// Overview / Examples / API / FAQ

REFLECT: C++20 Static Reflection library

https://en.wikipedia.org/wiki/Reflective_programming

Features

• Single header (https://raw.githubusercontent.com/qlibs/reflect/main/reflect - for integration see FAQ)
• Minimal API
• Verifies itself upon include (can be disabled with -DNTEST - see FAQ)
  • Basically guarantees no UB, no memory leaks*
• Compiles cleanly with (-fno-exceptions -fno-rtti -Wall -Wextra -Werror -pedantic -pedantic-errors | /W4 /WX)
• Agnostic to compiler changes (no ifdefs for the compiler specific implementations - see FAQ)
• Optimized run-time execution and binary size (see performance)
• Fast compilation times (see compilation times)

Requirements

• C++20 (gcc-12+, clang-15+, msvc-19.36+)

---

Overview

Hello world (https://godbolt.org/z/oYhh1hfeo)

#include <reflect>

enum E { A, B };
struct foo { int a; E b; };

constexpr auto f = foo{.a = 42, .b = B};

// reflect::size
static_assert(2 == reflect::size(f));

// reflect::type_id
static_assert(reflect::type_id(f.a) != reflect::type_id(f.b));

// reflect::type_name
static_assert("foo"sv == reflect::type_name(f));
static_assert("int"sv == reflect::type_name(f.a));
static_assert("E"sv   == reflect::type_name(f.b));

// reflect::enum_name
static_assert("B"sv == reflect::enum_name(f.b));

// reflect::member_name
static_assert("a"sv == reflect::member_name<0>(f));
static_assert("b"sv == reflect::member_name<1>(f));

// reflect::get
static_assert(42 == reflect::get<0>(f)); // by index
static_assert(B  == reflect::get<1>(f));

static_assert(42 == reflect::get<"a">(f)); // by name
static_assert(B  == reflect::get<"b">(f));

// reflect::to
constexpr auto t = reflect::to<std::tuple>(f);
static_assert(42 == std::get<0>(t));
static_assert(B  == std::get<1>(t));

int main() {
  reflect::for_each([](auto I) {
    std::print("{}.{}:{}={} ({}/{}/{})\n",
        reflect::type_name(f),                  // foo, foo
        reflect::member_name<I>(f),             // a  , b
        reflect::type_name(reflect::get<I>(f)), // int, E
        reflect::get<I>(f),                     // 42 , B
        reflect::size_of<I>(f),                 // 4  , 4
        reflect::align_of<I>(f),                // 4  , 4
        reflect::offset_of<I>(f));              // 0  , 4
  }, f);
}

// and more (see API)...

---

Examples

• [feature] Opt-in mixins - https://godbolt.org/z/sj7fYKoc3
• [feature] Meta-programming (https://github.com/qlibs/mp) - https://godbolt.org/z/ds3KMGhqP
• [future] Structured Bindings can introduce a Pack (https://wg21.link/P1061) - https://godbolt.org/z/Ga3bc3KKW
• [performance] Minimal Perfect Hashing based enum_name (https://github.com/qlibs/mph) - https://godbolt.org/z/WM155vTfv

---

Performance

Binary size (https://godbolt.org/z/7TbobjWfj)

struct foo { int bar; };
auto type_name(const foo& f) { return reflect::type_name(f); }

type_name(foo const&): // $CXX -O3 -DNDEBUG
  lea rdx, [rip + type_name<foo>]
  mov eax, 3
  ret

type_name<foo>
  .ascii  "foo"

struct foo { int bar; };
auto member_name(const foo& f) { return reflect::member_name<0>(f); }

member_name(foo const&): // $CXX -O3 -DNDEBUG
  lea rdx, [rip + member_name<0ul, foo>]
  mov eax, 3
  ret

member_name<0ul, foo>
  .ascii  "bar"

enum class E { A, B, };
auto enum_name(const E e) { return reflect::enum_name(e); }

enum_name(E): // $CXX -O3 -DNDEBUG (generates switch)
  xor     eax, eax
  xor     ecx, ecx
  cmp     edi, 1
  sete    cl
  lea     rdx, [rip + enum_name<0>]
  cmove   rax, rdx
  test    edi, edi
  lea     rdx, [rip + enum_name<1>]
  cmovne  rdx, rax
  mov     eax, 1
  cmovne  rax, rcx
  ret

enum_name<0ul>:
  .ascii  "A"

enum_name<1ul>:
  .ascii  "B"

Compilation times

[include] https://raw.githubusercontent.com/qlibs/reflect/main/reflect

time g++-13.2 -x c++ -std=c++20 reflect -c -DNTEST   # 0.113s
time g++-13.2 -x c++ -std=c++20 reflect -c           # 0.253s

time clang++-17 -x c++ -std=c++20 reflect -c -DNTEST # 0.119s
time clang++-17 -x c++ -std=c++20 reflect -c         # 0.322s

