Welcome to v0.2 of the meta::[[verse]]!

# Slide 1
# Slide 2
  • Lead C++ Developer
  • 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

Reflection in C++

# Slide 3
  • The ability of software to expose its internal structure
  • Static reflection - compiler exposes structure at compile time
     
  • Disclaimers:
    • May be 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. Usage examples (reflection-based library) (*)
      ​
  • Part II: Impact on our code bases
    1. Reflection as a Customization Point Mechanism
    2. Pipeline integration
    3. What's next?

(*) Examples from or derive from P2996, P3096, 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

2024

P2996R7

(1 year 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;
   auto res = std::meta::bases_of(refexpr);
}

* Note: This is a pseudo-code, does not work as is

# Slide 9

P2996: Reflection for C++26

  1. Reflection Operator: ^

  2. Splicers: [:…:]

  3. std::meta::info

  4. Metafunctions

  1. identifier_of

  2. display_string_of

  3. source_location_of

  1. type_of

  2. parent_of

  3. dealias
     

  4. Access modifiers: is_public, is_protected, is_private

  5. Inheritance: is_virtual, is_pure_virtual, is_override, ...

  6. Encapsulation: is_class_member, is_namespace_member, is_explicit, is_deleted, ...

  7. Advanced Type Queries: is_complete_type, is_template, is_special_member, ...

  1. template_of

  2. template_arguments_of

3. Template Queries:

  1. members_of

  2. bases_of

  3. (non)static_data_members_of

  4. accessible_members_of (P3293R2)

  5. enumerators_of

​2. Type Queries:

  1. Name & Location:

(+ Other Type Predicates)

​4. Member Queries:

5. substitute (template)

6. reflect Invoke (template)

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

8. test_type(s) ("is_same")

9. reflect_value (template)

10. define_class (injection)

11. Data Layout:

12. (+) Type Traits...?

P2996

  1. offset_of -> member_offsets

  2. size_of

  3. bit_size_of

  4. alignment_of

^^

auto rexpr = ^int;
# Slide 10

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 11

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
# Slide 12

P2996: Reflection for C++26

P2996

  1. Reflection Operator: ^

  2. Splicers: [:…:]

  3. std::meta::info

  4. Metafunctions

  1. identifier_of

  2. display_string_of

  3. source_location_of

  1. type_of

  2. parent_of

  3. dealias
     

  4. Access modifiers: is_public, is_protected, is_private

  5. Inheritance: is_virtual, is_pure_virtual, is_override, ...

  6. Encapsulation: is_class_member, is_namespace_member, is_explicit, is_deleted, ...

  7. Advanced Type Queries: is_complete_type, is_template, is_special_member, ...

  1. template_of

  2. template_arguments_of

3. Template Queries:

  1. members_of

  2. bases_of

  3. (non)static_data_members_of

  4. accessible_members_of (P3293R2)

  5. enumerators_of

​2. Type Queries:

  1. Name & Location:

(+ Other Type Predicates)

​4. Member Queries:

5. substitute (template)

6. reflect Invoke (template)

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

8. test_type(s) ("is_same")

9. reflect_value (template)

10. define_class (injection)

11. Data Layout:

12. (+) Type Traits...?

  1. offset_of -> member_offsets

  2. size_of

  3. bit_size_of

  4. alignment_of

# Slide 13

std::meta::info

From: "Let's talk about abstraction layers"

P2996
# Slide 14

std::meta::info

Represents:

  • Type and type alias
  • Function or member function
  • Variable, static data member, or structured binding
  • Non-static data member
  • Constant value
  • Constant expression
  • Template
  • Namespaces
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
# Slide 15

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)
}
P2996

https://godbolt.org/z/sYs39bj5e

# Slide 16

P2996: Reflection for C++26

P2996

  1. Reflection Operator: ^

  2. Splicers: [:…:]

  3. std::meta::info

  4. Metafunctions

  1. identifier_of

  2. display_string_of

  3. source_location_of

  1. type_of

  2. parent_of

  3. dealias
     

  4. Access modifiers: is_public, is_protected, is_private

  5. Inheritance: is_virtual, is_pure_virtual, is_override, ...

  6. Encapsulation: is_class_member, is_namespace_member, is_explicit, is_deleted, ...

  7. Advanced Type Queries: is_complete_type, is_template, is_special_member, ...

  1. template_of

  2. template_arguments_of

3. Template Queries:

  1. members_of

  2. bases_of

  3. (non)static_data_members_of

  4. accessible_members_of (P3293R2)

  5. enumerators_of

​2. Type Queries:

  1. Name & Location:

(+ Other Type Predicates)

​4. Member Queries:

5. substitute (template)

6. reflect Invoke (template)

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

8. test_type(s) ("is_same")

9. reflect_value (template)

10. define_class (injection)

11. Data Layout:

12. (+) Type Traits...?

