The term "SAX" originated from [Simple API for XML](http://en.wikipedia.org/wiki/Simple_API_for_XML). We borrowed this term for JSON parsing and generation.
### Handler
In RapidJSON, `Reader` (typedef of `GenericReader<...>`) is the SAX-style parser for JSON, and `Writer` (typedef of `GenericWriter<...>`) is the SAX-style generator for JSON.
### Parse Error
[TOC]
## Writer
# Reader {#Reader}
### PrettyWriter
`Reader` parses a JSON from a stream. While it reads characters from the stream, it analyze the characters according to the syntax of JSON, and publish events to a handler.
For example, here is a JSON.
~~~~~~~~~~js
{
"hello": "world",
"t": true ,
"f": false,
"n": null,
"i": 123,
"pi": 3.1416,
"a": [1, 2, 3, 4]
}
~~~~~~~~~~
While a `Reader` parses this JSON, it publishes the following events to the handler sequentially:
~~~~~~~~~~
BeginObject()
String("hello", 5, true)
String("world", 5, true)
String("t", 1, true)
Bool(true)
String("f", 1, true)
Bool(false)
String("n", 1, true)
Null()
String("i")
UInt(123)
String("pi")
Double(3.1416)
String("a")
BeginArray()
Uint(1)
Uint(2)
Uint(3)
Uint(4)
EndArray(4)
EndObject(7)
~~~~~~~~~~
These events can be easily matched with the JSON, except some event parameters need further explanation. Let's see the `simplereader` example which produces exactly the same output as above:
Note that, RapidJSON uses template to statically bind the `Reader` type and the handler type, instead of using class with virtual functions. This paradigm can improve the performance by inlining functions.
## Handler {#Handler}
As the previous example showed, user needs to implement a handler, which consumes the events (function calls) from `Reader`. The handler must contain the following member functions.
`Null()` is called when the `Reader` encounters a JSON null value.
`Bool(bool)` is called when the `Reader` encounters a JSON true or false value.
When the `Reader` encounters a JSON number, it chooses a suitable C++ type mapping. And then it calls *one* function out of `Int(int)`, `Uint(unsigned)`, `Int64(int64_t)`, `Uint64(uint64_t)` and `Double(double)`.
`String(const char* str, SizeType length, bool copy)` is called when the `Reader` encounters a string. The first parameter is pointer to the string. The second parameter is the length of the string (excluding the null terminator). Note that RapidJSON supports null character `'\0'` inside a string. If such situation happens, `strlen(str) < length`. The last `copy` indicates whether the handler needs to make a copy of the string. For normal parsing, `copy = true`. Only when *insitu* parsing is used, `copy = false`. And beware that, the character type depends on the target encoding, which will be explained later.
When the `Reader` encounters the beginning of an object, it calls `StartObject()`. An object in JSON is a set of name-value pairs. If the object contains members it first calls `String()` for the name of member, and then calls functions depending on the type of the value. These calls of name-value pairs repeats until calling `EndObject(SizeType memberCount)`. Note that the `memberCount` parameter is just an aid for the handler, user may not need this parameter.
Array is similar to object but simpler. At the beginning of an array, the `Reader` calls `BeginArary()`. If there is elements, it calls functions according to the types of element. Similarly, in the last call `EndArray(SizeType elementCount)`, the parameter `elementCount` is just an aid for the handler.
Every handler functions returns a `bool`. Normally it should returns `true`. If the handler encounters an error, it can return `false` to notify event publisher to stop further processing.
For example, when we parse a JSON with `Reader` and the handler detected that the JSON does not conform to the required schema, then the handler can return `false` and let the `Reader` stop further parsing. And the `Reader` will be in error state with error code `kParseErrorTermination`.
## GenericReader {#GenericReader}
As mentioned before, `Reader` is a typedef of a template class `GenericReader`:
The `Reader` uses UTF-8 as both source and target encoding. The source encoding means the encoding in the JSON stream. The target encoding means the encoding of the `str` parameter in `String()` calls. For example, to parse a UTF-8 stream and outputs UTF-16 string events, you can define a reader by:
~~~~~~~~~~cpp
GenericReader<UTF8<>, UTF16<> > reader;
~~~~~~~~~~
Note that, the default character type of `UTF16` is `wchar_t`. So this `reader`needs to call `String(const wchar_t*, SizeType, bool)` of the handler.
The third template parameter `Allocator` is the allocator type for internal data structure (actually a stack).
## Parsing {#Parsing}
The one and only one function of `Reader` is to parse JSON.
If an error occurs during parsing, it will return `false`. User can also calls `bool HasParseEror()`, `ParseErrorCode GetParseErrorCode()` and `size_t GetErrorOffset()` to obtain the error states. Actually `Document` uses these `Reader` functions to obtain parse errors. Please refer to [DOM](doc/dom.md) for details about parse error.
# Writer {#Writer}
`Reader` converts (parses) JSON into events. `Writer` does exactly the opposite. It converts events into JSON.
`Writer` is very easy to use. If your application only need to converts some data into JSON, it may be a good choice to use `Writer` directly, instead of building a `Document` and then stringifying it with a `Writer`.
In `simplewriter` example, we do exactly the reverse of `simplereader`.
There is two `String()` overloads. One is the same as defined in handler concept with 3 parameters. It can handle string with null characters. Another one is the simpler version used in the above example.
