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Wrote a part of SAX

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# SAX
## Reader
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}
`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 the JSON, it will publish 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 match up with the JSON, except some event parameters need further explanation. Let's see the code which produces exactly the same output as above:
~~~~~~~~~~cpp
#include "rapidjson/reader.h"
#include <iostream>
using namespace rapidjson;
using namespace std;
struct MyHandler {
void Null() { cout << "Null()" << endl; }
void Bool(bool b) { cout << "Bool(" << (b ? "true" : "false") << ")" << endl; }
void Int(int i) { cout << "Int(" << i << ")" << endl; }
void Uint(unsigned u) { cout << "Uint(" << u << ")" << endl; }
void Int64(int64_t i) { cout << "Int64(" << i << ")" << endl; }
void Uint64(uint64_t u) { cout << "Uint64(" << u << ")" << endl; }
void Double(double d) { { cout << "Double(" << d << ")" << endl; }
void String(const char* str, SizeType length, bool copy) {
cout << "String(" << str << ", " << length << ", " << (b ? "true" : "false") << ")" << endl; }
void StartObject() { cout << "StartObject()" << endl; }
void EndObject(SizeType memberCount) { cout << "EndObject(" << memberCount << ")" << endl; }
void StartArray() { cout << "StartArray()" << endl; }
void EndArray(SizeType elementCount) { cout << "EndArray(" << elementCount << ")" << endl; }
};
void main() {
const char* json = "...";
MyHandler handler;
Reader<MyHandler> reader;
StringStream ss(json);
reader.Parse(ss, handler);
}
~~~~~~~~~~
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 concept has the following member type and member functions.
~~~~~~~~~~cpp
concept Handler {
typename Ch;
void Null();
void Bool(bool b);
void Int(int i);
void Uint(unsigned i);
void Int64(int64_t i);
void Uint64(uint64_t i);
void Double(double d);
void String(const Ch* str, SizeType length, bool copy);
void StartObject();
void EndObject(SizeType memberCount);
void StartArray();
void EndArray(SizeType elementCount);
};
~~~~~~~~~~
`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.
## GenericReader {#GenericReader}
As mentioned before, `Reader` is a typedef of a template class `GenericReader`:
~~~~~~~~~~cpp
namespace rapidjson {
template <typename SourceEncoding, typename TargetEncoding, typename Allocator = MemoryPoolAllocator<> >
class GenericReader {
// ...
};
typedef GenericReader<UTF8<>, UTF8<> > Reader;
} // namespace rapidjson
~~~~~~~~~~
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.
~~~~~~~~~~cpp
template <unsigned parseFlags, typename InputStream, typename Handler>
bool Parse(InputStream& is, Handler& handler);
// with parseFlags = kDefaultParseFlags
template <typename InputStream, typename Handler>
bool Parse(InputStream& is, Handler& handler);
~~~~~~~~~~
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}
## PrettyWriter {#PrettyWriter}
# 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.
Example:
~~~~~~~~~~cpp
// Note: Ad hoc, not yet tested.
using namespace std;
using namespace rapidjson;
typedef map<string, string> MessageMap;
struct MessageHandler : public GenericBaseHandler<> {
MessageHandler() : mState(kExpectStart) {
}
bool Default() {
return false;
}
bool StartObject() {
if (!kBeforeStart)
return false;
mState = mExpectName;
return true;
}
bool String(const Ch* str, SizeType length, bool copy) {
if (mState == kExpectName) {
name_ = string(str, length);
return true;
}
else if (mState == kExpectValue) {
messages_.insert(MessageMap::value_type(name_, string(str, length)));
return true;
}
else
return false;
}
bool EndObject() {
return mState == kExpectName;
}
MessageMap messages_;
enum State {
kExpectObjectStart,
kExpectName,
kExpectValue,
}mState;
std::string name_;
};
void ParseMessages(const char* json, MessageMap& messages) {
Reader reader;
MessageHandler handler;
StringStream ss(json);
if (reader.Parse(ss, handler))
messages.swap(handler.messages_);
}
main() {
MessageMap messages;
ParseMessages("{ \"greeting\" : \"Hello!\", \"farewell\" : \"bye-bye!\" }", messages);
}
~~~~~~~~~~
~~~~~~~~~~cpp
// Parse a NxM array
const char* json = "[3, 4, [1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]"
~~~~~~~~~~
## Filtering of JSON {#Filtering}
### PrettyWriter