Welcome to v1.0 of the meta::[[verse]]

Welcome to v1.0 of the meta::[[verse]]!

# Slide 1

# Slide 2

C++ Software Engineer, OS Internals, Microsoft
ISO C++ foundation and Boost foundation board member
An active member of the ISO C++ Committee
Israeli National Body Chair
Library Evolution Work Group Chair

Inbal Levi

https://slides.com/inballevi/welcome-to-v1-0-of-the-meta-verse-pure-virtual/

Reflection in C++

# Slide 3

The ability of software to expose its internal structure
Static reflection - compiler exposes structure at compile time
Disclaimers:
- It's possible to "carry" the data into the runtime...
- But may introduce performance, security, or other issues
- This is out of scope for this talk (and for C++26, as of now)

This Talk

# Slide 4

Part I: Intro to reflection:
1. A brief history of "Reflection" proposals in C++
2. Latest proposal: "P2996: Reflection for C++26"
3. The complexity of reflection
Part II: Impact on our code bases
1. Reflection-based libraries
2. What's next?

(*) Examples from or derive from P2996, or EDG's implementation

2006

Template-based reflection
(Matúš Chochlik)
Mirror reflection library

2012

N3403: Use Cases for Compile-Time Reflection
(Mike Spertus)

# Slide 5

2014 - 2016

N3996: Static reflection (Matúš Chochlik)

N4113, N4239, N4027, N3984
(type traits, attributes, etc.)

2015

Boost.Hana
(Louis Dionne)
Template metaprog. lib

A Brief History

2018

P0954: What do we want to do with reflection?
(Bjarne)

2017

P0194|P0385: Static Reflection
(Matúš, Axel, David)
P0590: A design for static reflection
(Andrew, Herb)
P0633: Exploring the design space
(Daveed, Louis)

2018

N4747: Reflection TS
(David)

# Slide 6

2018

P0993: Value-based reflection
(Andrew, Herb)
P1240: Scalable Reflection in C++
(Andrew, Faisal, Daveed)

2019

P0953: constexpr reflexpr
(Matúš, Axel, David)
P1733: User-friendly and Evolution-friendly Reflection: A Compromise
(David, Daveed)

A Brief History

Typefull
vs.
Monotype

Type based
vs.
Value based

meta objects

2020

P2087: Reflection Naming: fix reflexpr
(Mihail Naydenov)
P2040: Reflection-based lazy-evaluation
(Corentin)

2021

P2320: The Syntax of Static Reflection
(Andrew, Wyatt, Daveed)
(+ implementation: lock3 meta)

2023

P2996: Reflection for C++26
(Barry, Wyatt, Peter, Andrew, Faisal, Daveed)

2022

P2560: Comparing Value- and type- based reflection
(Matus)

# Slide 7

2023

P2911: Python Bindings (...)
(Adam, Jagrut)
P3010: Using reflection (...) JS Bindings
(Dan Katz)

A Brief History

2025

P2996R11
(+Dan Katz)

(1.5 years later)

# Slide 8

What do I mean by "Reflection"?

// Lib.hpp
class LibType : public BaseOne, public BaseTwo
{
   int a;
   double b;
};

P2996

// main.cpp
#include <meta>
#include "Lib.hpp"

int main()
{
   constexpr std::meta::info refexpr = ^^LibType;
   constexpr auto res = std::meta::bases_of(refexpr)[0];
}

https://godbolt.org/z/WxdvKdcMY

# Slide 9

P2996: Reflection for C++26

P2996

Reflection Operator: ^^
Splicers: [:…:]
std::meta::info
Metafunctions

8. define_aggregate (inject)

Name & Location:

3. Template Queries:

identifier_of
display_string_of
source_location_of

template_of
template_arguments_of

9. Data Layout:

4. Member Queries:

offset_of -> member_offset
size_of
alignment_of
bit_size_of

2. Type Queries:

members_of, bases_of
(non)static_data_members_of
enumerators_of

type_of
parent_of
dealias
Access modifiers: is_public, is_protected, is_private
Inheritance: is_virtual, is_pure_virtual, is_override, ...
Encapsulation: is_class_member, is_namespace_member, is_explicit, is_deleted, ...
Advanced Type Queries: is_complete_type, is_template, is_special_member, ...

