wro.cpp
short

simdjson 5.0 reads a struct from JSON keys in any order

· english· audience: working-cpp· AI-generated, reviewed by Filip Sajdak

On Compiler Explorer, GCC 16.2 and simdjson 5.0.2 read a struct from a JSON object with shuffled keys. The program exits with 0 and prints every field. The June post covered the writing side, sb << my_struct. This one covers reading.

What the 5.0 release notes say

I read the simdjson release page for 5.0.0 to 5.0.3 on 10 October 2026. Three points matter here.

  • Reflection “is no longer guarded and is an officially supported feature”. simdjson detects a reflection-enabled compiler by itself.
  • Deserializing a struct through reflection uses key selectors by default. It reads the object in a single pass, whatever the order of the keys.
  • A positive integer in the range from 2^64 up to 10^20 is now a big integer instead of a malformed number.

The patch releases 5.0.1 (28 Sep), 5.0.2 (4 Oct) and 5.0.3 (8 Oct) list only fixes.

Requirements

The documentation for 5.0.3 builds with g++ -std=c++26 -freflection on GCC 16. The June post enabled reflection with a define, while the notes now say simdjson detects it. Key selectors need C++20.

Reading shuffled keys

The struct has a string, an int, a bool, a nested struct and a vector. The JSON lists the keys in an order that matches none of them. The program also asks for the number type of 2^64 - 1 and of 2^64.

Tested: GCC 16.2 + simdjson 5.0.2 only
Run on Compiler ExplorerReading a struct from shuffled JSON keys-std=c++26 -freflection -O2 · simdjson 5.0.2 · runs live

The run prints this, with exit code 0:

name=Ada age=42 active=true
home=Wroclaw 50001
tags=2 cpp,json
18446744073709551615 -> uint64
18446744073709551616 -> big_integer

The output shows correct fields. It does not show a single pass, which rests on the documentation.

What the documentation adds

The page says the key-selector path builds a compile-time perfect hash from the member names. It falls back to one lookup per member for names over 63 characters. It adds that neither path is a universal speedup, and the per-member path “can edge ahead” when keys follow declaration order.

The same page still labels simdjson::from and key selectors experimental.

What the 1.8x figure measures

The notes report a 1.8x speedup for parsing a file full of escaped Unicode, from 1.59 to 2.85 GB/s. The author compared simdjson 4.0 and 5.0 on DOM parsing, using GCC 16.1 at -O3 on one Xeon Gold 6548N core. It is not a reflection result.

What this has not been tested on

  • clang-p2996 or any other compiler than GCC 16.2.
  • simdjson 5.0.0, 5.0.1 or 5.0.3. The demo uses the 5.0.2 package on Compiler Explorer.
  • My own hardware: the demo ran on Compiler Explorer only.
  • Performance: I measured nothing, and the 1.8x figure above is the author’s.
  • Annotations. The June post sketched [[simdjson::rename("x")]]. The 5.0.0 notes show [[= simdjson::rename<"KEY">]], and I did not run either form.

Sources, all read on 10 October 2026: