
   pwszInt[strlen(pszExt)] = 0;  
   wcout << ( (res!=codecvt_base::error)  L"It worked! " : L"It didn't work! " )  
   << L"The converted string is:\n ["  
   << &pwszInt[0]  
   << L"]" << endl;  
   exit(-1);  
}  

It worked! The converted string is:  
 [This is the string to be converted!]

codecvt::intern_type

A character type that is used for internal representations.

typedef CharType intern_type;

Remarks

The type is a synonym for the template parameter CharType.

codecvt::length

Determines how many Bytes from a given sequence of external Bytes produce not more than a given number of internal CharTypes and returns that number of Bytes.

int length(
    const StateType& _State,  
    const Byte* first1,   
    const Byte* last1,  
    size_t _Len2) const;

Parameters

_State
The conversion state that is maintained between calls to the member function.

first1
Pointer to the beginning of the external sequence.

last1
Pointer to the end of the external sequence.

_Len2
The maximum number of Bytes that can be returned by the member function.

Return Value

An integer that represents a count of the maximum number of conversions, not greater than _Len2, defined by the external source sequence at [ first1, last1).

Remarks

The member function returns do_length( _State, first1, last1, _Len2).

Example

// codecvt_length.cpp  
// compile with: /EHsc  
#define _INTL  
#include <locale>  
#include <iostream>  
using namespace std;  
#define LEN 90  
int main( )     
{  
   char* pszExt = "This is the string whose length is to be measured!";  
   mbstate_t state = {0};  
   locale loc("C");//English_Britain");//German_Germany  
   int res = use_facet<codecvt<wchar_t, char, mbstate_t> >  
     ( loc ).length( state,  
          pszExt, &pszExt[strlen(pszExt)], LEN );  
   cout << "The length of the string is: ";  
   wcout << res;  
   cout << "." << endl;  
   exit(-1);  
}  

The length of the string is: 50.

codecvt::max_length

Returns the maximum number of external Bytes necessary to produce one internal CharType.

int max_length() const throw();

Return Value

The maximum number of Bytes necessary to produce one CharType.

Remarks

The member function returns do_max_length.

Example

// codecvt_max_length.cpp  
// compile with: /EHsc  
#define _INTL  
#include <locale>  
#include <iostream>  
using namespace std;  
  
int main( )     
{  
   locale loc( "C");//English_Britain" );//German_Germany  
   int res = use_facet<codecvt<char, char, mbstate_t> >  
     ( loc ).max_length( );  
   wcout << res << endl;  
}  

1

codecvt::out

Converts a sequence of internal CharTypes to a sequence of external Bytes.

result out(
    StateType& _State,  
    const CharType* first1,   
    const CharType* last1,  
    const CharType*& next1,  
    Byte* first2,   
    Byte* last2,   
    Byte*& next2) const;

Parameters

_State
The conversion state that is maintained between calls to the member function.

first1
Pointer to the beginning of the sequence to be converted.

last1
Pointer to the end of the sequence to be converted.

next1
Reference to a pointer to the first unconverted CharType after the last CharType converted.

first2
Pointer to the beginning of the converted sequence.

last2
Pointer to the end of the converted sequence.

next2
Reference to a pointer to the first unconverted Byte after the last converted Byte.

Return Value

The member function returns do_out( _State, first1, last1, next1, first2, last2, next2).

Remarks

For more information, see codecvt::do_out.

Example

// codecvt_out.cpp  
// compile with: /EHsc  
#define _INTL  
#include <locale>  
#include <iostream>  
#include <wchar.h>  
using namespace std;  
#define LEN 90  
int main( )     
{  
   char pszExt[LEN+1];  
   wchar_t *pwszInt = L"This is the wchar_t string to be converted.";  
   memset( &pszExt[0], 0, ( sizeof( char ) )*( LEN+1 ) );  
   char* pszNext;  
   const wchar_t* pwszNext;  
   mbstate_t state;  
   locale loc("C");//English_Britain");//German_Germany  
   int res = use_facet<codecvt<wchar_t, char, mbstate_t> >  
      ( loc ).out( state,  
      pwszInt, &pwszInt[wcslen( pwszInt )], pwszNext ,  
      pszExt, &pszExt[wcslen( pwszInt )], pszNext );  
   pszExt[wcslen( pwszInt )] = 0;  
   cout << ( ( res!=codecvt_base::error )  "It worked: " : "It didn't work: " )  
   << "The converted string is:\n ["  
   << &pszExt[0]  
   << "]" << endl;  
}  

It worked: The converted string is:  
 [This is the wchar_t string to be converted.]

codecvt::state_type

A character type that is used to represent intermediate states during conversions between internal and external representations.

typedef StateType state_type;

Remarks

The type is a synonym for the template parameter StateType.

codecvt::unshift

Provides the Bytes needed in a state-dependent conversion to complete the last character in a sequence of Bytes.

result unshift(
    StateType& _State,  
    Byte* first2,   
    Byte* last2,   
    Byte*& next2) const;

Parameters

_State
The conversion state that is maintained between calls to the member function.

first2
Pointer to the first position in the destination range.

last2
Pointer to the last position in the destination range.

next2
Pointer to the first unaltered element in the destination sequence.

Return Value

The function returns:

  • codecvt_base::error if state represents an invalid state.

  • codecvt_base::noconv if the function performs no conversion.

  • codecvt_base::ok if the conversion succeeds.

  • codecvt_base::partial if the destination is not large enough for the conversion to succeed.

Remarks

The protected virtual member function tries to convert the source element CharType(0) to a destination sequence that it stores within [ first2, last2), except for the terminating element Byte(0). It always stores in next2 a pointer to the first unaltered element in the destination sequence.

_State must represent the initial conversion state at the beginning of a new source sequence. The function alters its stored value, as needed, to reflect the current state of a successful conversion. Typically, converting the source element CharType(0) leaves the current state in the initial conversion state.

The member function returns do_unshift( _State, first2, last2, next2 ).

See Also

<locale>
Code Pages
Locale Names, Languages, and Country/Region Strings
Thread Safety in the C++ Standard Library