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Sajson: The Definitive Guide to Lightweight C++ JSON Parsing (2025 Analysis)

In the landscape of C++ JSON libraries, sajson occupies a distinctive niche: it trades features for raw speed and minimal memory footprint. Developed by Google and used internally across several production services, sajson has earned a reputation among performance-conscious engineers who need fast JSON deserialization without the bloat of heavier alternatives. This article provides a data-driven deep dive into sajson's architecture, performance characteristics, API design, and where it stands against the broader ecosystem.

What Is sajson?

sajson (pronounced "sah-json") is a header-only, zero-dependency C++ JSON parser created by Google. Unlike general-purpose JSON libraries such as nlohmann/json or RapidJSON, sajson was designed with a singular focus: extreme parse speed with negligible memory allocation.

The library parses an entire JSON document into a compact, self-describing memory arena in a single pass. Each value object (number, string, array, object) holds a tag and a pointer into the arena — there are no per-node heap allocations. This design mirrors the philosophy behind other high-performance parsers like simdjson, but with a more conservative, portable implementation.

Design Philosophy

sajson's architecture is built on three core principles:

1. Precision via arena model: All string data lives in the original input buffer. Numbers are stored as doubles. No round-trip conversion losses beyond IEEE 754 precision.
2. Safety through tags: Every value object carries an enum tag (NULL_TAG, BOOL_TAG, NUMBER_TAG, STRING_TAG, ARRAY_TAG, OBJECT_TAG) that enforces type-safe access at compile time.
3. Read-only access model: sajson deliberately omits serialization and mutation APIs. It is a parser and accessor — not a full JSON toolkit.

Architecture Deep Dive

The Arena Model

At the heart of sajson lies the arena allocator. When you invoke parse(), the library allocates one contiguous block of memory sized to hold:

• The raw input buffer (or a reference to it)
• A flat array of value structs, one per JSON node
• Metadata for nested structures (object key indices, array length fields)

This means a JSON document with 10,000 keys generates exactly one allocation — not 10,000. For comparison, a naive recursive parser might perform thousands of individual new or malloc calls.

The Value Type System

Every parsed node is represented by the sajson::value type, which is a small struct containing:

Field	Type	Purpose
tag_	uint8_t	JSON value type enum (6 possible values)
index_	uint32_t	Offset into the arena — points to children, string data, or object entries

This two-field representation keeps each value at just 5 bytes (with padding to 8 bytes for alignment), enabling cache-friendly traversal even for deeply nested documents.

Benchmark Comparison: sajson vs. Competitors

The following table compares sajson against widely used C++ JSON libraries across four critical dimensions. Data is synthesized from public benchmarks, GitHub repository metrics, and community-reported timings on a standardized Intel Core i7-12700K platform using a 5MB JSON payload (mixed nested objects and arrays).

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Library	Parse Speed (MB/s)	Memory Allocations	Serialization Support	Header-Only	License
sajson	~850	1 (arena)	No	Yes	MIT
nlohmann/json	~120	10,000+	Yes	Yes	MIT
RapidJSON	~620	0–50	Yes	Yes	MIT
simdjson	~2,100	0–2	No (read-only)	Yes	Apache 2.0
fastjson (Facebook)	~780	Minimal	Limited	No	BSD