alt.hn

5/13/2026 at 8:41:30 AM

Cost of enum-to-string: C++26 reflection vs. the old ways

 https://vittorioromeo.com/index/blog/refl_enum_to_string.html

by sagacity

5/13/2026 at 8:37:12 PM

> The header is the cost. Not the reflection. The reflection algorithm is fast – asymptotically ~0.07 ms per enumerator, essentially the same as the hand-rolled switch in the X-macro version (~0.06 ms). What makes reflection look expensive is <meta>: just including it costs ~155 ms per TU over the baseline.

So speaking of old ways, I'm not a C++ dev, but a while ago saw someone comment that they still organize their C++ projects using tips from John Lakos' Large-scale C++ software design from 1997, and that their compile times are incredibly fast. So I decided to find a digital copy on the high seas and read it out of historical curiosity. While I didn't finish it, one wild thing stood out to me: he advised for using redundant external include guards around every include, e.g.

     #ifndef INCLUDED_MATH
     #include <math>
     #define INCLUDED_MATH
     #endif
The reason for this being that (in 1997) every include required that the pre-processor opened the file just to check for an include guard and reading it all the way to the end to find the closing #endif, causing potentially O(N*2) disk read overhead (if anyone feels like verifying this, it's explained on pages 85 to 87).

Again, that was in 1997. I have no idea what mitigations for this problem exist in compilers by now, but I hope at least a few, right?

This conclusion is making me wonder if following that advice still would have a positive impact on compile times today after all though. Surely not, right? Can anyone more knowledgeable about this comment on that?

by vanderZwan

5/13/2026 at 8:46:10 PM

This cost is not significant nowadays, it's the frontend/parsing time.

You can also use `#pragma once` which works everywhere, is nicer, and technically needs less work by the compiler, but compilers have optimized for include guards since a long time ago.

Some random measurements I found: https://github.com/Return-To-The-Roots/s25client/issues/1073

by SuperV1234

5/13/2026 at 9:14:15 PM

Yes, I've heard that before, but comments like this one in your linked issue still make me wonder:

> at least for gcc and Visual Studio using #pragma once has a significant impact. The fact is, the compiler does not need to continue parsing the whole file when reaching a #pragma once. otherwise the compiler always needs to do it even if the include guard afterwards will avoid double processing of the content afterwards.

As written the explanation for these optimizationst suggest that both "pragma once" and include guard optimization still requires opening and closing the file each time an include is encountered, even if you bail after parsing the first line. Is that overhead zero? Or are the optimizations explained poorly and is repeatedly opening/closing the file also avoided?

Either way, do you know what causes the slowdown as a result of including <meta>?

by vanderZwan

5/14/2026 at 6:53:44 AM

The compiler doesn't need to open the same file multiple times. It can remember if a a file is guarded or not every time it sees its name.

My understanding is that this is an optimization that has been available for a very long time now.

The only issue is if a file is referred through multiple names (because of hard links, symlinks, mounts). That might cause the file to be opened again, and can actually break pragma once.

by gpderetta

5/13/2026 at 10:38:48 PM

The overhead isn't zero, but with SSDs (and filesystem caches in the gigabytes these days) it's damn near insignificant in pure terms of opening files and such.

by Quekid5

5/14/2026 at 12:03:37 AM

What I found (so far on MSVC) is that #pragma once does only process the file once, where as include guards still open the file each time it is included. Though it takes almost no time to do so but it still appears on the traces.

I'm going to experiment with other compilers and figure out how they handle it.

by daemin

5/13/2026 at 8:43:49 AM

Oof, that first example (the idiomatic C++26 way) looks so foreign if you're mostly used to C++11.

by sagacity

5/13/2026 at 2:31:48 PM

I was very curious to see what C++ 26 brings to the table, since I haven't used C++ in a while.

When I saw the 'no boilerplate' example, the very first thought that came to my mind:

This is the ugliest, most cryptic and confusing piece of code I've ever seen. Calling this 'no boilerplate' is an insult to the word 'boilerplate'.

Yeah, I can parse it for a minute or two and I mostly get it.

But if given the choice, I'd choose the C-macro implementation (which is 30+ years old) over this, every time. Or the good old switch case where I understand what's going on.

I understand that reflection is a powerful capability for C++, but the template-meta-cryptic-insanity is just too much to invite me back to this version of the language.

by delegate

5/13/2026 at 7:38:40 PM

> But if given the choice, I'd choose the C-macro implementation (which is 30+ years old) over this, every time.

Why? The implementation is not pretty, but you only need to write it once and then it works for all enums. The actual usage is trivial, it's just a function call.