---

API

template <class Fn, class T> requires std::is_aggregate_v<std::remove_cvref_t<T>>
[[nodiscard]] constexpr auto visit(Fn&& fn, T&& t) noexcept;

struct foo { int a; int b; };
static_assert(2 == visit([](auto&&... args) { return sizeof...(args); }, foo{}));

template<class T> requires std::is_aggregate_v<T>
[[nodiscard]] constexpr auto size() -> std::size_t;

template<class T> requires std::is_aggregate_v<T>
[[nodiscard]] constexpr auto size(const T&) -> std::size_t;

struct foo { int a; int b; } f;
static_assert(2 == size<foo>());
static_assert(2 == size(f));

template <class T> [[nodiscard]] constexpr auto type_name() noexcept;
template <class T> [[nodiscard]] constexpr auto type_name(const T&) noexcept;

struct foo { int a; int b; };
static_assert(std::string_view{"foo"} == type_name<foo>());
static_assert(std::string_view{"foo"} == type_name(foo{}));

template <class T> [[nodiscard]] constexpr auto type_id() noexcept;
template <class T> [[nodiscard]] constexpr auto type_id(T&&) noexcept;

struct foo { };
struct bar { };
static_assert(type_id(foo{}) == type_id(foo{}));
static_assert(type_id(bar{}) != type_id<foo>());

template<class E>
[[nodiscard]] constexpr auto to_underlying(const E e) noexcept;

template<class E> requires std::is_enum_v<E>
consteval auto enum_min(const E = {}) { return REFLECT_ENUM_MIN; }

template<class E> requires std::is_enum_v<E>
consteval auto enum_max(const E = {}) { return REFLECT_ENUM_MAX; }

template<class E,
         fixed_string unknown = "",
         auto Min = enum_min(E{}),
         auto Max = enum_max(E{})>
  requires (std::is_enum_v<E> and Max > Min)
[[nodiscard]] constexpr auto enum_name(const E e) noexcept -> std::string_view {

enum class Enum { foo = 1, bar = 2 };
static_assert(std::string_view{"foo"} == enum_name(Enum::foo));
static_assert(std::string_view{"bar"} == enum_name(Enum::bar));

enum class Enum { foo = 1, bar = 1024 };
consteval auto enum_min(Enum) { return Enum::foo; }
consteval auto enum_max(Enum) { return Enum::bar; }

static_assert(std::string_view{"foo"} == enum_name(Enum::foo));
static_assert(std::string_view{"bar"} == enum_name(Enum::bar));

template <std::size_t N, class T>
  requires (std::is_aggregate_v<T> and N < size<T>())
[[nodiscard]] constexpr auto member_name(const T& = {}) noexcept;

struct foo { int a; int b; };
static_assert(std::string_view{"a"} == member_name<0, foo>());
static_assert(std::string_view{"a"} == member_name<0>(foo{}));
static_assert(std::string_view{"b"} == member_name<1, foo>());
static_assert(std::string_view{"b"} == member_name<1>(foo{}));

template<std::size_t N, class T>
  requires (std::is_aggregate_v<std::remove_cvref_t<T>> and
            N < size<std::remove_cvref_t<T>>())
[[nodiscard]] constexpr decltype(auto) get(T&& t) noexcept;

struct foo { int a; bool b; };
constexpr auto f = foo{.i=42, .b=true};
static_assert(42 == get<0>(f));
static_assert(true == get<1>(f));

template <class T, fixed_string Name> requires std::is_aggregate_v<T>
concept has_member_name = /*unspecified*/

struct foo { int a; int b; };
static_assert(has_member_name<foo, "a">);
static_assert(has_member_name<foo, "b">);
static_assert(not has_member_name<foo, "c">);

template<fixed_string Name, class T> requires has_member_name<T, Name>
constexpr decltype(auto) get(T&& t) noexcept;

struct foo { int a; int b; };
constexpr auto f = foo{.i=42, .b=true};
static_assert(42 == get<"a">(f));
static_assert(true == get<"b">(f));

template<fixed_string... Members, class TSrc, class TDst>
  requires (std::is_aggregate_v<TSrc> and std::is_aggregate_v<TDst>)
constexpr auto copy(const TSrc& src, TDst& dst) noexcept -> void;

struct foo { int a; int b; };
struct bar { int a{}; int b{}; };

bar b{};
foo f{};

copy(f, b);
assert(b.a == f.a);
assert(b.b == f.b);

copy<"a">(f, b);
assert(b.a == f.a);
assert(0 == b.b);

template<template<class...> class R, class T>
  requires std::is_aggregate_v<std::remove_cvref_t<T>>
[[nodiscard]] constexpr auto to(T&& t) noexcept;

struct foo { int a; int b; };

constexpr auto t = to<std::tuple>(foo{.a=4, .b=2});
static_assert(4 == std::get<0>(t));
static_assert(2 == std::get<1>(t));

