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

93 points by sagacity

by vanderZwan

3 subcomments

> 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 sagacity

6 subcomments

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

by jsd1982

3 subcomments

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 w4rh4wk5

5 subcomments

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 HarHarVeryFunny

1 subcomments

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 randusername

1 subcomments

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 cv5005

8 subcomments

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 miguel_martin

0 subcomment

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 Panzerschrek

3 subcomments

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 drzaiusx11

0 subcomment

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 mentos

1 subcomments

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

by dataflow

1 subcomments

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 king_geedorah

3 subcomments

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

by psyclobe

0 subcomment

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 TZubiri

1 subcomments

"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 jesgran

0 subcomment

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