The C macro version is horrendous in comparison. Why would I want to declare my enums like that just because I might want to print them?

by spacechild1

5/14/2026 at 5:28:46 AM

Then why isn't part of the stdlib? Why should everybody maintain their own version?

by madduci

5/13/2026 at 8:42:13 PM

As a developer who doesn't really write C++ code I'm inclined to agree, but I think Herb Sutter's "syntax 2" project might provide a nice way out of that mess eventually.

I played around with cppfront over Christmas and it was a lot more ergonomic than my distant memories of C++11, which I don't even have negative memories of per se.

[0] https://github.com/hsutter/cppfront

by vanderZwan

5/14/2026 at 6:08:05 AM

That isn't going anywhere official.

It is no different from any other language that compiles via C or C++ code generation, it got sold a bit differently due to his former position at WG21.

by pjmlp

5/13/2026 at 2:50:19 PM

It is "cryptic" and "ugly" to you just because you're not familiar with it. You'd pick the macro-based implementation because you are familiar with it.

Seeing this argumentation is so tiresome, because it feels like there is a lack of self-awareness regarding what is "familiar" and what isn't, which is subconsciously translated to "ugly" and "bad".

by SuperV1234

5/14/2026 at 7:46:34 AM

No, it is objectively cryptic and ugly. I honestly don’t understand how can anyone keep up with this garbage, but the ship has sailed long time ago. It is just a soup of symbols at this point.

by wiseowise

5/13/2026 at 3:05:36 PM

Have you ever used other (modern) programming languages ?

In a lot of languages, you achieve the same with 1 line of code. It's not about familiarity, it's about the fact that it's a long and convoluted incantation to get the name of an enum.

Why do I have to be familiar with all those weird symbols just to do a trivial thing ?

Update:

Zig:

const Color = enum { red, green, blue };

const name = @tagName(Color.red); // "red"

Rust:

#[derive(Display)]

enum Color { Red, Green, Blue }

let name = Color::Red.to_string(); // "Red"

Clojure:

(name :red) => "red"

by delegate

5/13/2026 at 7:11:13 PM

And what if you want to implement something like Rust's "derive"? That is what the article shows.

As far as I understand you would have to mess with individual parser tokens in Rust instead of high-level structures like "enum" (C++ reflection). It would be much, much uglier to implement anything like "to_enum_string" in Rust as you would have to re-implement parts of the compiler to get the "enum" concept out of a list of tokens.

by throwaway7356

5/13/2026 at 3:39:50 PM

C++:

  enum Color { red, green, blue };
  auto name = to_enum_string(Color::Red); // "Red"

by SuperV1234

5/13/2026 at 4:56:59 PM

... and where does that `to_enum_string` come from exactly? It doesn't seem to be built-in, which is the point of the parent comment.

by shooly

5/13/2026 at 6:11:18 PM

It's a fair comparison. The parent comment isn't showing the compiler source code for the built-in reflection mechanisms.

You won't have to care about ^^ and [:X:] if you just want to consume reflection-based utils, which was the whole point of my comment.

by SuperV1234

5/13/2026 at 10:08:24 PM

What? No. Parent comment is comparing C++ to modern programming languages, showcasing how they provide commonly used utilities out-of-the-box instead of making every programmer re-implement them again and again and again and again and again.

by shooly

5/13/2026 at 10:19:31 PM

The parent comment is quite clear:

> Why do I have to be familiar with all those weird symbols just to do a trivial thing ?

And my answer demonstrates that you do not have to.

by SuperV1234

5/13/2026 at 11:11:18 PM

> And my answer demonstrates that you do not have to

Then again - "where does that `to_enum_string` come from exactly?".

by shooly

5/14/2026 at 6:11:00 AM

A library that you install via vcpkg or conan.

How many libraries do you read the source code after installing them with the package manager?

by pjmlp

5/14/2026 at 7:49:22 AM

Typical C++ dev schizophrenia. In one thread complain about Node and its death-by-a-thousand-packages, then suggest the same in another.

by wiseowise

5/14/2026 at 8:25:28 AM

Don't put assumptions in others heads.

First of all, the only correct way to use package managers is with validated repos, don't vibe install, that goes for node, and goes for C++ as well.

Second this thread was all about how code lands in one's computer.

by pjmlp

5/13/2026 at 7:24:48 PM

The whole point of reflection is that it doesn't have to be builtin.

by gpderetta

5/13/2026 at 12:41:12 PM

I was a fool to assume that the same forces shaping the ugliness of C++ syntax would not also be at work in C++ 26.

by randusername

5/13/2026 at 8:13:23 PM

Reflect/reify, quasiquote/unquote, etc. are the final boss of syntax design. Even Template Haskell looks rather bad.

by mananaysiempre

5/13/2026 at 8:42:45 PM

I wish I understood the reason for the `std::define_static_array`... Why can't `std::meta::enumerators_of` just return something that can be iterated through????

by mort96

5/13/2026 at 8:50:13 PM

It is kind of weird at first but the reason is that `std::vector` requires heap allocation and transient allocations are not allowed in `constexpr` contexts. The purpose of `std::define_static_array` is to promote the storage of the vector to static storage to eliminate the transient allocation issue, and so that the `template for` can work properly with it.