auto f = foo{.a=4, .b=2};
auto t = to<std::tuple>(f);
std::get<0>(t) *= 10;
f.b = 42;
assert(40 == std::get<0>(t) and 40 == f.a);
assert(42 == std::get<1>(t) and 42 == f.b);

template<class R, class T>
[[nodiscard]] constexpr auto to(T&& t);

struct foo { int a; int b; };
struct baz { int a{}; int c{}; };

const auto b = to<baz>(foo{.a=4, .b=2});
assert(4 == b.a and 0 == b.c);

template<std::size_t N, class T> requires std::is_aggregate_v<T>
[[nodiscard]] constexpr auto size_of() -> std::size_t;

template<std::size_t N, class T> requires std::is_aggregate_v<T>
[[nodiscard]] constexpr auto size_of(T&&) -> std::size_t;

template<std::size_t N, class T> requires std::is_aggregate_v<T>
[[nodiscard]] constexpr auto align_of() -> std::size_t;

template<std::size_t N, class T> requires std::is_aggregate_v<T>
[[nodiscard]] constexpr auto align_of(T&&) -> std::size_t;

template<std::size_t N, class T> requires std::is_aggregate_v<T>
[[nodiscard]] constexpr auto offset_of() -> std::size_t;

template<std::size_t N, class T> requires std::is_aggregate_v<T>
[[nodiscard]] constexpr auto offset_of(T&&) -> std::size_t;

struct foo { int a; bool b; };

static_assert(4 == size_of<0, foo>());
static_assert(1 == size_of<1, foo>());
static_assert(4 == align_of<0, foo>());
static_assert(1 == align_of<1, foo>());
static_assert(0 == offset_of<0, foo>());
static_assert(4 == offset_of<1, foo>());

template<class T, class Fn>
  requires std::is_aggregate_v<std::remove_cvref_t<T>>
constexpr auto for_each(Fn&& fn) -> void;

template<class Fn, class T>
  requires std::is_aggregate_v<std::remove_cvref_t<T>>
constexpr auto for_each(Fn&& fn, T&& t) -> void;

struct { int a; int b; } f;

reflect::for_each([&f](const auto I) {
  std::print("{}:{}={}", member_name<I>(f), get<I>(f)); // prints a:int=4, b:int=2
}, f);

Configuration

#define REFLECT_ENUM_MIN 0      // Min size for enum name (can be overridden)
                                // For example: `-DREFLECT_ENUM_MIN=-1`
#define REFLECT_ENUM_MAX 128    // Max size for enum name (can be overridden)
                                // For example: `-DREFLECT_ENUM_MAX=32`

---

FAQ

• How does reflect compare to https://wg21.link/P2996?

  reflect library only provides basic reflection primitives, mostly via hacks and workarounds to deal with lack of the reflection. https://wg21.link/P2996 is a language proposal with many more features and capabilities.

• How does reflect work under the hood?

  There are many different ways to implement reflection. reflect uses C++20's structure bindings, concepts and source_location to do it. See visit implementation for more details.

• How can reflect be agnostic to compiler changes?

  reflect precomputes required prefixes/postfixes to find required names from the source_location::function_name() output for each compiler upon inclusion. Any compiler change will end up with new prefixes/postfixes and wont require additional maintenance.

• What does it mean that reflect tests itself upon include?

  reflect runs all tests (via static_asserts) upon include. If the include compiles it means all tests are passing and the library works correctly on given compiler, environment.

• What is compile-time overhead of reflect library?

  reflect include takes ~.2s (that includes running all tests). The most expensive calls are visit and enum_to_name whose timing will depend on the number of reflected elements and/or min/max values provided. There are no recursive template instantiations in the library.

• Can I disable running tests at compile-time for faster compilation times?

  When -DNTEST is defined static_asserts tests wont be executed upon inclusion. Note: Use with caution as disabling tests means that there are no guarantees upon inclusion that the given compiler/env combination works as expected.

• How to extend the number of members to be reflected (default: 64)?

  Override visit, for example - https://godbolt.org/z/Ga3bc3KKW

  template <class Fn, class T> // requires https://wg21.link/P1061
  [[nodiscard]] constexpr decltype(auto) visit(Fn&& fn, T&& t) noexcept {
    auto&& [... ts] = std::forward<T>(t);
    return std::forward<Fn>(fn)(std::forward_like<T>(ts)...);
  }

• How to integrate with CMake.FetchContent?

  include(FetchContent)
  
  FetchContent_Declare(
    qlibs.reflect
    GIT_REPOSITORY https://github.com/qlibs/reflect
    GIT_TAG v1.2.4
  )
  
  FetchContent_MakeAvailable(qlibs.reflect)
  

  target_link_libraries(${PROJECT_NAME} PUBLIC qlibs.reflect);
  

• Similar projects?

  boost.pfr, glaze, reflect-cpp, magic_enum