// Overview / Examples / API / FAQ
- 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)
- C++20 (gcc-12+, clang-15+, msvc-19.36+)
- STL headers (array, string_view, source_location, type_traits, utility, [tuple])
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)...
- [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"
[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
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 Fn, class T>
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`
-
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. Seevisit
implementation for more details. -
How can
reflect
be agnostic to compiler changes?reflect
precomputes required prefixes/postfixes to find required names from thesource_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 arevisit
andenum_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/Ga3bc3KKWtemplate <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/CPM?
CPMAddPackage( Name reflect GITHUB_REPOSITORY qlibs/reflect GIT_TAG v1.2.2 ) add_library(reflect INTERFACE) target_include_directories(reflect SYSTEM INTERFACE ${reflect_SOURCE_DIR}) add_library(reflect::reflect ALIAS reflect)
target_link_libraries(${PROJECT_NAME} reflect::reflect);
-
Similar projects?