See wg21.link/P3491

by SuperV1234

5/13/2026 at 8:52:54 PM

Is there a reason why `std::meta::enumerators_of`, a reflection feature that's surely almost exclusively going to be used in constexpr contexts, returns a value which doesn't work in constexpr contexts?

by mort96

5/13/2026 at 10:03:09 PM

It works generally, but not with expansion statements. See section 3.2 here: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p13...

It seems that this is being worked on, and eventually the `define_static_array` won't be needed anymore

by SuperV1234

5/13/2026 at 10:13:20 PM

Just another example where C++ language features are incompatible with each other, to be fixed "in a later version" which may or may not happen. There are so many of those in C++. I desperately wish they'd just do it properly initially.

by mort96

5/14/2026 at 6:13:39 AM

Me too, unfortunately the old guard sees no value in implementation before standardisation for each single feature.

So it is as it is, plenty of software in C++ isn't going to be rewriten into something else.

Maybe someone can do a Claude rewrite from LLVM into something else. /s

by pjmlp

5/13/2026 at 7:34:00 PM

I find it quite readable. I can understand what it does even though I haven't written reflection code yet myself.

by spacechild1

5/13/2026 at 1:13:06 PM

You realize c++11 is closer in age to C++98 than C++26?

by bluGill

5/13/2026 at 8:21:44 PM

I’m not sure the nominal publication date of a standard is all that relevant when the implementors’ reaction is as lukewarm as it has been to C++ ≥20.

by mananaysiempre

5/13/2026 at 12:17:11 PM

Is it? I'm mostly used to (pre-)C++11 and the only unusual operators I see are ^^T (which I presume accesses the metadata info of T) and [:e:] (which I assume somehow casts the enumerator metadata 'e' to a constant value of T).

And template for but I assume that's like inline for like in zig.

by ginko

5/13/2026 at 12:45:47 PM

requires is also new (not sure exactly when that appeared, it's after the last time I wrote C++ in anger) although I think it's fairly clear what it means. I can only guess at the other two.

Not familiar with Zig but AFAICT `inline for` is about instructing the compiler to unroll the loop, whereas `template for` means it can be evaluated at compile time and each loop iteration can have a different type for the iteration variable. It's a bit crazy but necessary for reflection to work usefully in the way the language sets it up.

by CamouflagedKiwi

5/13/2026 at 2:26:35 PM

requires is concept, one of big-C-words of ++20

by NooneAtAll3

5/13/2026 at 12:53:50 PM

Zig's inline for is also evaluated at comptime:

https://ziglang.org/documentation/master/#inline-for

by ginko

5/13/2026 at 8:31:08 PM

Well yes, but the _effect_ is to unroll the loop for runtime, if the inline-for survives that long.

A for loop executed during comptime is just

    const stuff = comptime stuff: {
       for (0...8) |i| {
         // etc, build up some stuff
       }
       break :stuff some_stuff;
    };
The difference is that a comptime block won't leave behind runnable 'residue', only whatever data is constructed for later. An inline for might not leave behind an unrolled loop either, but it can.

by samatman

5/13/2026 at 12:14:52 PM

I've been wondering about debug-ability of code using reflection. X-Macros are quite annoying to step through in most debuggers, though possible. While the code in the first example is evaluated fully at compile-time, how would you approach debugging it?

by w4rh4wk5

5/13/2026 at 6:46:46 PM

The answer is being debated at the moment in c++ papers and building on experience from other languages with extensive compile time evaluators like D. One thing that is happening is that we will get compile time exceptions (a paper for that is aiming to add this to the language in c++29 has come out) which may help us in reporting problems. Which will be important as there is also a lot of papers and talk about an extension of reflection allowing for better output generation which as far as I know was deferred until reflection had been accepted.

But there is also good news that with the advent of JIT like components for compile time evaluation in progress and the like of CLion having the beginnings of a compile debugger in combination with concepts there is a chance some help is available and on the way.

However right now you have to rely on compiler errors and static_asserts which is not ideal of course.

by theICEBeardk

5/13/2026 at 2:46:03 PM

Nothing that makes it straightforward. Testing via `static_assert` is a good strategy, but it's not debugging. I believe there are some ways of printing custom diagnostics during compilation, but I am not aware of any step-by-step debugging tool that runs at compile-time.

