[maccard]

Serialization is the canonical example. Being able to turn

    struct MyStruct {
      int val = 42;
      string name = "my name";
    };
    

into

    {
      "val": 42, // if JSON had integers, and comments of course
      "name": "my name",
    }

is incredibly powerfuly. If reflection supported attributes (i can't believe it shipped without, honestly), then you could also mark members as [[ignore]] and skip them.

[SuperV1234]

You can achieve that since C++20 (or C++17 if you don't care about the member names). E.g. https://www.linkedin.com/posts/vittorioromeo_cpp-gamedev-ref...

(The link above shows ImGui generation, but the same exact logic can be applied for serialiation to JSON/YAML/whatever.)

[maccard]

Sure, but

> The magic sauce? Boost.PFR! An incredibly clever library that enables reflections on aggregates, even in C++17.

That's not vanilla C++!

[SuperV1234]

...so what? It's just a header you have to #include.

[maccard]

By that logic why would anything have to be standardised?

[gpderetta]

The question is whether something belong in the language or in a library (possibly the standard library).

A guiding principle of C++ is that if something can be implemented cleanly and efficiently in a library, the language should not be extended to support the use case.

Now boost.pfr is exceedingly clever, but relying on speculative pack expansions or using stateful metaprogramming hacks is not something I would call clean and efficient, so proper reflection is warranted.

I do worry about the compile time impact though.

[jcelerier]

the only thing that should be standardized are things that cannot be done through libraries in an efficient way. Boost.PFR is great, I built a lot of things on it, but eventually you hit the limits of what a pure library approach can do -> language feature.

[SuperV1234]

By your logic we shouldn't ever use external libraries.

PFR has given us reflection since C++14.

I also don't think the Standard Library is particularly well-defined nor well-implemented, as demonstrated by the atrocious compilation times.

[bluGill]

The standard library shows the roots of the mid-1990s and there are a lot of things we would definitely do different today. However, it is still extremely well defined compared to most everything else. C++ is one of the few languages where the library actually guarantees how an algorithm works, which is both good and bad. The bad part is some things that made perfect sense in 1995 simply don't make sense in our modern CPUs where cache is important.

[bluGill]

It is powerful, but I'm not sure it is a good idea. Other languages have it, and there is lots of experience in all the ways things go wrong in the real world. I'm inclined to say you should hand write this code because eventually you will discover something weird anyway.

[electroly]

Can you give an example of a language ecosystem that went with reflection-based JSON serialization/deserialization and then went on to regret it? I can't think of any, and don't agree with your conclusion. It works great, and manually writing serialization and matching deserialization code is terrible, annoying, error-prone work.

[maccard]

I disagree. Rust's defacto default is serde, golang comes with batteries included, dotnet/java have had it for _years_, and all the dynamic languages do it.

[SuperV1234]

I think this is a very bad take -- once you write it by hand you have to manually keep it in sync with the actual struct and ensure you made no mistakes. Reflection guarantees 1-1 future-proof mapping with the actual C++ struct, avoids boilerplate, and ensures that the serialization logic is correct.

[bluGill]

The protocol is important though, not the internal structure. When you only have exactly one version of a program talking to the same version of itself you don't care. However when you are mixing versions or worse programming language (and thus can't mix structs which are implementation details of your language) the protocol is what matters.

That is if you are worried about doing this by hand reflection is not the answer, something like protobuf where your data structures are generated is the answer.

[gpderetta]

I completely understand your point. Then again you might be able to use reflection to verify that your manually rolled implementation actually serializes all fields.