Parsing JSON at compile time with C++26 static reflection

Suppose that you have a configuration file in JSON. Something like this:

{ "width": 1920, "height": 1080, "fullscreen": true,
  "title": "My Game", "volume": 0.8 }

Normally you ship this file alongside your program, open it at startup, read it, and parse it. That is a lot of work for data that never changes. What if the file is fixed at build time? Could the compiler read it, parse it, and bake the result directly into the executable as a constant?

With C++26, the answer is yes. We need two new ingredients, all of which are usable right now with the latest version of the GCC compiler (16).

1. #embed to pull the file into the program at compile time,
2. A software library supporting static reflection like simdjson.

Let me show you how far we can take this.

The new #embed directive reads a file and expands it into a comma-separated list of byte values. To read the file data.json at compile time and keep it around as a constant, we write:

constexpr const char json_data[] = {
#embed "data.json"
    , 0
};

I use constexpr because I want the compiler to be allowed to inspect these bytes during constant evaluation. The trailing , 0 simply appends a null terminator, so the array can be treated as an ordinary C string.

There is no run-time input/output of any kind. The bytes are part of the program.

But embedded bytes are not yet useful by themselves. What I really want is a typed C++ object. In my example, the target type is this configuration struct:

struct Window {
  int         width;
  int         height;
  bool        fullscreen;
  std::string title;
  double      volume;
};

The traditional way to populate such a struct from JSON is to write, by hand, one line per field: read "width", store it into width, read "height", store it into height, and so on. It is tedious. And because it runs at startup, a malformed file becomes a run-time error, discovered by your users rather than by you.

Recent versions of simdjson can parse JSON at compile time using C++26 static reflection. The entry point is simdjson::compile_time::parse_json, and it does something I still find slightly magical: it reads the JSON and, from the keys it finds, and synthesises the struct type for you.

#define SIMDJSON_STATIC_REFLECTION 1
#include "simdjson.h"
constexpr const char json_data[] = {
#embed "data.json"
    , 0
};
constexpr auto window = simdjson::compile_time::parse_json<json_data>();

The variable window is a value computed entirely by the compiler. Its type is generated from the document: it has a width and a height (both 64-bit integers), a bool fullscreen, a double volume, and a title. From here on I write window.width and it behaves like any ordinary field.

How do I know the parsing really happened at compile time? Because I can assert things about the result that the compiler must check before the program even exists:

static_assert(window.width      == 1920);
static_assert(window.height     == 1080);
static_assert(window.fullscreen == true);

If I corrupt the JSON — delete a brace, misspell true, leave a trailing comma — the program no longer compiles, and the error points at the parse_json line. The broken file is caught at build time, on my machine, instead of at startup on someone else’s.

Because window is a genuine compile-time constant, any computation over it is a constant too. Consider this function:

int  screen_area()   { return window.width * window.height; }

Compiled with -O3, there is no multiplication, no field access, and certainly no parsing left — only the answers, as immediate values (here on my macBook):

screen_area:    mov  w0, #0xa400        // 0x1fa400 = 2073600
                movk w0, #0x1f, lsl #16
                ret

The JSON has vanished from the binary. It was read and parsed exactly once, by the compiler, and all that survives is the number 2073600.

Because static reflection is so new, when building with GCC 16, you need to pass the flags -std=c++26 -freflection: the -freflection flag is necessary to activate compile-time reflection You must also set the simdjson macro SIMDJSON_STATIC_REFLECTION=1 before importing the simdjson.h. It is a temporary safeguard.

The source code to reproduce these examples is available.

Reference: P2996 — Reflection for C++26 and the simdjson library.

Credit: The simdjson implementation is joint work with Francisco Geiman Thiesen.

Generated by RSStT. The copyright belongs to the original author.

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