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Difference between revisions of "cpp/locale/codecvt mode"

From cppreference.com
< cpp‎ | locale
m (Text replace - "{{example cpp" to "{{example")
m (P2871R2.)
 
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{{cpp/title | codecvt_mode}}
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{{cpp/title|codecvt_mode}}
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{{cpp/locale/sidebar}}
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{{cpp/locale/navbar}}
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{{ddcl | header=locale | 1=
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{{ddcl|since=c++11|deprecated=c++17|removed=c++26|header=codecvt|1=
 
enum codecvt_mode {
 
enum codecvt_mode {
 
     consume_header = 4,
 
     consume_header = 4,
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}}
 
}}
  
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The facets {{cpp|std::codecvt_utf8}}, {{cpp|std::codecvt_utf16}}, and {{cpp|std::codecvt_utf8_utf16}} accept an optional value of type {{cpp|std::codecvt_mode}} as a template argument, which specifies optional features of the unicode string conversion.
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The facets {{lc|std::codecvt_utf8}}, {{lc|std::codecvt_utf16}}, and {{lc|std::codecvt_utf8_utf16}} accept an optional value of type {{tt|std::codecvt_mode}} as a template argument, which specifies optional features of the unicode string conversion.
  
 
===Constants===
 
===Constants===
−
 
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{{dsc begin}}
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{{tdcl list begin}}
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{{dsc header|locale}}
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{{tdcl list header | locale}}
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{{dsc hitem|Value|Meaning}}
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{{tdcl list hitem | Value | Meaning }}
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{{dsc|{{tt|little_endian}}|assume the input is in little-endian byte order (applies to UTF-16 input only, the default is big-endian)}}
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{{tdcl list item | {{tt|little_endian}} | assume the input is in little-endian byte order (applies to UTF-16 input only, the default is big-endian) }}
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{{dsc|{{tt|consume_header}}|consume the byte order mark, if present at the start of input sequence, and (in case of UTF-16), rely on the byte order it specifies for decoding the rest of the input}}
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{{tdcl list item | {{tt|consume_header}} | consume the byte order mark, if present at the start of input sequence, and (in case of UTF-16), rely on the byte order it specifies for decoding the rest of the input}}
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{{dsc|{{tt|generate_header}}|output the byte order mark at the start of the output sequence}}
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{{tdcl list item | {{tt|generate_header}} | output the byte order mark at the start of the output sequence }}
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{{dsc end}}
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{{tdcl list end}}
+
  
 
The recognized byte order marks are:
 
The recognized byte order marks are:
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{{tdcl list begin}}
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{{dsc begin}}
−
{{tdcl list item | {{tt|0xfe 0xff}} | UTF-16 big-endian }}
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{{dsc|{{tt|0xfe 0xff}}|UTF-16 big-endian}}
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{{tdcl list item | {{tt|0xff 0xfe}} | UTF-16 little-endian }}
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{{dsc|{{tt|0xff 0xfe}}|UTF-16 little-endian}}
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{{tdcl list item | {{tt|0xef 0xbb 0xbf}} | UTF-8 (no effect on endianness)}}
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{{dsc|{{tt|0xef 0xbb 0xbf}}|UTF-8 (no effect on endianness)}}
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{{tdcl list end}}
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{{dsc end}}
  
−
If {{cpp|std::consume_header}} is not selected when reading a file beginning with byte order mark, the Unicode character U+FEFF (Zero width non-breaking space) will be read as the first character of the string content.
+
If {{tt|std::consume_header}} is not selected when reading a file beginning with byte order mark, the Unicode character U+FEFF (Zero width non-breaking space) will be read as the first character of the string content.
  
 
===Example===
 
===Example===
 
{{example
 
{{example
−
| The following example demonstrates consuming the UTF-8 BOM
+
|The following example demonstrates consuming the UTF-8 BOM:
−
| code=
+
|code=
 +
#include <codecvt>
 +
#include <cwchar>
 
#include <fstream>
 
#include <fstream>
 
#include <iostream>
 
#include <iostream>
−
#include <string>
 
 
#include <locale>
 
#include <locale>
−
#include <codecvt>
+
#include <string>
 +
 
 
int main()
 
int main()
 
