1 Revision History

Since [P2996R1], several changes to the overall library API:

• added qualified_name_of (to partner with name_of)
• removed is_static for being ambiguous, added has_internal_linkage (and has_linkage and has_external_linkage) and is_static_member instead
• added is_class_member, is_namespace_member, and is_concept
• added reflect_invoke
• added all the type traits

Other paper changes: * some updates to examples, including a new examples which add a named tuple and emulate typeful reflection. * more discussion of syntax, constant evaluation order, aliases, and freestanding. * adding lots of wording

Since [P2996R0]:

• added links to Compiler Explorer demonstrating just about all of the examples
• respecified synth_struct to define_class
• respecified a few metafunctions to be functions instead of function templates
• introduced section on error handling mechanism and our preference for exceptions (removing invalid reflections)
• added ticket counter and variant examples
• collapsed entity_ref and pointer_to_member into value_of

2 Introduction

This is a proposal for a reduced initial set of features to support static reflection in C++. Specifically, we are mostly proposing a subset of features suggested in [P1240R2]:

• the representation of program elements via constant-expressions producing reflection values — reflections for short — of an opaque type std::meta::info,
• a reflection operator (prefix ^) that produces a reflection value for its operand construct,
• a number of consteval metafunctions to work with reflections (including deriving other reflections), and
• constructs called splicers to produce grammatical elements from reflections (e.g., [: refl :]).

(Note that this aims at something a little broader than pure “reflection”. We not only want to observe the structure of the program: We also want to ease generating code that depends on those observations. That combination is sometimes referred to as “reflective metaprogramming”, but within WG21 discussion the term “reflection” has often been used informally to refer to the same general idea.)

This proposal is not intended to be the end-game as far as reflection and compile-time metaprogramming are concerned. Instead, we expect it will be a useful core around which more powerful features will be added incrementally over time. In particular, we believe that most or all the remaining features explored in P1240R2 and that code injection (along the lines described in [P2237R0]) are desirable directions to pursue.

Our choice to start with something smaller is primarily motivated by the belief that that improves the chances of these facilities making it into the language sooner rather than later.

2.1 Notable Additions to P1240

While we tried to select a useful subset of the P1240 features, we also made a few additions and changes. Most of those changes are minor. For example, we added a std::meta::test_type interface that makes it convenient to use existing standard type predicates (such as is_class_v) in reflection computations.

One addition does stand out, however: We have added metafunctions that permit the synthesis of simple struct and union types. While it is not nearly as powerful as generalized code injection (see [P2237R0]), it can be remarkably effective in practice.

2.2 Why a single opaque reflection type?

Perhaps the most common suggestion made regarding the framework outlined in P1240 is to switch from the single std::meta::info type to a family of types covering various language elements (e.g., std::meta::variable, std::meta::type, etc.).

We believe that doing so would be a mistake with very serious consequences for the future of C++.

Specifically, it would codify the language design into the type system. We know from experience that it has been quasi-impossible to change the semantics of standard types once they were standardized, and there is no reason to think that such evolution would become easier in the future. Suppose for example that we had standardized a reflection type std::meta::variable in C++03 to represent what the standard called “variables” at the time. In C++11, the term “variable” was extended to include “references”. Such an change would have been difficult to do given that C++ by then likely would have had plenty of code that depended on a type arrangement around the more restricted definition of “variable”. That scenario is clearly backward-looking, but there is no reason to believe that similar changes might not be wanted in the future and we strongly believe that it behooves us to avoid adding undue constraints on the evolution of the language.

Other advantages of a single opaque type include:

• it makes no assumptions about the representation used within the implementation (e.g., it doesn’t advantage one compiler over another),
• it is trivially extensible (no types need to be added to represent additional language elements and meta-elements as the language evolves), and
• it allows convenient collections of heterogeneous constructs without having to surface reference semantics (e.g., a std::vector<std::meta::info> can easily represent a mixed template argument list — containing types and nontypes — without fear of slicing values).

2.3 Implementation Status

Lock3 implemented the equivalent of much that is proposed here in a fork of Clang (specifically, it worked with the P1240 proposal, but also included several other capabilities including a first-class injection mechanism).

EDG has an ongoing implementation of this proposal that is currently available on Compiler Explorer (thank you, Matt Godbolt). Nearly all of the examples below have links to compiler explorer demonstrating them.

The implementation is not complete (notably, for debugging purposes, name_of(^int) yields an empty string and name_of(^std::optional<std::string>) yields "optional", neither of which are what we want). The implementation will evolve along with this paper. The EDG implementation also lacks some of the other language features we would like to be able to take advantage of. In particular, it does not support expansion statements. A workaround that will be used in the linked implementations of examples is the following facility:

namespace __impl {
  template<auto... vals>
  struct replicator_type {
    template<typename F>
      constexpr void operator>>(F body) const {
        (body.template operator()<vals>(), ...);
      }
  };

  template<auto... vals>
  replicator_type<vals...> replicator = {};
}

template<typename R>
consteval auto expand(R range) {
  std::vector<std::meta::info> args;
  for (auto r : range) {
    args.push_back(reflect_value(r));
  }
  return substitute(^__impl::replicator, args);
}