In practice, I haven't really needed to ever debug `consteval` functions -- it's quite easy to get the right behavior down thanks to `static_assert`-based testing and thanks to the fact that they do not depend on external state (simpler).

by SuperV1234

5/13/2026 at 8:17:57 PM

Keep macro generated code isolated in self-contained wrapper functions that just return a static object corresponding to an argument. Then you can treat them like black boxes that never fail and never need to be stepped over.

by kevin_thibedeau

5/13/2026 at 1:20:39 PM

I mean it's still C++ that's compiled and executed, surely the compiler would be able to provide a way to hook into that?

by cenamus

5/13/2026 at 2:18:05 PM

I don't recall the source, but I don't believe most (any?) c++ compilers implement compile-time code evaluation by compiling and running code.

For one thing they are required to disallow all undefined behavior for compile time execution, and some forms of UB only occur when the code is run.

by usefulcat

5/14/2026 at 6:16:04 AM

Basically nowadays they ship an interpreter in the box as well.

by pjmlp

5/13/2026 at 5:22:02 PM

Why people are still using debuggers?

I never felt the need for them when doing TDD.

by varispeed

5/13/2026 at 8:43:51 PM

Because sometimes you have bugs and you haven't narrowed down the cause enough to write a proper test for it?

by mort96

5/14/2026 at 6:17:52 AM

[dead]

by pjmlp

5/13/2026 at 1:32:32 PM

I think the conclusion section should indicate that they are based entirely on GCC 16's behavior and current implementation. We should avoid generalizing one compiler's behavior and performance. Curious how this same test would behave once clang ships C++26 reflection.

by jsd1982

5/13/2026 at 2:39:47 PM

I explicitly mentioned that GCC 16.1 was the compiler used in the benchmarking section, do you think I also need to add a disclaimer in the conclusion section as well?

Regardless, I don't think things are going to differ much with Clang. Without PCH/modules, standard header inclusion is still the "slow part" of C++ compilation, regardless of the compiler used and the standard library used (libstdc++ vs libc++). `#include` is fundamentally the same on any modern compiler.

Because the reflection feature itself seems quite fast on GCC (compared to the cost of the header), I predict the results will be similar on Clang as well.

by SuperV1234

5/14/2026 at 6:19:07 AM

Or VC++ if ever, which has the best modules support, but it is still trailing behind in C++23.

by pjmlp

5/13/2026 at 1:36:20 PM

I was thinking the same thing. Modules are still not widely used, it is a reasonable guess that there are a lot of optimization opportunities left.

by bluGill

5/13/2026 at 2:42:09 PM

That is true, but on the other hand Modules were standardized more than 6 years ago.

Promises and claims have been made for longer than that on how Modules would have improved compilation times and made everyone's lives easier. In 2026, I still have to see any real evidence of that, especially when PCH + unity builds are much easier to use (except on damn Bazel, which supports neither) and deliver great results.

If after 6+ years of development Modules are still so far behind, it is fair to question if the problem is with the design/implementability of the feature itself.

by SuperV1234

5/13/2026 at 7:53:28 PM

> it is fair to question if the problem is with the design/implementability of the feature itself.

The module story is just insane. How was it possible to get such a big feature into the standard without any working reference implementation? Isn't this the requirement for standard proposals to get accepted? If I compare this with how they treated JeanHeyd and his #embed proposal, the difference is staggering. To me it seems like a few powerful comittee members wanted to get modules into C++20 at any cost. This was just irresponsible.

by spacechild1

5/13/2026 at 11:37:20 PM

There was in visual studio which has had it other than minor details.the real problem is tools are needed to make modules work and those needed a lot of work. The work was already partially there because it's the same work that Fortran needs which tools supported but there were just enough details different to be annoying. Fortran modules were something that were always an afterthought and when tools started realizing that this is going to be a big deal, they decided they had to do it right, which took a lot of time too.

Maybe you forget Hacker News of 10 years ago, but in 2015-2016, everyone was complaining C++ doesn't have modules and how awful it must be because they're not modules. Now that C++ has modules, they're complaining about how it has modules.

by bluGill

5/14/2026 at 12:43:25 AM

I don't remember because I wasn't there :)

People are not complaining about the fact that C++ has modules, but about their usability and effectiveness. The compile time benefits seem modest and I have seen reports that it breaks Intellisense. (Maybe that's not true anymore?)

As Vittorio said, if it takes compiler vendors so long to implement them properly, maybe the design wasn't that good after all?

My point was: if you add such a big feature, shouldn't the standard require a sufficiently complete implementation? Otherwise, how can they assess whether the proposal actually works in practice and lives up to its promises?

by spacechild1

5/14/2026 at 8:13:47 AM

Agree that proof of implementation and real world experience should be a requirement for standardization. But it is a catch-22: implement it's are probably not always too keen to spend time on a large feature if it is not clear that it will be standardized.