10. (+) Type Traits

(+ Other Type Predicates)

5. substitute (template)

6. extract<T>(info) (constexpr not required)

7. reflect_x (extended)

reflect_value
reflect_object
reflect_function

auto rexpr = ^ int;

# Slide 10

The Reflection Operator (^^) (Lift)

P2996

AKA Unibrow

auto rexpr = ^^int;

# Slide 11

The Reflection Operator (^^) (Lift)

Shifts expressions into a "meta" - "reflection info" object
Object can then to be used as input to reflection utilities

error: meta type variables must be constexpr

constexpr auto rexpr = ^^int;

P2996

# Slide 12

Splicers

Splice extract the C++ expression back from "meta::info"...
...To be then used regularly to write the C++ program

constexpr auto rexpr = ^^int;
typename[:rexpr:] a  = 42;

What are you?

rexpr

meta::info obj, contains info on "int" type

typename[:rexpr:]

int

P2996

* More about the typename in P2996/[dcl.type.splice]

# Slide 13

P2996: Reflection for C++26

P2996

Reflection Operator: ^^
Splicers: [:…:]
std::meta::info
Metafunctions

8. define_aggregate (inject)

Name & Location:

3. Template Queries:

identifier_of
display_string_of
source_location_of

template_of
template_arguments_of

9. Data Layout:

4. Member Queries:

offset_of -> member_offset
size_of
alignment_of
bit_size_of

2. Type Queries:

members_of, bases_of
(non)static_data_members_of
enumerators_of

type_of
parent_of
dealias
Access modifiers: is_public, is_protected, is_private
Inheritance: is_virtual, is_pure_virtual, is_override, ...
Encapsulation: is_class_member, is_namespace_member, is_explicit, is_deleted, ...
Advanced Type Queries: is_complete_type, is_template, is_special_member, ...

10. (+) Type Traits

(+ Other Type Predicates)

5. substitute (template)

6. extract<T>(info) (constexpr not required)

7. reflect_x (extended)

reflect_value
reflect_object
reflect_function

# Slide 14

std::meta::info

From: "Let's talk about abstraction layers"

P2996

# Slide 15

std::meta::info

int a = 42;
constexpr auto b = ^^a;
std::cout << [:b:] << "\n";       // OK
std::cout << [:^^(a*2):] << "\n"; // Error

constexpr int a = 42;
constexpr auto b = ^^a;
std::cout << [:b:] << "\n";       // OK
std::cout << [:^^(a*2):] << "\n"; // OK

P2996

Represents:

Type and type alias
Function or member function
Variable, static data member, or structured binding
Non-static data member
Enumerators
Object that is a permitted result of a constant expression
Value with structural type that is a permitted result of a const evaluation
Template
Namespaces, namespace alias

# Slide 16

P2996: Reflection for C++26

Reflection Operator: ^^
Splicers: [:…:]
std::meta::info
Metafunctions

P2996

8. define_aggregate (inject)

Name & Location:

3. Template Queries:

identifier_of
display_string_of
source_location_of

template_of
template_arguments_of

9. Data Layout:

4. Member Queries:

offset_of -> member_offset
size_of
alignment_of
bit_size_of

2. Type Queries:

members_of, bases_of
(non)static_data_members_of
enumerators_of

type_of
parent_of
dealias
Access modifiers: is_public, is_protected, is_private
Inheritance: is_virtual, is_pure_virtual, is_override, ...
Encapsulation: is_class_member, is_namespace_member, is_explicit, is_deleted, ...
Advanced Type Queries: is_complete_type, is_template, is_special_member, ...

10. (+) Type Traits

(+ Other Type Predicates)

5. substitute (template)

6. extract<T>(info) (constexpr not required)

7. reflect_x (extended)

reflect_value
reflect_object
reflect_function

# Slide 17

Metafunctions

constexpr auto a = 42;
std::cout << meta::name_of(^^a) << "\n";

All* the metafunctions accept "info" and return:

string_view (e.g. identifier_of)
std::meta::info (e.g. type_of)
vector<std::meta::info> (e.g. bases_of)
bool (e.g. is_concept)
size_t (e.g. alignment_of)
T (e.g. extract(info))
source_location, member_offset (e.g. offset_of)

* Besides "reflect_value/object/function", which accepts type T

https://godbolt.org/z/9YW4bWGM7

constexpr auto a = 42;
std::cout << meta::identifier_of(^^a) << "\n";

P2996

# Slide 18

std::meta::info

#include <iostream>
#include <experimental/meta>

class R;

constexpr std::meta::info res1 = ^^R;
constexpr auto print1 = std::meta::
				is_complete_type(res1); // (1)

class R {
    int a;
};

constexpr std::meta::info res2 = ^^R;
constexpr auto print2 = std::meta::
				is_complete_type(res2); // (2)
constexpr auto print3 = std::meta::
				is_complete_type(res1); // (3)

int main()
{
    std::cout << "Res: " << print1 << "\n"; // (1)
    std::cout << "Res: " << print2 << "\n"; // (2)
    std::cout << "Res: " << print3 << "\n"; // (3)
}

https://godbolt.org/z/cafxMPo4o

Complexity of Reflection

# Slide 19

Type Traits

Two types of "Type Traits":
- Query the type
- Modify the type
P2996 proposes the first type (e.g. is_nothrow_swappable)
Interesting functionality available by the second type
...But this is more challenging, as it interferes with the compiler's representation of the program