Used like:

template <typename E>
  requires std::is_enum_v<E>
constexpr std::string enum_to_string(E value) {
  template for (constexpr auto e : std::meta::enumerators_of(^E)) {
    if (value == [:e:]) {
      return std::string(std::meta::name_of(e));
    }
  }

  return "<unnamed>";
}

template<typename E>
  requires std::is_enum_v<E>
constexpr std::string enum_to_string(E value) {
  std::string result = "<unnamed>";
  [:expand(std::meta::enumerators_of(^E)):] >> [&]<auto e>{
    if (value == [:e:]) {
      result = std::meta::name_of(e);
    }
  };
  return result;
}

3 Examples

We start with a number of examples that show off what is possible with the proposed set of features. It is expected that these are mostly self-explanatory. Read ahead to the next sections for a more systematic description of each element of this proposal.

A number of our examples here show a few other language features that we hope to progress at the same time. This facility does not strictly rely on these features, and it is possible to do without them - but it would greatly help the usability experience if those could be adopted as well:

• expansion statements [P1306R1]
• non-transient constexpr allocation [P0784R7] [P1974R0] [P2670R1]

3.1 Back-And-Forth

Our first example is not meant to be compelling but to show how to go back and forth between the reflection domain and the grammatical domain:

constexpr auto r = ^int;
typename[:r:] x = 42;       // Same as: int x = 42;
typename[:^char:] c = '*';  // Same as: char c = '*';

The typename prefix can be omitted in the same contexts as with dependent qualified names (i.e., in what the standard calls type-only contexts). For example:

using MyType = [:sizeof(int)<sizeof(long)? ^long : ^int:];  // Implicit "typename" prefix.

On Compiler Explorer.

3.2 Selecting Members

Our second example enables selecting a member “by number” for a specific type:

struct S { unsigned i:2, j:6; };

consteval auto member_number(int n) {
  if (n == 0) return ^S::i;
  else if (n == 1) return ^S::j;
}

int main() {
  S s{0, 0};
  s.[:member_number(1):] = 42;  // Same as: s.j = 42;
  s.[:member_number(5):] = 0;   // Error (member_number(5) is not a constant).
}

This example also illustrates that bit fields are not beyond the reach of this proposal.

On Compiler Explorer

Note that a “member access splice” like s.[:member_number(1):] is a more direct member access mechanism than the traditional syntax. It doesn’t involve member name lookup, access checking, or — if the spliced reflection value denotes a member function — overload resolution.

This proposal includes a number of consteval “metafunctions” that enable the introspection of various language constructs. Among those metafunctions is std::meta::nonstatic_data_members_of which returns a vector of reflection values that describe the nonstatic members of a given type. We could thus rewrite the above example as:

struct S { unsigned i:2, j:6; };

consteval auto member_number(int n) {
  return std::meta::nonstatic_data_members_of(^S)[n];
}

int main() {
  S s{0, 0};
  s.[:member_number(1):] = 42;  // Same as: s.j = 42;
  s.[:member_number(5):] = 0;   // Error (member_number(5) is not a constant).
}

On Compiler Explorer

This proposal specifies that namespace std::meta is associated with the reflection type (std::meta::info); the std::meta:: qualification can therefore be omitted in the example above.

Another frequently-useful metafunction is std::meta::name_of, which returns a std::string_view describing the unqualified name of an entity denoted by a given reflection value. With such a facility, we could conceivably access nonstatic data members “by string”:

struct S { unsigned i:2, j:6; };

consteval auto member_named(std::string_view name) {
  for (std::meta::info field : nonstatic_data_members_of(^S)) {
    if (name_of(field) == name) return field;
  }
}

int main() {
  S s{0, 0};
  s.[:member_named("j"):] = 42;  // Same as: s.j = 42;
  s.[:member_named("x"):] = 0;   // Error (member_named("x") is not a constant).
}

On Compiler Explorer

3.3 List of Types to List of Sizes

Here, sizes will be a std::array<std::size_t, 3> initialized with {sizeof(int), sizeof(float), sizeof(double)}:

constexpr std::array types = {^int, ^float, ^double};
constexpr std::array sizes = []{
  std::array<std::size_t, types.size()> r;
  std::ranges::transform(types, r.begin(), std::meta::size_of);
  return r;
}();

Compare this to the following type-based approach, which produces the same array sizes:

template<class...> struct list {};

using types = list<int, float, double>;

constexpr auto sizes = []<template<class...> class L, class... T>(L<T...>) {
    return std::array<std::size_t, sizeof...(T)>{{ sizeof(T)... }};
}(types{});

On Compiler Explorer.

3.4 Implementing make_integer_sequence

We can provide a better implementation of make_integer_sequence than a hand-rolled approach using regular template metaprogramming (although standard libraries today rely on an intrinsic for this):

#include <utility>
#include <vector>

template<typename T>
consteval std::meta::info make_integer_seq_refl(T N) {
  std::vector args{^T};
  for (T k = 0; k < N; ++k) {
    args.push_back(std::meta::reflect_value(k));
  }
  return substitute(^std::integer_sequence, args);
}

template<typename T, T N>
  using make_integer_sequence = [:make_integer_seq_refl<T>(N):];

On Compiler Explorer.