{
 
{
 
     // UTF-8 data with BOM
 
     // UTF-8 data with BOM
−
     std::ofstream("text.txt") << u8"\ufeffz\u6c34\U0001d10b";
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     std::ofstream{"text.txt"} << "\ufeffz\u6c34\U0001d10b";
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     // read the UTF8 file, skipping the BOM
+
   
−
     std::wifstream fin("text.txt");
+
     // read the UTF-8 file, skipping the BOM
 +
     std::wifstream fin{"text.txt"};
 
     fin.imbue(std::locale(fin.getloc(),
 
     fin.imbue(std::locale(fin.getloc(),
 
                           new std::codecvt_utf8<wchar_t, 0x10ffff, std::consume_header>));
 
                           new std::codecvt_utf8<wchar_t, 0x10ffff, std::consume_header>));
−
     for(wchar_t c; fin.get(c); )
+
   
−
         std::cout << std::hex << std::showbase << c << '\n';
+
     for (wchar_t c; fin.get(c);)
 +
         std::cout << std::hex << std::showbase << (std::wint_t)c << '\n';
 
}
 
}
−
| output=
+
|output=
 
0x7a
 
0x7a
 
0x6c34
 
0x6c34
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===See also===
 
===See also===
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{{dcl list begin}}
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{{dsc begin}}
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{{dcl list template | cpp/locale/dcl list codecvt}}
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{{dsc inc|cpp/locale/dsc codecvt}}
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{{dcl list template | cpp/locale/dcl list codecvt_utf8}}
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{{dsc inc|cpp/locale/dsc codecvt_utf8}}
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{{dcl list template | cpp/locale/dcl list codecvt_utf16}}
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{{dsc inc|cpp/locale/dsc codecvt_utf16}}
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{{dcl list template | cpp/locale/dcl list codecvt_utf8_utf16}}
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{{dsc inc|cpp/locale/dsc codecvt_utf8_utf16}}
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{{dcl list end}}
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{{dsc end}}
 +
 
 +
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 10:20, 22 November 2023

 
C++
 
Localization library
 
Defined in header <codecvt>
enum codecvt_mode {

    consume_header = 4,
    generate_header = 2,
    little_endian = 1

};
 (since C++11)
(deprecated in C++17)
(removed in C++26)

The facets std::codecvt_utf8, std::codecvt_utf16, and std::codecvt_utf8_utf16 accept an optional value of type std::codecvt_mode as a template argument, which specifies optional features of the unicode string conversion.

[edit] Constants

Defined in header <locale>
Value Meaning
little_endian assume the input is in little-endian byte order (applies to UTF-16 input only, the default is big-endian)
consume_header consume the byte order mark, if present at the start of input sequence, and (in case of UTF-16), rely on the byte order it specifies for decoding the rest of the input
generate_header output the byte order mark at the start of the output sequence

The recognized byte order marks are:

0xfe 0xff UTF-16 big-endian
0xff 0xfe UTF-16 little-endian
0xef 0xbb 0xbf UTF-8 (no effect on endianness)

If std::consume_header is not selected when reading a file beginning with byte order mark, the Unicode character U+FEFF (Zero width non-breaking space) will be read as the first character of the string content.

[edit] Example

The following example demonstrates consuming the UTF-8 BOM:

Run this code
#include <codecvt>
#include <cwchar>
#include <fstream>
#include <iostream>
#include <locale>
#include <string>
 
int main()
{
    // UTF-8 data with BOM
    std::ofstream{"text.txt"} << "\ufeffz\u6c34\U0001d10b";
 
    // read the UTF-8 file, skipping the BOM
    std::wifstream fin{"text.txt"};
    fin.imbue(std::locale(fin.getloc(),
                          new std::codecvt_utf8<wchar_t, 0x10ffff, std::consume_header>));
 
    for (wchar_t c; fin.get(c);)
        std::cout << std::hex << std::showbase << (std::wint_t)c << '\n';
}

Output: 

0x7a
0x6c34
0x1d10b

[edit] See also

 converts between character encodings, including UTF-8, UTF-16, UTF-32
(class template) [edit]
(C++11)(deprecated in C++17)(removed in C++26)
 converts between UTF-8 and UCS-2/UCS-4
(class template) [edit]
(C++11)(deprecated in C++17)(removed in C++26)
 converts between UTF-16 and UCS-2/UCS-4
(class template) [edit]
(C++11)(deprecated in C++17)(removed in C++26)
 converts between UTF-8 and UTF-16
(class template) [edit]
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