In practice both clang and VS have had some form of module support for quite a while, but the final standard ended up being different from either implementation (shaped by their experience, and certainly with inevitable last minute inventions).

I wonder if for some features the committee should vote for general guidelines, the delegate a third party (one or more implementors) to come up with both an implementation and standardese with the understanding that it will be fast-tracked in wit too much bike-shedding

by gpderetta

5/14/2026 at 1:02:57 AM

Again, they had a sufficiently complete implementation. That implementation was in Visual Studio, clang had a very different implementation. The standard decided to take the Microsoft version. There are pros and cons to both and I will not fault the decision but either way one of the two had to lose and there is no surprise that for something complex it will take a long time to reimplement it to whatever the new standard is.

by bluGill

5/14/2026 at 1:46:45 AM

If the implementation really was sufficiently complete, then this is even worse! Why did they choose to vote something into the standard that is very complex and difficult to implement, but does not live up to the promises? Maybe they thought it would improve in the future, but isn't this a huge gamble?

I have heard rumors that certain people in the Visual Studio team have exaggerated the state of their modules implementation to speedrun the standardization process. I have no idea if that is really true, but it would explain a lot of things...

I'm not the only one who is asking these questions:

> I don’t know if they exaggerated their claims at the time, or if they didn’t properly fund the Visual Studio team since or what, but you can’t tell me 8 years wasn’t enough to make syntax highlighting work with modules. And if it is, then maybe there was something deeply wrong in their proposal and the committee should have asked to see the receipts before voting yes.

https://mropert.github.io/2026/04/13/modules_in_2026/

by spacechild1

5/14/2026 at 5:54:51 AM

Its misleading to call it "cost". In the C++ world only runtime cost matters. If using reflection allows to generate faster result code, it doesn't matter how long it takes to compile.

by Panzerschrek

5/14/2026 at 6:06:45 AM

It has a direct impact on the amount of emails and slack messages I get to reply to.

by pjmlp

5/13/2026 at 12:32:00 PM

No doubt reflection has been built with other use cases in mind, but it sure would have been nice just to have std::to_string(enum)

by HarHarVeryFunny

5/13/2026 at 1:11:59 PM

C++ conference speakers (including keynotes) are now begging everyone to stop using enum to string in their example. While they are a simple and easy to understand example, reflection is for much more interesting problems. I can't think of any other example that I would type into a comment box or put on a slide.

by bluGill

5/13/2026 at 2:02:49 PM

It comes up pretty frequently in java. Serialization/Deserialization, adding capabilities based on type, Adding new capabilities to a type, general tuning (for example, adding a timing or logging call onto methods).

Almost all the Java web frameworks are giant balls of reflection. Name a function the right way or add the right magic annotation and the framework will autowire it correctly.

It's a pretty powerful tool. (IDK if C++'s reflection is as capable, but this is what was enabled by java's reflection).

by cogman10

5/13/2026 at 2:48:56 PM

Java reflection is another beast altogether as it is runtime reflection. C++26 reflection is purely compile-time, which not only means it adds zero runtime cost, but also prevents those kind-of-insane use cases you see in Java and C#.

by SuperV1234

5/14/2026 at 6:23:04 AM

I think C++ devs have to eventually update their knowledge how Java and .NET work when talking about reflection.

Yes, originally they only supported runtime reflection.

Nowadays they have compile time tooling as well, via plugins, annotation processors, and code generators.

Which is exactly how you can have a Spring like frameworks that do all the AOP magic at compile time, for native code with GraalVM or OpenJ9, like Quarkus or Micronaut.

by pjmlp

5/13/2026 at 2:11:49 PM

> Almost all the Java web frameworks are giant balls of reflection. Name a function the right way or add the right magic annotation and the framework will autowire it correctly.

I find this to be very powerful, and also very unintuitive/undiscoverable at the same time.

by david422

5/13/2026 at 2:43:36 PM

Initially, but it very quickly becomes discoverable once you are familiar with how things are working.

Most frameworks in Java are very similar. The ones that aren't are effectively doing what "expressjs" does in terms of setup, which is still pretty discoverable.