  1. offset_of -> member_offsets

  2. size_of

  3. bit_size_of

  4. alignment_of

# Slide 17

Metafunctions

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

All* the metafunctions accept "info" and return:

  1. string_view (e.g. identifier_of)
  2. std::meta::info (e.g. type_of)
  3. vector<std::meta::info> (e.g. bases_of)
  4. bool (e.g. is_concept)
  5. size_t (e.g. alignment_of)
  6. T (e.g. extract(info))
  7. source_location, member_offset (e.g. offset_of)

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

P2996

* Note: This is a pseudo-code, does not work as is

# Slide 18

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
P2996
# Slide 19

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(name_of(type).data());

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

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

static_assert(LogMembers(^Student));

https://godbolt.org/z/oobfx9E7h

Reflection-Based Library

Based on example from EDG's CE implementation

# Slide 20

Command Line Args

// Members represent Args (should be known at compile time)
using namespace clap;
struct Args : Clap 
{
  Option<std::string, Flags{.use_short=true, .use_long=true}> name = "";
  Option<int, Flags{.use_short=true, .use_long=true}> count = 0;
};

int main(int argc, char** argv) 
{
  auto opts = Args{}.parse(argc, argv);   

  for (int i = 0; i < opts.count; ++i) 
  {
    std::cout << "Hello " << opts.name << "!\n";
  }    
}

https://godbolt.org/z/9GE3ePK71

Reflection-Based Library

Based on example from P2996

# Slide 21

Function Param Names

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

consteval auto PrintParamK(std::meta::info r, size_t k) 
{
    return name_of(parameters_of(r)[k]);
}

// Function Declaration
void func(int first, int last);

void PrintFuncParamAfterDeclaration() 
{
    std::cout << "Param names: " 
    << PrintParamK(^func, 0) << ", "
    << PrintParamK(^func, 1) << "\n";
}

// Function Definition
void func(int n, int ln)
{}

void PrintFuncParamAfterDefinition() 
{
    std::cout << "Param names: "
    << PrintParamK(^func, 0) << ", "
    << PrintParamK(^func, 1) << "\n";
}


int main() 
{
    PrintFuncParamAfterDeclaration();   // Param names: first, last
    PrintFuncParamAfterDefinition();    // Param names: n, ln
}
Reflection-Based Library

https://godbolt.org/z/M84Ea6P88

Based on example from P3096

# Slide 22

Reflection Based Library

void PrintFuncParams() 
{
  std::cout << "Param names: " 
  << PrintParamK(^func, 0) << ", "
  << PrintParamK(^func, 1) << "\n";
}
  • Interaction between library code and user(*) code
void func(int first, int last);
// PrintParamsLocation1() { ... }
void func(int a, int b) { ... }
// PrintParamsLocation2() { ... }

int main() 
{
  PrintFuncParamsLocation1();
  PrintFuncParamsLocation2();
}

Library Code

User(*) Code

"P3096: Function Parameter Reflection in Reflection for C++26"
 (Adam Lach, Walter Genovese)

Reflection-Based Library
# Slide 23

Function Param Names

  • P3096 Introduces the following options:
    1. Compile, No Guarantees
      (Different compilers' output may be inconsistent)
    2. Enforce Consistent Naming
      (Don't compile if more than one option exists)
    3. Mark by attribute (e.g. [[canonical]])
      (Explicitly mark, otherwise ill-formed)
Reflection-Based Library
# Slide 24