Complexity of Reflection

# Slide 20

Interaction between library code and user(*) code

int main()
{
    type.[:get_member_i(0):] = 1;
    type.[:get_member_i(1):] = 1;

    type.[:member_named("pub"):] = 2;
    type.[:member_named("prv"):] = 2;
}

Library Code

User(*) Code

ReflectionLibraryDemo

Reflection as Customization

class LibType {
public:
    int pub;
private:
    int prv;
public:
    void Print();
};

Complexity of Reflection

# Slide 21

Reflection Logger

#include <string>
#include <experimental/meta>

consteval bool LogMembers(std::meta::info type) 
{
  // Verify type is a class/struct
  // Ignore usecase of unnamed members (union/struct etc.)
  std::__report_constexpr_value(identifier_of(type).data());

  for (auto r : nonstatic_data_members_of(type)) 
  {
      std::__report_constexpr_value("\n\tmember:");
      std::__report_constexpr_value(identifier_of(r).data());
  }
  return true;
}

// User code
struct Student 
{
  std::string name;
  int id;
};

static_assert(LogMembers(^^Student));

Reflection-Based Library

Based on example from EDG's CE implementation

https://godbolt.org/z/Yddfxdq8j

# Slide 22

Detour: But what about AI?

Summary

Generate "MyTypeSerializer" class in a header (you can add .cpp file if needed), based on "MyType.h" which provide "serialize" and "deserialize" methods for "MyType", turning it into bytes buffer and back into a "MyType" class.

# Slide 23

Detour: But what about AI?

Summary

# Slide 24

Detour: But what about AI?

Summary

# Slide 25

Detour: But what about AI?

LLMs are trained on data to create weights
Algorithm aims for the "most fitting" result
There's non-determinism inherently in the process
Definetly with new models / versions

Summary

(Disclaimer: as of now...)

No guarantees between executions

# Slide 26

Detour: But what about "Rust"?

Rust's toolkit:

"macros" - variadic, used as templates, expend pre-compiler
"syn" - library for injecting tokens into the AST
"procedural macro" - used to process AST into AST

Summary

Apr 2023: a-plan-for-generic-compile-time-introspection-in-rust
Jul 2024: First-class CT type manipulation: rfcs#3669
March 2025: Reflect lib (source, crate)

	C++	Rust
Approach	Functions / traits	Token manipulation
Failure Outcome	Ill-formed / Compile	Compile
Error indication	constexpr exception	Runtime / tests

# Slide 27

What should we expect from "reflection libraries"?

Guarantees we do not provide:

The outcome of "breaking the contract":

The outcome of failures:
1. Program is ill-formed (fails to compile)
2. Program contains UB (undefined behavior)
Errors at compile time:
- Should indicate to "reflection" stage (NOT "old" compilation)
- Throw constexpr exceptions (Thanks to Hana Dusíková!)
Warnings: Which level of feedback is given to correct mistakes?

Summary

No compatibility between standard versions
No backward compatibility for source code
No guarantees for existing code

# Slide 28

Summary

Which "reflection libs" should go into the standard library?

Would love your input!

P3157R0: Generative Extensions (...) (Andrei Alexandrescu, Bryce Lelbach, Michael Garland)
P3095R0: ABI comparison with reflection (Saksham Sharma)
P2996's Implementation (Bloomberg)
Comparing AI models (suggested uses)
ACCU 2025: Learning to Stop Writing Code (Daisy Hollman)

More info:

A brief history of "Reflection" proposals in C++
Latest proposal: "P2996: Reflection for C++26"
The complexity of reflection
Reflection-based libraries
What's next?

Summary

# Slide 29

Thank you!

Inbal Levi
sinbal2lextra@gmail.com

linkedin.com/in/inballevi/

Thank you!

Stay in touch!

Thanks to:

CoreC++ user group
Hana Dusíková (SG7 chair)
Matus Chochlik
David Sankel
Corentin Jabot
Lewis Baker
Adi Shavit

Thank you for being passionate about C++!

Reflection Papers' authors!
Wyatt Childers, Peter Dimov, Dan Katz, Barry Revzin, Andrew Sutton, Faisal Vali, Daveed Vandevoorde, Matus Chochlik, Herb Sutter, Bjarne Stroustrup, David Sankel, Axel Naumann, Andrei Alexandrescu, Bryce Lelbach, Michael Garland, Louis Dionne, Adam Lach, Jagrut Dave, Walter Genovese, Saksham Sharma