Most java frameworks rely on annotations rather than naming schemes which makes everything a lot easier to grok.

by cogman10

5/13/2026 at 2:42:26 PM

Reflection is simply a syntax vinegar for duck typing.

by kuboble

5/13/2026 at 2:08:58 PM

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.

by maccard

5/13/2026 at 2:47:00 PM

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.)

by SuperV1234

5/13/2026 at 3:27:26 PM

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++!

by maccard

5/13/2026 at 3:38:06 PM

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

by SuperV1234

5/13/2026 at 6:50:25 PM

By that logic why would anything have to be standardised?

by maccard

5/13/2026 at 7:37:18 PM

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.

by gpderetta

5/14/2026 at 3:02:08 AM

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.

by jcelerier

5/13/2026 at 8:53:48 PM

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.

by SuperV1234

5/13/2026 at 11:39:24 PM

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.

by bluGill

5/13/2026 at 2:25:17 PM

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.

by bluGill

5/13/2026 at 2:48:03 PM

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.

by electroly

5/13/2026 at 3:31:35 PM

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.

by maccard

5/13/2026 at 2:47:58 PM

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.

by SuperV1234

5/13/2026 at 3:55:14 PM

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.

by bluGill

5/13/2026 at 7:39:20 PM

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.

by gpderetta

5/13/2026 at 1:16:48 PM

Anybody the derive traits rust has are a good demo.

by surajrmal

5/13/2026 at 6:51:35 PM

I mean a readable implementation of tuple with minimal overhead is a great case for me (went from around 1.6k lines to approximately 250 lines). I wrote an implementation including the normally difficult to implement tuple_cat based on c++26 within a few hours.

My favorite thing is that I will get to remove and replace most of the cryptic template recursion stuff I have with "template for" and maybe a bit of reflection. Debugging the unrolled stuff will be a joy in comparison.

by theICEBeardk

5/13/2026 at 1:16:17 PM

I can't imagine myself using reflection much, but maybe it will eliminate a lot of feature proposals bogging down the committee and they can focus on harder problems.

It would be cool if the stated goal of C++29 was compile times.

by randusername

5/13/2026 at 1:30:10 PM

I'd argue reflection is very much a feature for libraries. You wouldn't use it directly, but your JSON / YAML serialize is then built on top of it. So are your bindings for scripting engines like Lua.

by w4rh4wk5

5/13/2026 at 2:43:10 PM

You can already automatically serialize/deserialize arbitrarily nested structs since C++17 (using Boost.PFR). Since C++20, you can also serialize/deserialize the struct data member names automatically.

For many useful use cases, you don't need C++26 reflection at all. E.g. https://www.linkedin.com/posts/vittorioromeo_cpp-gamedev-ref...

by SuperV1234

5/13/2026 at 2:08:52 PM

There are a lot of things that are very very important for a tiny niche. In any non-trivial project you will end up with a lot of custom libraries and some of them really benefit from some obscure feature that no place else in your project would want.

by bluGill

5/13/2026 at 2:16:55 PM

Also nice for UI tooling; game tools, debuggers, etc. Pull apart a struct and display it on screen and not have to patch the UI tool every time you change the struct is pretty nice.

by agentultra

5/14/2026 at 2:50:06 AM

No surprise here that the macro + char* approach wins hands down. I'm not really an active C++ user but I did use a VERY similar trick in my custom C code generator DSL (writing in Ruby) just this week. Easy and no "magic" involved.

by drzaiusx11

5/13/2026 at 6:41:37 PM

Never quite understood why people are so obsessed with meta programming capabilities in a language, be it templates, comptime, macros, whatever.

I program mostly in C, if I need 'meta' programming I just write another C program that processes C source code (I've written a simple C parser), then in my build script I build in two stages, build meta program, run it, build rest of program.

Simple, effective, debuggable (the meta program is just normal C), infinite capabilities - can nest this to arbitritary depths, need meta-meta programming? Make a program that generates a meta program.

by cv5005

5/13/2026 at 7:00:27 PM

One obvious answer is that people probably don’t want to write a whole parser and wire up new steps in their build pipeline just to do something simple like get the name of enum cases as a string.

Without taking a stance on whether in-language meta programming facilities are good or bad, it’s not hard to find examples of cases where people find it useful to have them.

by rddbs

5/14/2026 at 5:58:30 AM

C++ has templates, which means, that some meta-code generation needs to be executed for arbitrary types. Doing so with an external tool is impossible.

by Panzerschrek

5/13/2026 at 8:35:41 PM

Actually why even specify metaprogram as C like source code? It must be convenience. But there is little practical use, like a good program always models a lot of different representations of more or less the same things, just recombined and processed a little differently. Why would we want to deal with semantics of C types for example, if we can model a much clearer and better constrained universe of types used in e.g. a de/serialization framework? Even only pointers are quite special, and often only of very immediate use, but there is no point in e.g. persisting them to disk or sending them over the network.

by jstimpfle

5/13/2026 at 6:49:35 PM

Meta programming in C++ can enable you to remove lots of runtime branching in your code at the cost binary size.

by ironman1478

5/13/2026 at 7:42:06 PM

This works for extreme needs.

But you're probably not doing s ton of metaprogramming all the time like you should be, and would with a language that allows it.