Note that, the example code does not pass any parameters in `EndArray()` and `EndObject()`. An `SizeType` can be passed but it will be simply ignored by `Writer`.
You may doubt that, why not just using `sprintf()` or `std::stringstream` to build a JSON?
There are various reasons:
1.`Writer` must output a well-formed JSON. If there is incorrect event sequence (e.g. `Int()` just after `StartObject()`), it generates assertion fail in debug mode.
2.`Writer::String()` can handle string escaping (e.g. converting code point `U+000A` to `\n`) and Unicode transcoding.
3.`Writer` handles number output consistently. For example, user can set precision for `Double()`.
4.`Writer` implements the event handler concept. It can be used to handle events from `Reader`, `Document` or other event publisher.
5.`Writer` can be optimized for different platforms.
Anyway, using `Writer` API is even simpler than generating a JSON by ad hoc methods.
## Template {#WriterTemplate}
`Writer` has a minor design difference to `Reader`. `Writer` is a template class, not a typedef. There is no `GenericWriter`. The following is the declaration.
The `OutputStream` template parameter is the type of output stream. It cannot be deduced and must be specified by user.
The `SourceEncoding` template parameter specifies the encoding to be used in `String(const Ch*, ...)`.
The `TargetEncoding` template parameter specifies the encoding in the output stream.
The last one, `Allocator` is the type of allocator, which is used for allocating internal data structure (a stack).
Besides, the constructor of `Writer` has a `levelDepth` parameter. This parameter affects the initial memory allocated for storing information per hierarchy level.
## Precision (#WriterPrecision)
When using `Double()`, the precision of output can be specified, for example:
~~~~~~~~~~cpp
writer.SetDoublePrecision(4);
writer.StartArary();
writer.Double(3.14159265359);
writer.EndArray();
~~~~~~~~~~
~~~~~~~~~~
[3.1416]
~~~~~~~~~~
## PrettyWriter {#PrettyWriter}
While the output of `Writer` is the most condensed JSON without white-spaces, suitable for network transfer or storage, it is not easily readable by human.
Therefore, RapidJSON provides a `PrettyWriter`, which adds indentation and line feeds in the output.
The usage of `PrettyWriter` is exactly the same as `Writer`, expect that `PrettyWriter` provides a `SetIndent(Ch indentChar, unsigned indentCharCount)` function. The default is 4 spaces.
## Completeness and Reset {#CompletenessReset}
A `Writer` can only output a single JSON, which can be either an object or array at the root. Once the last matching `EndObject()` or `EndArray()` event is handled, the output JSON is well-formed and complete. User can detect this state by calling `Writer::IsComplete()`.
When a JSON is complete, the `Writer` cannot accept any new events. Otherwise the output will be invalid (i.e. having more than one root). To reuse the `Writer` object, user can call `Writer::Reset(OutputStream& os)` to reset all internal states of the `Writer` with a new output stream.
# Techniques {#Techniques}
## Parsing JSON to Custom Data Structure {#CustomDataStructure}
`Document`'s parsing capability is completely based on `Reader`. Actually `Document` is a handler which receives events from a reader to build a DOM during parsing.
User may uses `Reader` to build other data structures directly. This eliminates building of DOM, thus reducing memory and improving performance.
In the following `messagereader` example, `ParseMessages()` parses a JSON which should be an object with key-string pairs.
~~~~~~~~~~cpp
#include "rapidjson/reader.h"
#include "rapidjson/error/en.h"
#include <iostream>
#include <string>
#include <map>
using namespace std;
using namespace rapidjson;
typedef map<string, string> MessageMap;
struct MessageHandler : public BaseReaderHandler<> {
The first JSON (`json1`) was successfully parsed into `MessageMap`. Since `MessageMap` is a `std::map`, the printing order are sorted by the key. This order is different from the JSON's order.
In the second JSON (`json2`), `foo`'s value is an empty object. As it is an object, `MessageHandler::StartObject()` will be called. However, at that moment `state_ = kExpectValue`, so that function returns `false` and cause the parsing process be terminated. The error code is `kParseErrorTermination`.
## Filtering of JSON {#Filtering}
As mentioned earlier, `Writer` can handle the events published by `Reader`. `condense` example simply set a `Writer` as handler of a `Reader`, so it can remove all white-spaces in JSON. `pretty` example uses the same relationship, but replacing `Writer` by `PrettyWriter`. So `pretty` can be used to reformat a JSON with indentation and line feed.
Actually, we can add intermediate layer(s) to filter the contents of JSON via these SAX-style API. For example, `capitalize` example capitalize all strings in a JSON.
Note that, it is incorrect to simply capitalize the JSON as a string. For example:
~~~~~~~~~~
["Hello\nWorld"]
~~~~~~~~~~
Simply capitalizing the whole JSON would contain incorrect escape character:
~~~~~~~~~~
["HELLO\NWORLD"]
~~~~~~~~~~
The correct result by `capitalize`:
~~~~~~~~~~
["HELLO\nWORLD"]
~~~~~~~~~~
More complicated filters can be developed. However, since SAX-style API can only provide information about a single event at a time, user may need to book-keeping the contextual information (e.g. the path from root value, storage of other related values). Some processing may be easier to be implemented in DOM than SAX.