Note that the memoization implicit in the template substitution process still applies. So having multiple uses of, e.g., make_integer_sequence<int, 20> will only involve one evaluation of make_integer_seq_refl<int>(20).

3.5 Getting Class Layout

struct member_descriptor
{
  std::size_t offset;
  std::size_t size;
};

// returns std::array<member_descriptor, N>
template <typename S>
consteval auto get_layout() {
  constexpr auto members = nonstatic_data_members_of(^S);
  std::array<member_descriptor, members.size()> layout;
  for (int i = 0; i < members.size(); ++i) {
      layout[i] = {.offset=offset_of(members[i]), .size=size_of(members[i])};
  }
  return layout;
}

struct X
{
    char a;
    int b;
    double c;
};

/*constexpr*/ auto Xd = get_layout<X>();

/*
where Xd would be std::array<member_descriptor, 3>{{
  { 0, 1 }, { 4, 4 }, { 8, 8 }
}}
*/

On Compiler Explorer.

3.6 Enum to String

One of the most commonly requested facilities is to convert an enum value to a string (this example relies on expansion statements):

template <typename E>
  requires std::is_enum_v<E>
constexpr std::string enum_to_string(E value) {
  template for (constexpr auto e : std::meta::enumerators_of(^E)) {
    if (value == [:e:]) {
      return std::string(std::meta::name_of(e));
    }
  }

  return "<unnamed>";
}

enum Color { red, green, blue };
static_assert(enum_to_string(Color::red) == "red");
static_assert(enum_to_string(Color(42)) == "<unnamed>");

We can also do the reverse in pretty much the same way:

template <typename E>
  requires std::is_enum_v<E>
constexpr std::optional<E> string_to_enum(std::string_view name) {
  template for (constexpr auto e : std::meta::enumerators_of(^E)) {
    if (name == std::meta::name_of(e)) {
      return [:e:];
    }
  }

  return std::nullopt;
}

But we don’t have to use expansion statements - we can also use algorithms. For instance, enum_to_string can also be implemented this way (this example relies on non-transient constexpr allocation), which also demonstrates choosing a different algorithm based on the number of enumerators:

template <typename E>
  requires std::is_enum_v<E>
constexpr std::string enum_to_string(E value) {
  constexpr auto get_pairs = []{
    return std::meta::enumerators_of(^E)
      | std::views::transform([](std::meta::info e){
          return std::pair<E, std::string>(std::meta::value_of<E>(e), std::meta::name_of(e));
        })
  };

  constexpr auto get_name = [](E value) -> std::optional<std::string> {
    if constexpr (enumerators_of(^E).size() <= 7) {
      // if there aren't many enumerators, use a vector with find_if()
      constexpr auto enumerators = get_pairs() | std::ranges::to<std::vector>();
      auto it = std::ranges::find_if(enumerators, [value](auto const& pr){
        return pr.first == value;
      };
      if (it == enumerators.end()) {
        return std::nullopt;
      } else {
        return it->second;
      }
    } else {
      // if there are lots of enumerators, use a map with find()
      constexpr auto enumerators = get_pairs() | std::ranges::to<std::map>();
      auto it = enumerators.find(value);
      if (it == enumerators.end()) {
        return std::nullopt;
      } else {
        return it->second;
      }
    }
  };

  return get_name(value).value_or("<unnamed>");
}

Note that this last version has lower complexity: While the versions using an expansion statement use an expected O(N) number of comparisons to find the matching entry, a std::map achieves the same with O(log(N)) complexity (where N is the number of enumerator constants).

On Compiler Explorer.

Many many variations of these functions are possible and beneficial depending on the needs of the client code. For example:

• the "<unnamed>" case could instead output a valid cast expression like "E(5)"
• a compact two-way persistent data structure could be generated to support both enum_to_string and string_to_enum with a minimal footprint
• etc.

3.7 Parsing Command-Line Options

Our next example shows how a command-line option parser could work by automatically inferring flags based on member names. A real command-line parser would of course be more complex, this is just the beginning.

template<typename Opts>
auto parse_options(std::span<std::string_view const> args) -> Opts {
  Opts opts;
  template for (constexpr auto dm : nonstatic_data_members_of(^Opts)) {
    auto it = std::ranges::find_if(args,
      [](std::string_view arg){
        return arg.starts_with("--") && arg.substr(2) == name_of(dm);
      });

    if (it == args.end()) {
      // no option provided, use default
      continue;
    } else if (it + 1 == args.end()) {
      std::print(stderr, "Option {} is missing a value\n", *it);
      std::exit(EXIT_FAILURE);
    }

    using T = typename[:type_of(dm):];
    auto iss = std::ispanstream(it[1]);
    if (iss >> opts.[:dm:]; !iss) {
      std::print(stderr, "Failed to parse option {} into a {}\n", *it, display_name_of(^T));
      std::exit(EXIT_FAILURE);
    }
  }
  return opts;
}

struct MyOpts {
  std::string file_name = "input.txt";  // Option "--file_name <string>"
  int    count = 1;                     // Option "--count <int>"
};

int main(int argc, char *argv[]) {
  MyOpts opts = parse_options<MyOpts>(std::vector<std::string_view>(argv+1, argv+argc));
  // ...
}

This example is based on a presentation by Matúš Chochlík.

