According to ECMA-404,
(in Introduction) JSON text is a sequence of Unicode code points.
The earlier RFC4627 stated that,
(in §3) JSON text SHALL be encoded in Unicode. The default encoding is UTF-8.
(in §6) JSON may be represented using UTF-8, UTF-16, or UTF-32. When JSON is written in UTF-8, JSON is 8bit compatible. When JSON is written in UTF-16 or UTF-32, the binary content-transfer-encoding must be used.
RapidJSON supports various encodings. It can also validate the encodings of JSON, and transconding JSON among encodings. All these features are implemented internally, without the need for external libraries (e.g. ICU).
Unicode provides a unique number for every character, no matter what the platform, no matter what the program, no matter what the language.
Those unique numbers are called code points, which is in the range
There are various encodings for storing Unicode code points. These are called Unicode Transformation Format (UTF). RapidJSON supports the most commonly used UTFs, including
For UTF-16 and UTF-32, the byte order (endianness) does matter. Within computer memory, they are often stored in the computer's endianness. However, when it is stored in file or transferred over network, we need to state the byte order of the byte sequence, either little-endian (LE) or big-endian (BE).
RapidJSON provide these encodings via the structs in
For processing text in memory, we normally use
UTF32. For processing text via I/O, we may use
When using the DOM-style API, the
Encoding template parameter in
GenericDocument<Encoding> indicates the encoding to be used to represent JSON string in memory. So normally we will use
UTF32 for this template parameter. The choice depends on operating systems and other libraries that the application is using. For example, Windows API represents Unicode characters in UTF-16, while most Linux distributions and applications prefer UTF-8.
Example of UTF-16 DOM declaration:
For a detail example, please check the example in DOM's Encoding section.
As shown in the declaration, each encoding has a
CharType template parameter. Actually, it may be a little bit confusing, but each
CharType stores a code unit, not a character (code point). As mentioned in previous section, a code point may be encoded to 1–4 code units for UTF-8.
CharType must be integer type of at least 2 and 4 bytes respectively.
Note that C++11 introduces
char32_t, which can be used for
Previous encodings are statically bound in compile-time. In other words, user must know exactly which encodings will be used in the memory or streams. However, sometimes we may need to read/write files of different encodings. The encoding needed to be decided in runtime.
AutoUTF is an encoding designed for this purpose. It chooses which encoding to be used according to the input or output stream. Currently, it should be used with
Although the JSON standards did not mention about ASCII, sometimes we would like to write 7-bit ASCII JSON for applications that cannot handle UTF-8. Since any JSON can represent unicode characters in escaped sequence
\uXXXX, JSON can always be encoded in ASCII.
Here is an example for writing a UTF-8 DOM into ASCII:
ASCII can be used in input stream. If the input stream contains bytes with values above 127, it will cause
ASCII cannot be used in memory (encoding of
Document or target encoding of
Reader), as it cannot represent Unicode code points.
When RapidJSON parses a JSON, it can validate the input JSON, whether it is a valid sequence of a specified encoding. This option can be turned on by adding
parseFlags template parameter.
If the input encoding and output encoding is different,
Writer will automatically transcode (convert) the text. In this case,
kParseValidateEncodingFlag is not necessary, as it must decode the input sequence. And if the sequence was unable to be decoded, it must be invalid.
Although the encoding functions in RapidJSON are designed for JSON parsing/generation, user may abuse them for transcoding of non-JSON strings.
Here is an example for transcoding a string from UTF-8 to UTF-16:
You may also use
AutoUTF and the associated streams for setting source/target encoding in runtime.