Function Param Names

  • Forward competability
void PrintFuncParams() 
{
  std::cout << "Param names: " 
  << PrintParamK(^func, 0) << ", "
  << PrintParamK(^func, 1) << "\n";
}

void func(int first, int last);
// PrintParamsLocation1() { ... }
void func(int first, int last) 
{ ... }
// PrintParamsLocation2() { ... }

int main() 
{
  PrintFuncParamsLocation1();
  PrintFuncParamsLocation2();
}

Library Code

New User Code

void func(int first, int last);
// PrintParamsLocation1() { ... }
void func(int a, int b) 
{ ... }
// PrintParamsLocation2() { ... }

int main() 
{
  PrintFuncParamsLocation1();
  PrintFuncParamsLocation2();
}

"P3096: Function Parameter Reflection in Reflection for C++26"
 (Adam Lach, Walter Genovese)

Reflection-Based Library
# Slide 25

Guarantees and requirements provided by the library
can be expressed as "Customization Points"

Reflection as CP

Detour: Customization Points

C++Now 2022, Cpp India 2022, CoreC++ 2022, CppCon 2023
"Customization Methods: Connecting User and C++ Library Code"

# Slide 26

Detour: Customization Points

Reflection as CP
  • Interaction between library code and user (*) code:
    • Virtual functions:
      • Library side: declaration of virtual functions
      • User side: overriding virtual functions 
    • Template instantiation:
      • Library side: declare a template
      • User side: instantiate template for their types
    • Provide functionality to be detected by ADL:
      • Library side: allows/expects functions by name
      • User side: declares a function within their type to be detected by ADL
    • And now - Reflection?

C++Now 2022, Cpp India 2022, CoreC++ 2022, CppCon 2023
"Customization Methods: Connecting User and C++ Library Code"

# Slide 27

Reflection as Customization

template<class_type T, structural_subtype_of<T> U> 
void LibFunc(const T& src, U& dst)
{   
  constexpr auto members = meta::data_members_of(reflexpr(src));   
  template for (constexpr meta::info a : members)
  {
    constexpr meta::info b = meta::lookup(dst, meta::name_of(a));
    dst.|b| = src.|a|;   
  }
}   // `structural_copy`

Example from "P2237: Metaprogramming" (Andrew Sutton)

C++Now 2022, Cpp India 2022, CoreC++ 2022, CppCon 2023
"Customization Methods: Connecting User and C++ Library Code"

Reflection as CP
# Slide 28
  • 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();
};
Reflection as CP
# Slide 29

Reflection Libraries

What will be the guarantees for "reflection libraries"?

  • P3157R0: Generative Extensions for Reflection 
    (Andrei Alexandrescu, Bryce Lelbach, Michael Garland)
  • P3095R0: ABI comparison with reflection
    (Saksham Sharma)
  • P4329R0: should minimize standard library dependency
    (Jonathan Muller)

More interesting papers:

  • Scheduled next week for LEWG, the discussion is ongoing
     
  • We would love your input!
Reflection as CP
# Slide 30

Pipeline Integration

Pipeline Integration

AppCode

LoggerLib

Compiler

LoggerLib.o

AppCode.o

Linker

version / macros

  • The compiler provides:
    • Standard version flag and implementation macros
  • The library can then "ifdef" based on it to figure out
    • Standard version
    • Avaliable features

main.out

# Slide 31

Pipeline Integration

Pipeline Integration

main.out

Linker

main.o

AppCode

LoggerLib

Compiler

version / macros

Linker

OldAppCode

LoggerLib

Compiler

version / macros

Backward Compatibility

# Slide 32

What should we expect from "reflection libraries"?

What's Next?

Which guarantees do we need to provide?

  1. No compatibility between standard versions
  2. No backward compatibility for source code
  3. No guarantees for existing code?

What should be the outcome of "breaking the contract"

  1. The outcome of failures:
    1. Program is ill-formed (fails to compile)
    2. Program contains UB (undefined behavior)
  2. Errors at compile time:
    • Should indicate to "reflection" stage (NOT "old" compilation)
    • Throw exceptions(?)
  3. Warnings: Which level of feedback is given to correct mistakes?
# Slide 33

C++ 26 Reflection

How will Reflection impact our code bases?

Summary

?

From: https://isocpp.org/std/status (by Herb Sutter)

# Slide 34

Thank you!

Inbal Levi
sinbal2lextra@gmail.com

linkedin.com/in/inballevi/

Thank you!

Stay in touch!

Thanks to:

  • CoreC++ user group
  • Matus Chochlik
  • David Sankel
  • Corentin Jabot
  • Lewis Baker
  • Amir Kirsh
  • 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