On Compiler Explorer.

3.8 A Simple Tuple Type

#include <meta>

template<typename... Ts> struct Tuple {
  struct storage;

  static_assert(is_type(define_class(^storage, {data_member_spec(^Ts)...})));
  storage data;

  Tuple(): data{} {}
  Tuple(Ts const& ...vs): data{ vs... } {}
};

template<typename... Ts>
  struct std::tuple_size<Tuple<Ts...>>: public integral_constant<size_t, sizeof...(Ts)> {};

template<std::size_t I, typename... Ts>
  struct std::tuple_element<I, Tuple<Ts...>> {
    static constexpr std::array types = {^Ts...};
    using type = [: types[I] :];
  };

consteval std::meta::info get_nth_field(std::meta::info r, std::size_t n) {
  return nonstatic_data_members_of(r)[n];
}

template<std::size_t I, typename... Ts>
  constexpr auto get(Tuple<Ts...> &t) noexcept -> std::tuple_element_t<I, Tuple<Ts...>>& {
    return t.data.[:get_nth_field(^decltype(t.data), I):];
  }
// Similarly for other value categories...

This example uses a “magic” std::meta::define_class template along with member reflection through the nonstatic_data_members_of metafunction to implement a std::tuple-like type without the usual complex and costly template metaprogramming tricks that that involves when these facilities are not available. define_class takes a reflection for an incomplete class or union plus a vector of nonstatic data member descriptions, and completes the give class or union type to have the described members.

On Compiler Explorer.

3.9 A Simple Variant Type

Similarly to how we can implement a tuple using define_class to create on the fly a type with one member for each Ts..., we can implement a variant that simply defines a union instead of a struct. One difference here is how the destructor of a union is currently defined:

union U1 {
  int i;
  char c;
};

union U2 {
  int i;
  std::string s;
};

U1 has a trivial destructor, but U2’s destructor is defined as deleted (because std::string has a non-trivial destructor). This is a problem because we need to define this thing… somehow. However, for the purposes of define_class, there really is only one reasonable option to choose here:

template <class... Ts>
union U {
  // all of our members
  Ts... members;

  // a defaulted destructor if all of the types are trivially destructible
  constexpr ~U() requires (std::is_trivially_destructible_v<Ts> && ...) = default;

  // ... otherwise a destructor that does nothing
  constexpr ~U() { }
};

If we make define_class for a union have this behavior, then we can implement a variant in a much more straightforward way than in current implementations. This is not a complete implementation of std::variant (and cheats using libstdc++ internals, and also uses Boost.Mp11’s mp_with_index) but should demonstrate the idea:

template <typename... Ts>
class Variant {
    union Storage;
    struct Empty { };

    static_assert(is_type(define_class(^Storage, {
        data_member_spec(^Empty, {.name="empty"}),
        data_member_spec(^Ts)...
    })));

    static constexpr std::array<std::meta::info, sizeof...(Ts)> types = {^Ts...};

    static consteval std::meta::info get_nth_field(std::size_t n) {
        return nonstatic_data_members_of(^Storage)[n+1];
    }

    Storage storage_;
    int index_ = -1;

    // cheat: use libstdc++'s implementation
    template <typename T>
    static constexpr size_t accepted_index = std::__detail::__variant::__accepted_index<T, std::variant<Ts...>>;

    template <class F>
    constexpr auto with_index(F&& f) const -> decltype(auto) {
        return mp_with_index<sizeof...(Ts)>(index_, (F&&)f);
    }

public:
    constexpr Variant() requires std::is_default_constructible_v<[: types[0] :]>
        // should this work: storage_{. [: get_nth_field(0) :]{} }
        : storage_{.empty={}}
        , index_(0)
    {
        std::construct_at(&storage_.[: get_nth_field(0) :]);
    }

    constexpr ~Variant() requires (std::is_trivially_destructible_v<Ts> and ...) = default;
    constexpr ~Variant() {
        if (index_ != -1) {
            with_index([&](auto I){
                std::destroy_at(&storage_.[: get_nth_field(I) :]);
            });
        }
    }

    template <typename T, size_t I = accepted_index<T&&>>
        requires (!std::is_base_of_v<Variant, std::decay_t<T>>)
    constexpr Variant(T&& t)
        : storage_{.empty={}}
        , index_(-1)
    {
        std::construct_at(&storage_.[: get_nth_field(I) :], (T&&)t);
        index_ = (int)I;
    }

