Implementation:Alibaba MNN RapidJSON Writer

Knowledge Sources	Alibaba_MNN RapidJSON
Domains	JSON_Parsing, Serialization
Last Updated	2026-02-10 12:00 GMT

Overview

3rd_party/rapidjson/writer.h (709 lines) implements a JSON writer/serializer for the vendored RapidJSON library within Alibaba MNN. The Writer class implements the Handler concept, generating JSON text from programmatic API calls or from DOM tree traversal via Document::Accept(). It produces compact (non-pretty-printed) JSON output.

Usage note: Vendored dependency used internally by MNN for JSON configuration parsing (model configs, LLM configs). Not directly imported by end users.

Key Class

Writer

template<typename OutputStream,
         typename SourceEncoding = UTF8<>,
         typename TargetEncoding = UTF8<>,
         typename StackAllocator = CrtAllocator,
         unsigned writeFlags = kWriteDefaultFlags>
class Writer {
public:
    typedef typename SourceEncoding::Ch Ch;
    static const int kDefaultMaxDecimalPlaces = 324;

    explicit Writer(OutputStream& os,
                    StackAllocator* stackAllocator = 0,
                    size_t levelDepth = kDefaultLevelDepth);

    explicit Writer(StackAllocator* allocator = 0,
                    size_t levelDepth = kDefaultLevelDepth);

    // Reset for reuse
    void Reset(OutputStream& os);

    // Handler interface
    bool Null();
    bool Bool(bool b);
    bool Int(int i);
    bool Uint(unsigned u);
    bool Int64(int64_t i);
    bool Uint64(uint64_t u);
    bool Double(double d);
    bool RawNumber(const Ch* str, SizeType length, bool copy = false);
    bool String(const Ch* str, SizeType length, bool copy = false);
    bool StartObject();
    bool Key(const Ch* str, SizeType length, bool copy = false);
    bool EndObject(SizeType memberCount = 0);
    bool StartArray();
    bool EndArray(SizeType elementCount = 0);
    bool RawValue(const Ch* json, size_t length, Type type);

    // Configuration
    void SetMaxDecimalPlaces(int maxDecimalPlaces);
    bool IsComplete() const;
};

Write Flags

enum WriteFlag {
    kWriteNoFlags = 0,
    kWriteValidateEncodingFlag = 1,  // Validate encoding of JSON strings
    kWriteNanAndInfFlag = 2,         // Allow writing of Infinity, -Infinity and NaN
    kWriteDefaultFlags = RAPIDJSON_WRITE_DEFAULT_FLAGS
};

Number Serialization

The writer uses optimized internal routines for number output:

• Integers -- Uses internal/itoa.h for fast integer-to-string conversion via digit lookup tables
• Doubles -- Uses internal/dtoa.h implementing the Grisu2 algorithm for fast and accurate double-to-string conversion
• Decimal places -- Configurable via SetMaxDecimalPlaces() (default: 324, i.e., full precision)

SIMD Acceleration

Like the Reader, the Writer supports SIMD-accelerated string scanning for characters that need escaping:

• SSE4.2 (via RAPIDJSON_SSE42)
• SSE2 (via RAPIDJSON_SSE2)
• NEON (via RAPIDJSON_NEON)

String Escaping

JSON strings are output with proper escaping for control characters and Unicode:

Character	Escape Sequence
"	\"
\	\\
Backspace (U+0008)	\b
Form feed (U+000C)	\f
Line feed (U+000A)	\n
Carriage return (U+000D)	\r
Tab (U+0009)	\t
Other control chars (U+0000-U+001F)	\uXXXX

Dependencies

• stream.h -- Output stream abstractions
• internal/meta.h -- Template metaprogramming utilities
• internal/stack.h -- Internal stack for nesting level tracking
• internal/strfunc.h -- String utility functions
• internal/dtoa.h -- Double-to-string (Grisu2 algorithm)
• internal/itoa.h -- Integer-to-string conversion
• stringbuffer.h -- In-memory string output stream

License

MIT License. Copyright (C) 2015 THL A29 Limited (Tencent) and Milo Yip.

See Also

• Alibaba_MNN_RapidJSON_Reader -- SAX parser (inverse operation)
• Alibaba_MNN_RapidJSON_Document -- DOM document with Accept() for serialization
• Alibaba_MNN_RapidJSON_Itoa -- Integer-to-string conversion used by Writer