The lack of metaprogramming is also why C is so slow compared to C++

by jmalicki

5/13/2026 at 8:36:32 PM

C is not slow compared to C++. C++ compilation time are slow though.

by uecker

5/13/2026 at 8:36:18 PM

C is not slow compared to C++, that is a strange myth.

by jstimpfle

5/13/2026 at 9:17:29 PM

In practice, C means you end up with generic data structures with pointers to what they contain, rather than being inline.

You do see a lot of macro use to deal with this, but that is just primitive, non-typesafe metaprogramming, and it gets unwieldy enough that in practice, you see people add an extra pointer. This is why it gets slower.

by jmalicki

5/14/2026 at 6:07:05 AM

In practice, I see people write very performance C code where it matters, while moving on quickly where it does not. C++ code is often highly templated with annoying compile times, but still often slow because it still does not use the right data structures, and the amount of instruction bloat by specializing everything does not help for anything which is not a toy benchmark.

by uecker

5/13/2026 at 9:40:25 PM

If you need callbacks and generics, you're not writing performance code.

99% of code in the wild is comically inefficient and is doing the wrong thing, using way too generic data structures and algorithms for very concrete problems. C++ templates may be one way to make comically slow code faster by spending a lot of compile time. But it's often much quicker to just write straightforward concrete code that the compiler can easily optimize.

IMO C++ makes for slow programs for the sole fact that it compiles so slow (if you use its modern features), so you have much less time to actually iterate and improve.

by jstimpfle

5/13/2026 at 9:49:16 PM

If compilation is even more than 10% of the time it takes you to run your tests, you're probably not writing correct code. Compilation times don't even measure.

by jmalicki

5/13/2026 at 10:25:56 PM

So every time you compile, you run your test suite? I don't. And you trust that I have experience writing and compiling programs too...?

It should be a goal to keep rebuild times around 1 second (often not quite possible, but 3-5 seconds, even for full rebuilds, is often realistic). I edit, compile, run, edit, compile, run. Editing and running can often take as little as 1-3 seconds, and I sometimes do it dozens of times working in a row, working on a single improvement. That's why there is a 1 second rebuild time goal.

In practice I often work on codebases I don't fully control, but when the build times are excessively high, I will complain and try to improve. Build times longer than 10-15 seconds break the flow, they are a significant productivity hit. But they are quite common with C++ codebases (it can also be bad with C codebases by the way, but C++ is typically much worse because of templates and metaprogramming which is very slow).

> Compilation times don't even measure.

You must be joking. Do you even program?

by jstimpfle

5/13/2026 at 10:58:02 PM

You run your code before running tests? IMO that's bad practice.

1 second, seriously? Even the Linux kernel is based on C, and it doesn't even have compilation times approaching that.

I guess I also work on a lot of big data projects, where getting results will take... 48 hours or so, so anything shorter than that is basically some sort of unit test or dry run... so in that context, compilation times do not even register on the things slowing me down.

by jmalicki

5/13/2026 at 7:09:59 PM

Writing a C++ parser is much harder than a C parser to the point there had been just 3 parsers used among all C++ compilers for quite a while. So you'd need to use some library for parsing. So now you are looking into the library's parser compatibility with the compiler you are using (it might not support the C++ standard you are on at all, it can have bugs preventing it from parsing the code that the compiler parses just fine) and not just on your code but on the library headers you include in your code. What are you going to do when cindex/libclang or whatever chokes on a libstdc++ header? You also have the issue with builtin macros: are they are the same in your library parser? Most likely not. Good luck testing all that.

Two-stage compilation is just a bonus on top: you add a sequential dependency in your build graph and if you have enough of these parsing programs you are going to wait till they are all built before your build can go wide.

by pandaman

5/13/2026 at 7:56:35 PM

Absolutely spot on, easier, and way more effective

by psyclobe

5/13/2026 at 8:01:12 PM

Why would I write a parser that almost-but-not-quite matches the compiler's own parser, when I could just use the compiler's parser directly? I don't want to write a parser, and I especially don't want to debug weird corner cases where my implementation diverges somehow. I just want to write some code that goes like, for each field in T, do X.

C++ metaprogramming is bad, but the problem there is the C++ part, not the metaprogramming-in-the-language part.

by wat10000

5/13/2026 at 9:41:17 PM

Cause its dead simple. Shell out, run a quick sed or something, then compile it in. It is quite amazing what 'magic meta' stuff you can do with that shit. Meanwhile 10 years in we are finally getting reflection.....

by psyclobe

5/13/2026 at 5:35:45 PM

I agree with some other's in this thread: this is example is not great, but I get why it was used: to compare with X-macros. How about something that would require code-generation e.g. via libclang?

For example, what does https://miguelmartin.com/blog/nim2-review#implementing-a-sim... look like with C++26's std::meta::info?