    // you can't actually express this constraint nicely until P2963
    constexpr Variant(Variant const&) requires (std::is_trivially_copyable_v<Ts> and ...) = default;
    constexpr Variant(Variant const& rhs)
            requires ((std::is_copy_constructible_v<Ts> and ...)
                and not (std::is_trivially_copyable_v<Ts> and ...))
        : storage_{.empty={}}
        , index_(-1)
    {
        rhs.with_index([&](auto I){
            constexpr auto field = get_nth_field(I);
            std::construct_at(&storage_.[: field :], rhs.storage_.[: field :]);
            index_ = I;
        });
    }

    constexpr auto index() const -> int { return index_; }

    template <class F>
    constexpr auto visit(F&& f) const -> decltype(auto) {
        if (index_ == -1) {
            throw std::bad_variant_access();
        }

        return mp_with_index<sizeof...(Ts)>(index_, [&](auto I) -> decltype(auto) {
            return std::invoke((F&&)f,  storage_.[: get_nth_field(I) :]);
        });
    }
};

Effectively, Variant<T, U> synthesizes a union type Storage which looks like this:

union Storage {
    Empty empty;
    T unnamed0;
    U unnamed1;

    ~Storage() requires std::is_trivially_destructible_v<T> && std::is_trivially_destructible_v<U> = default;
    ~Storage() { }
}

The question here is whether we should be should be able to directly initialize members of a defined union using a splicer, as in:

: storage{.[: get_nth_field(0) :]={}}

Arguably, the answer should be yes - this would be consistent with how other accesses work.

On Compiler Explorer.

3.10 Struct to Struct of Arrays

#include <meta>
#include <array>

template <typename T, std::size_t N>
struct struct_of_arrays_impl;

consteval auto make_struct_of_arrays(std::meta::info type,
                                     std::meta::info N) -> std::meta::info {
  std::vector<std::meta::info> old_members = nonstatic_data_members_of(type);
  std::vector<std::meta::info> new_members = {};
  for (std::meta::info member : old_members) {
    auto array_type = substitute(^std::array, {type_of(member), N });
    auto mem_descr = data_member_spec(array_type, {.name = name_of(member)});
    new_members.push_back(mem_descr);
  }
  return std::meta::define_class(
    substitute(^struct_of_arrays_impl, {type, N}),
    new_members);
}

template <typename T, size_t N>
using struct_of_arrays = [: make_struct_of_arrays(^T, ^N) :];

Example:

struct point {
  float x;
  float y;
  float z;
};

using points = struct_of_arrays<point, 30>;
// equivalent to:
// struct points {
//   std::array<float, 30> x;
//   std::array<float, 30> y;
//   std::array<float, 30> z;
// };

Again, the combination of nonstatic_data_members_of and define_class is put to good use.

On Compiler Explorer.

3.11 Parsing Command-Line Options II

Now that we’ve seen a couple examples of using std::meta::define_class to create a type, we can create a more sophisticated command-line parser example.

This is the opening example for clap (Rust’s Command Line Argument Parser):

struct Args : Clap {
  Option<std::string, {.use_short=true, .use_long=true}> name;
  Option<int, {.use_short=true, .use_long=true}> count = 1;
};

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

  for (int i = 0; i < opts.count; ++i) {  // opts.count has type int
    std::print("Hello {}!", opts.name);   // opts.name has type std::string
  }
}

Which we can implement like this:

struct Flags {
  bool use_short;
  bool use_long;
};

template <typename T, Flags flags>
struct Option {
  std::optional<T> initializer = {};

  // some suitable constructors and accessors for flags
};

// convert a type (all of whose non-static data members are specializations of Option)
// to a type that is just the appropriate members.
// For example, if type is a reflection of the Args presented above, then this
// function would evaluate to a reflection of the type
// struct {
//   std::string name;
//   int count;
// }
consteval auto spec_to_opts(std::meta::info opts,
                            std::meta::info spec) -> std::meta::info {
  std::vector<std::meta::info> new_members;
  for (std::meta::info member : nonstatic_data_members_of(spec)) {
    auto new_type = template_arguments_of(type_of(member))[0];
    new_members.push_back(data_member_spec(new_type, {.name=name_of(member)}));
  }
  return define_class(opts, new_members);
}

struct Clap {
  template <typename Spec>
  auto parse(this Spec const& spec, int argc, char** argv) {
    std::vector<std::string_view> cmdline(argv+1, argv+argc)

    // check if cmdline contains --help, etc.

    struct Opts;
    static_assert(is_type(spec_to_opts(^Opts, ^Spec)));
    Opts opts;

    template for (constexpr auto [sm, om] : std::views::zip(nonstatic_data_members_of(^Spec),
                                                            nonstatic_data_members_of(^Opts))) {
      auto const& cur = spec.[:sm:];
      constexpr auto type = type_of(om);

      // find the argument associated with this option
      auto it = std::ranges::find_if(cmdline,
        [&](std::string_view arg){
          return (cur.use_short && arg.size() == 2 && arg[0] == '-' && arg[1] == name_of(sm)[0])
              || (cur.use_long && arg.starts_with("--") && arg.substr(2) == name_of(sm));
        });

      // no such argument
      if (it == cmdline.end()) {
        if constexpr (has_template_arguments(type) and template_of(type) == ^std::optional) {
          // the type is optional, so the argument is too
          continue;
        } else if (cur.initializer) {
          // the type isn't optional, but an initializer is provided, use that
          opts.[:om:] = *cur.initializer;
          continue;
        } else {
          std::print(stderr, "Missing required option {}\n", name_of(sm));
          std::exit(EXIT_FAILURE);
        }
      } else if (it + 1 == cmdline.end()) {
        std::print(stderr, "Option {} for {} is missing a value\n", *it, name_of(sm));
        std::exit(EXIT_FAILURE);
      }

      // found our argument, try to parse it
      auto iss = ispanstream(it[1]);
      if (iss >> opts.[:om:]; !iss) {
        std::print(stderr, "Failed to parse {:?} into option {} of type {}\n",
          it[1], name_of(sm), display_name_of(type));
        std::exit(EXIT_FAILURE);
      }
    }
    return opts;
  }
};

On Compiler Explorer.

