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The term "SAX" originated from Simple API for XML. We borrowed this term for JSON parsing and generation.

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.


Reader parses a JSON from a stream. While it reads characters from the stream, it analyzes the characters according to the syntax of JSON, and publishes events to a handler.

For example, here is a JSON.

"hello": "world",
"t": true ,
"f": false,
"n": null,
"i": 123,
"pi": 3.1416,
"a": [1, 2, 3, 4]

When a Reader parses this JSON, it publishes the following events to the handler sequentially:

Key("hello", 5, true)
String("world", 5, true)
Key("t", 1, true)
Key("f", 1, true)
Key("n", 1, true)

These events can be easily matched with the JSON, but some event parameters need further explanation. Let's see the simplereader example which produces exactly the same output as above:

#include <iostream>
using namespace rapidjson;
using namespace std;
struct MyHandler : public BaseReaderHandler<UTF8<>, MyHandler> {
bool Null() { cout << "Null()" << endl; return true; }
bool Bool(bool b) { cout << "Bool(" << boolalpha << b << ")" << endl; return true; }
bool Int(int i) { cout << "Int(" << i << ")" << endl; return true; }
bool Uint(unsigned u) { cout << "Uint(" << u << ")" << endl; return true; }
bool Int64(int64_t i) { cout << "Int64(" << i << ")" << endl; return true; }
bool Uint64(uint64_t u) { cout << "Uint64(" << u << ")" << endl; return true; }
bool Double(double d) { cout << "Double(" << d << ")" << endl; return true; }
bool String(const char* str, SizeType length, bool copy) {
cout << "String(" << str << ", " << length << ", " << boolalpha << copy << ")" << endl;
return true;
bool StartObject() { cout << "StartObject()" << endl; return true; }
bool Key(const char* str, SizeType length, bool copy) {
cout << "Key(" << str << ", " << length << ", " << boolalpha << copy << ")" << endl;
return true;
bool EndObject(SizeType memberCount) { cout << "EndObject(" << memberCount << ")" << endl; return true; }
bool StartArray() { cout << "StartArray()" << endl; return true; }
bool EndArray(SizeType elementCount) { cout << "EndArray(" << elementCount << ")" << endl; return true; }
void main() {
const char json[] = " { \"hello\" : \"world\", \"t\" : true , \"f\" : false, \"n\": null, \"i\":123, \"pi\": 3.1416, \"a\":[1, 2, 3, 4] } ";
MyHandler handler;
Reader reader;
StringStream ss(json);
reader.Parse(ss, handler);

Note that RapidJSON uses templates to statically bind the Reader type and the handler type, instead of using classes with virtual functions. This paradigm can improve performance by inlining functions.


As shown in the previous example, the user needs to implement a handler which consumes the events (via function calls) from the Reader. The handler must contain the following member functions.

class Handler {
bool Null();
bool Bool(bool b);
bool Int(int i);
bool Uint(unsigned i);
bool Int64(int64_t i);
bool Uint64(uint64_t i);
bool Double(double d);
bool RawNumber(const Ch* str, SizeType length, bool copy);
bool String(const Ch* str, SizeType length, bool copy);
bool StartObject();
bool Key(const Ch* str, SizeType length, bool copy);
bool EndObject(SizeType memberCount);
bool StartArray();
bool 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). If kParseNumbersAsStrings is enabled, Reader will always calls RawNumber() instead.

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 be aware 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 Key() for the name of member, and then calls functions depending on the type of the value. These calls of name-value pairs repeat until calling EndObject(SizeType memberCount). Note that the memberCount parameter is just an aid for the handler; users who do not need this parameter may ignore it.

Arrays are similar to objects, 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 function returns a bool. Normally it should return true. If the handler encounters an error, it can return false to notify the event publisher to stop further processing.

For example, when we parse a JSON with Reader and the handler detects that the JSON does not conform to the required schema, the handler can return false and let the Reader stop further parsing. This will place the Reader in an error state, with error code kParseErrorTermination.


As mentioned before, Reader is a typedef of a template class GenericReader:

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 output UTF-16 string events, you can define a reader by:

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).

The main function of Reader is used to parse JSON.

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 call bool HasParseError(), ParseErrorCode GetParseErrorCode() and size_t GetErrorOffset() to obtain the error states. In fact, Document uses these Reader functions to obtain parse errors. Please refer to DOM for details about parse errors.

Token-by-Token Parsing

Some users may wish to parse a JSON input stream a single token at a time, instead of immediately parsing an entire document without stopping. To parse JSON this way, instead of calling Parse, you can use the IterativeParse set of functions:

void IterativeParseInit();
template <unsigned parseFlags, typename InputStream, typename Handler>
bool IterativeParseNext(InputStream& is, Handler& handler);
bool IterativeParseComplete();

Here is an example of iteratively parsing JSON, token by token:

while (!reader.IterativeParseComplete()) {
reader.IterativeParseNext<kParseDefaultFlags>(is, handler);
// Your handler has been called once.


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.

#include "rapidjson/writer.h"
#include "rapidjson/stringbuffer.h"
#include <iostream>
using namespace rapidjson;
using namespace std;
void main() {
for (unsigned i = 0; i < 4; i++)
cout << s.GetString() << endl;

There are two String() and Key() 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.
  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.

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.

namespace rapidjson {
template<typename OutputStream, typename SourceEncoding = UTF8<>, typename TargetEncoding = UTF8<>, typename Allocator = CrtAllocator<>, unsigned writeFlags = kWriteDefaultFlags>
class Writer {
Writer(OutputStream& os, Allocator* allocator = 0, size_t levelDepth = kDefaultLevelDepth)
// ...
} // namespace rapidjson

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 Allocator is the type of allocator, which is used for allocating internal data structure (a stack).

The writeFlags are combination of the following bit-flags:

Parse flags Meaning
kWriteNoFlags No flag is set.
kWriteDefaultFlags Default write flags. It is equal to macro RAPIDJSON_WRITE_DEFAULT_FLAGS, which is defined as kWriteNoFlags.
kWriteValidateEncodingFlag Validate encoding of JSON strings.
kWriteNanAndInfFlag Allow writing of Infinity, -Infinity and NaN.

Besides, the constructor of Writer has a levelDepth parameter. This parameter affects the initial memory allocated for storing information per hierarchy level.


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

A Writer can only output a single JSON, which can be any JSON type at the root. Once the singular event for root (e.g. String()), or 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.