My guess is: libclang is more suited for this situation if you care about compile times, even if Python is used.

by miguel_martin

5/13/2026 at 2:42:00 PM

I don't see how a library like Enchantum could handle everything reflection does. (How) does it figure out duplicate enum values, for example? And (how) does it discover arbitrarily large, discontiguous ranges? And (how) does it do these on MSVC?

by dataflow

5/13/2026 at 2:53:52 PM

In short, it probes enum values in a pre-defined range (e.g. [-256; 256]), and parses the `__PRETTY_FUNCTION__` macro at compile-time to extract the name of the enumerator.

Once you have that in place, you can easily detect duplicates, etc...

Of course, there are major limitations, as it's all a big hack: https://github.com/ZXShady/enchantum/blob/main/docs/limitati...

Similarly interesting is Boost.PFR, which gives you reflection superpowers since C++14: https://github.com/boostorg/pfr

by SuperV1234

5/13/2026 at 2:28:52 PM

Curious to see if Epic Games ever refactors their reflection in Unreal Engine to use C++ 26 reflections or not.

by mentos

5/14/2026 at 12:58:59 AM

That'll never happen. The engine's entire serialization system is built around their custom reflection layer and UHT. Not to mention how this would affect licensees. PLUS, they just laid off a bunch of people, and the leftovers are focused on Tim's Verse fiasco. I hate to use jargon here, but there's no "business value" to switching.

EDIT: and based on these compilation time results, this would be a major setback for building the engine, which already takes an eternity.

by LugosFergus

5/13/2026 at 1:00:12 PM

Another win for X macros and for C style in general, though the author didn’t declare it as such.

by king_geedorah

5/13/2026 at 9:40:06 PM

Pretty much. Was hoping it would've been a 'reflection slam dunk' but no... same 'ol same 'ol.

by psyclobe

5/13/2026 at 7:58:32 PM

The downside is, of course, that it's ugly and very awkward to use.

That's the essence of C++: you're basically trading ergonomics for compile times.

by spacechild1

5/13/2026 at 8:35:28 PM

Are X macros awkward? I find them very straightforward and clear.

by uecker

5/13/2026 at 8:42:11 PM

The implementation doesn't look too bad, but the usage is terrible:

  #define E_LIST(X) \
      X(V0) X(V1) X(V2) X(V3)

  DEFINE_ENUM(E, E_LIST)
That's not how I want to declare my enums...

by spacechild1

5/14/2026 at 6:39:18 AM

In practice it is written like like this:

  #define MY_ENUM(x) x(MY_A) x(MY_B) x(MY_C)
  enum my_enum { MY_ENUM(ENUM_ENTRIES) };

  static const char *my_enum_names[] = { MY_ENUM(ENUM_NAMES) };
but one could also make it even more compact if one cared.

by uecker

5/13/2026 at 8:56:17 PM

To be honest there are ways to make that much nicer. I believe that if you use recursive macros using the VA_OPT feature, you should be able to provide enumerators directly to define enum as a list.

The underlying machinery implementation is going to be much uglier and complex, though.

See https://www.scs.stanford.edu/~dm/blog/va-opt.html

by SuperV1234

5/14/2026 at 12:32:56 AM

Oh, I didn't know about __VA_OPT__(), thanks for that!

That looks much nicer indeed, but I still vastly prefer the other solutions, simply because I can just declare regular enums.

by spacechild1

5/13/2026 at 9:40:32 PM

They are gross but... effective so shrug

by psyclobe

5/13/2026 at 1:28:38 PM

Author here. It isn't a clear "win" at all, there are tradeoffs to each approach.

by SuperV1234

5/13/2026 at 6:08:10 PM

Man that aucks was looking forward to some kind of speed improvement. Using magic enum atm and I guess we'll continue to do so.

C++ build times are hard pill to swallow when migrating from c. This is just another reason we'll probably stick to writing c as t the company where I work. It's like asking someone to give up instant compilation for cleaner easier to read apps?

Also now that we have cleanup handlers in c (destructors) even less of a reason to move...

by psyclobe

5/13/2026 at 1:32:09 PM

"Enum to string"

We've come full circle huh?

Why do you need this, logging? In that case I would rather reflect the logging statement to pribt any variable name, or hell, just write out the string.

If saving for db, maybe store as string, there's more incentive for an enum in the db, if that's a string you might as well. At any rate it doesn't seem a great idea to depend on a variable name, imagine changing a variable name and stuff breaks.

by TZubiri

5/13/2026 at 2:51:33 PM

Logging, debugging, auto-generation of UIs/editors, etc... This is an extremely common operation and for a good reason.

by SuperV1234

5/13/2026 at 9:06:37 AM

[dead]

by jesgran