3.12 A Universal Formatter

This example is taken from Boost.Describe:

struct universal_formatter {
  constexpr auto parse(auto& ctx) { return ctx.begin(); }

  template <typename T>
  auto format(T const& t, auto& ctx) const {
    auto out = std::format_to(ctx.out(), "{}{{", name_of(^T));

    auto delim = [first=true]() mutable {
      if (!first) {
        *out++ = ',';
        *out++ = ' ';
      }
      first = false;
    };

    template for (constexpr auto base : bases_of(^T)) {
      delim();
      out = std::format_to(out, "{}", static_cast<[:base:] const&>(t));
    }

    template for (constexpr auto mem : nonstatic_data_members_of(^T)) {
      delim();
      out = std::format_to(out, ".{}={}", name_of(mem), t.[:mem:]);
    }

    *out++ = '}';
    return out;
  }
};

struct X { int m1 = 1; };
struct Y { int m2 = 2; };
class Z : public X, private Y { int m3 = 3; int m4 = 4; };

template <> struct std::formatter<X> : universal_formatter { };
template <> struct std::formatter<Y> : universal_formatter { };
template <> struct std::formatter<Z> : universal_formatter { };

int main() {
    std::println("{}", Z()); // Z{X{.m1 = 1}, Y{.m2 = 2}, .m3 = 3, .m4 = 4}
}

This example is not implemented on compiler explorer at this time, but only because of issues compiling both std::format and fmt::format.

3.13 Implementing member-wise hash_append

Based on the [N3980] API:

template <typename H, typename T> requires std::is_standard_layout_v<T>
void hash_append(H& algo, T const& t) {
    template for (constexpr auto mem : nonstatic_data_members_of(^T)) {
        hash_append(algo, t.[:mem:]);
    }
}

3.14 Converting a Struct to a Tuple

This approach requires allowing packs in structured bindings [P1061R5], but can also be written using std::make_index_sequence:

template <typename T>
constexpr auto struct_to_tuple(T const& t) {
  constexpr auto members = nonstatic_data_members_of(^T);

  constexpr auto indices = []{
    std::array<int, members.size()> indices;
    std::ranges::iota(indices, 0);
    return indices;
  }();

  constexpr auto [...Is] = indices;
  return std::make_tuple(t.[: members[Is] :]...);
}

An alternative approach is:

consteval auto struct_to_tuple_type(info type) -> info {
  return substitute(^std::tuple,
                    nonstatic_data_members_of(type)
                    | std::ranges::transform(std::meta::type_of)
                    | std::ranges::transform(std::meta::remove_cvref)
                    | std::ranges::to<std::vector>());
}

template <typename To, typename From, std::meta::info ... members>
constexpr auto struct_to_tuple_helper(From const& from) -> To {
  return To(from.[:members:]...);
}

template<typename From>
consteval auto get_struct_to_tuple_helper() {
  using To = [: struct_to_tuple_type(^From): ];

  std::vector args = {^To, ^From};
  for (auto mem : nonstatic_data_members_of(^From)) {
    args.push_back(reflect_value(mem));
  }

  /*
  Alternatively, with Ranges:
  args.append_range(
    nonstatic_data_members_of(^From)
    | std::views::transform(std::meta::reflect_value)
    );
  */

  return value_of<To(*)(From const&)>(
    substitute(^struct_to_tuple_helper, args));
}

template <typename From>
constexpr auto struct_to_tuple(From const& from) {
  return get_struct_to_tuple_helper<From>()(from);
}

Here, struct_to_tuple_type takes a reflection of a type like struct { T t; U const& u; V v; } and returns a reflection of the type std::tuple<T, U, V>. That gives us the return type. Then, struct_to_tuple_helper is a function template that does the actual conversion — which it can do by having all the reflections of the members as a non-type template parameter pack. This is a constexpr function and not a consteval function because in the general case the conversion is a run-time operation. However, determining the instance of struct_to_tuple_helper that is needed is a compile-time operation and has to be performed with a consteval function (because the function invokes nonstatic_data_members_of), hence the separate function template get_struct_to_tuple_helper().

Everything is put together by using substitute to create the instantiation of struct_to_tuple_helper that we need, and a compile-time reference to that instance is obtained with value_of. Thus f is a function reference to the correct specialization of struct_to_tuple_helper, which we can simply invoke.

On Compiler Explorer, with a different implementation than either of the above.

