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.
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.
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.
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.
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.
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.
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?
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.
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.
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.
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).
> 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.
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.
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.
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.
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.
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.
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.
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.
Curious to see if Epic Games ever refactors their reflection in Unreal Engine to use C++ 26 reflections or not.
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.
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.
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.
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.
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?
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?
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.
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)
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.
Serialization is the canonical example. Being able to turn
into 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.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.
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).
> 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.
Anybody the derive traits rust has are a good demo.
Oof, that first example (the idiomatic C++26 way) looks so foreign if you're mostly used to C++11.
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.
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.
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.
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.
requires is concept, one of big-C-words of ++20
Zig's inline for is also evaluated at comptime:
https://ziglang.org/documentation/master/#inline-for
You realize c++11 is closer in age to C++98 than C++26?
Another win for X macros and for C style in general, though the author didn’t declare it as such.
Author here. It isn't a clear "win" at all, there are tradeoffs to each approach.
"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.