C++26 marks a transformative milestone with the adoption of full compile-time reflection, enabling powerful new metaprogramming capabilities. In this post, we’ll explore how reflection lets you turn a JSON file directly into a fully-typed C++ object — all at compile time.

Reflecting JSON into C++ Objects

by Barry Revzin

From the article:

Last week, C++26 was finalized in Sofia, Bulgaria — and C++26 will include all of the reflection papers that we were pushing for:

1. P2996R13: Reflection for C++26
2. P3394R4: Annotations for Reflection
3. P3293R3: Splicing a Base Class Subobject
4. P3491R3: define_static_{string,object,array}
5. P1306R5: Expansion Statements
6. P3096R12: Function Parameter Reflection in Reflection for C++26
7. P3560R2: Error Handling in Reflection

Those are in the order in which they were adopted, not in the order of their impact (otherwise splicing base classes would go last). This is a pretty incredible achievement that couldn’t have happened without lots of people’s work, but no one person is more responsible for Reflection in C++26 than Dan Katz.

So today I wanted to talk about a very cool example that Dan put together on the flight home from Sofia, while I was unconscious a few seats over: the ability to, at compile time, ingest a JSON file and turn it into a C++ object. That is, given a file test.json that looks like this:

{ "outer": "text", 
"inner": { "field": "yes", "number": 2996 } 
} 

We can write this:

constexpr const char data[] = { 
     #embed "test.json" 
     , 0 

}; constexpr auto v = json_to_object<data>;