3.15 Named Tuple

The tricky thing with implementing a named tuple is actually strings as non-type template parameters. Because you cannot just pass "x" into a non-type template parameter of the form auto V, that leaves us with two ways of specifying the constituents:

1. Can introduce a pair type so that we can write make_named_tuple<pair<int, "x">, pair<double, "y">>(), or
2. Can just do reflections all the way down so that we can write make_named_tuple<^int, ^"x", ^double, ^"y">().

We do not currently support splicing string literals (although that may change in the next revision), and the pair approach follows the similar pattern already shown with define_class (given a suitable fixed_string type):

template <class T, fixed_string Name>
struct pair {
    static constexpr auto name() -> std::string_view { return Name.view(); }
    using type = T;
};

template <class... Tags>
consteval auto make_named_tuple(std::meta::info type, Tags... tags) {
    std::vector<std::meta::info> nsdms;
    auto f = [&]<class Tag>(Tag tag){
        nsdms.push_back(data_member_spec(
            dealias(^typename Tag::type),
            {.name=Tag::name()}));

    };
    (f(tags), ...);
    return define_class(type, nsdms);
}

struct R;
static_assert(is_type(make_named_tuple(^R, pair<int, "x">{}, pair<double, "y">{})));

static_assert(type_of(nonstatic_data_members_of(^R)[0]) == ^int);
static_assert(type_of(nonstatic_data_members_of(^R)[1]) == ^double);

int main() {
    [[maybe_unused]] auto r = R{.x=1, .y=2.0};
}

On Compiler Explorer.

3.16 Compile-Time Ticket Counter

The features proposed here make it a little easier to update a ticket counter at compile time. This is not an ideal implementation (we’d prefer direct support for compile-time —– i.e., consteval — variables), but it shows how compile-time mutable state surfaces in new ways.

class TU_Ticket {
  template<int N> struct Helper {
    static constexpr int value = N;
  };
public:
  static consteval int next() {
    // Search for the next incomplete Helper<k>.
    std::meta::info r;
    for (int k = 0;; ++k) {
      r = substitute(^Helper, { std::meta::reflect_value(k) });
      if (is_incomplete_type(r)) break;
    }
    // Return the value of its member.  Calling static_data_members_of
    // triggers the instantiation (i.e., completion) of Helper<k>.
    return value_of<int>(static_data_members_of(r)[0]);
  }
};

int x = TU_Ticket::next();  // x initialized to 0.
int y = TU_Ticket::next();  // y initialized to 1.
int z = TU_Ticket::next();  // z initialized to 2.

Note that this relies on the fact that a call to substitute returns a specialization of a template, but doesn’t trigger the instantiation of that specialization. Thus, the only instantiations of TU_Ticket::Helper occur because of the call to nonstatic_data_members_of (which is a singleton representing the lone value member).

On Compiler Explorer.

3.17 Emulating typeful reflection

Although we believe a single opaque std::meta::info type to be the best and most scalable foundation for reflection, we acknowledge the desire expressed by SG7 for future support for “typeful reflection”. The following demonstrates one possible means of assembling a typeful reflection library, in which different classes of reflections are represented by distinct types, on top of the facilities proposed here.

// Represents a 'std::meta::info' constrained by a predicate.
template <std::meta::info Pred>
  requires (type_of(^([:Pred:](^int))) == ^bool)
struct metatype {
  std::meta::info value;

  // Construction is ill-formed unless predicate is satisfied.
  consteval metatype(std::meta::info r) : value(r) {
    if (![:Pred:](r))
      throw "Reflection is not a member of this metatype";
  }

  // Cast to 'std::meta::info' allows values of this type to be spliced.
  consteval operator std::meta::info() const { return value; }

  static consteval bool check(std::meta::info r) { return [:Pred:](r); }
};

// Type representing a "failure to match" any known metatypes.
struct unmatched {
  consteval unmatched(std::meta::info) {}
  static consteval bool check(std::meta::info) { return true; }
};

// Returns the given reflection "enriched" with a more descriptive type.
template <typename... Choices>
consteval std::meta::info enrich(std::meta::info r) {
  // Because we control the type, we know that the constructor taking info is
  // the first constructor. The copy/move constructors are added at the }, so
  // will be the last ones in the list.
  std::array ctors = {members_of(^Choices, std::meta::is_constructor)[0]...,
                      members_of(^unmatched, std::meta::is_constructor)[0]};
  std::array checks = {^Choices::check..., ^unmatched::check};

  std::meta::info choice;
  for (auto [check, ctor] : std::views::zip(checks, ctors))
    if (value_of<bool>(reflect_invoke(check, {reflect_value(r)})))
      return reflect_invoke(ctor, {reflect_value(r)});

  std::unreachable();
}

We can leverage this machinery to select different function overloads based on the “type” of reflection provided as an argument.

using type_t = metatype<^std::meta::is_type>;
using fn_t = metatype<^std::meta::is_function>;

// Example of a function overloaded for different "types" of reflections.
void PrintKind(type_t) { std::println("type"); }
void PrintKind(fn_t) { std::println("function"); }
void PrintKind(unmatched) { std::println("unknown kind"); }

int main() {
  // Classifies any reflection as one of: Type, Function, or Unmatched.
  auto enrich = [](std::meta::info r) { return ::enrich<type_t, fn_t>(r); };

  // Demonstration of using 'enrich' to select an overload.
  PrintKind([:enrich(^main):]);  // "function"
  PrintKind([:enrich(^int):]);   // "type"
  PrintKind([:enrich(^3):]);     // "unknown kind"
}

Note that the metatype class can be generalized to wrap values of any literal type, or to wrap multiple values of possibly different types. This has been used, for instance, to select compile-time overloads based on: whether two integers share the same parity, the presence or absence of a value in an optional, the type of the value held by a variant or an any, or the syntactic form of a compile-time string.

Achieving the same in C++23, with the same generality, would require spelling the argument(s) twice: first to obtain a “classification tag” to use as a template argument, and again to call the function, i.e.,

Printer::PrintKind<classify(^int)>(^int).
// or worse...
fn<classify(Arg1, Arg2, Arg3)>(Arg1, Arg2, Arg3).

4 Proposed Features

4.1 The Reflection Operator (^)

The reflection operator produces a reflection value from a grammatical construct (its operand):

unary-expression:
      …
      ^ ::
      ^ namespace-name
      ^ type-id
      ^ cast-expression

Note that cast-expression includes id-expression, which in turn can designate templates, member names, etc.

The current proposal requires that the cast-expression be:

• a primary-expression referring to a function or member function, or
• a primary-expression referring to a variable, static data member, or structured binding, or
• a primary-expression referring to a nonstatic data member, or
• a primary-expression referring to a template, or
• a constant-expression.

In a SFINAE context, a failure to substitute the operand of a reflection operator construct causes that construct to not evaluate to constant.

4.1.1 Syntax discussion

The original TS landed on reflexpr(...) as the syntax to reflect source constructs and [P1240R0] adopted that syntax as well. As more examples were discussed, it became clear that that syntax was both (a) too “heavy” and (b) insufficiently distinct from a function call. SG7 eventually agreed upon the prefix ^ operator. The “upward arrow” interpretation of the caret matches the “lift” or “raise” verbs that are sometimes used to describe the reflection operation in other contexts.

The caret already has a meaning as a binary operator in C++ (“exclusive OR”), but that is clearly not conflicting with a prefix operator. In C++/CLI (a Microsoft C++ dialect) the caret is also used as a new kind of ptr-operator (9.3.1 [dcl.decl.general]) to declare “handles”. That is also not conflicting with the use of the caret as a unary operator because C++/CLI uses the usual prefix * operator to dereference handled.

Apple also uses the caret in the syntax “blocks” and unfortunately we believe that does conflict with our proposed use of the caret.

Since the syntax discussions in SG7 landed on the use of the caret, new basic source characters have become available: @, `, and $. Of those, @ seems the most likely substitute for the caret, because $ is used for splice-like operations in other languages and ` is suggestive of some kind of quoting (which may be useful in future metaprogramming syntax developments).

Another option might be the use of the backslash (\). It currently has a meaning at the end of a line of source code, but we could still use it as a prefix operator with the constraint that the reflected operand has to start on the same source line. If we were to opt for that choice, it could make sense to use the slash (/) as a unary operator denoting splicing (see Splicers below) so that \ would correspond to “raise” and / would correspond to “lower”.

4.2 Splicers ([:…:])

A reflection can be “spliced” into source code using one of several splicer forms:

• [: r :] produces an expression evaluating to the entity or constant value represented by r in grammatical contexts that permit expressions. In type-only contexts (13.8.1 [temp.res.general]/4), [: r :] produces a type (and r must be the reflection of a type). In contexts that only permit a namespace name, [: r :] produces a namespace (and r must be the reflection of a namespace or alias thereof).
• typename[: r :] produces a simple-type-specifier corresponding to the type represented by r.
• template[: r :] produces a template-name corresponding to the template represented by r.
• [:r:]:: produces a nested-name-specifier corresponding to the namespace, enumeration type, or class type represented by r.

The operand of a splicer is implicitly converted to a std::meta::info prvalue (i.e., if the operand expression has a class type that with a conversion function to convert to std::meta::info, splicing can still work).

Attempting to splice a reflection value that does not meet the requirement of the splice is ill-formed. For example:

typename[: ^:: :] x = 0;  // Error.

4.2.1 Range Splicers

The splicers described above all take a single object of type std::meta::info (described in more detail below). However, there are many cases where we don’t have a single reflection, we have a range of reflections - and we want to splice them all in one go. For that, we need a different form of splicer: a range splicer.

Construct the struct-to-tuple example from above. It was demonstrated using a single splice, but it would be simpler if we had a range splice:

template <typename T>
constexpr auto struct_to_tuple(T const& t) {
  constexpr auto members = nonstatic_data_members_of(^T);

  constexpr auto indices = []{
    std::array<int, members.size()> indices;
    std::ranges::iota(indices, 0);
    return indices;
  }();

  constexpr auto [...Is] = indices;
  return std::make_tuple(t.[: members[Is] :]...);
}

template <typename T>
constexpr auto struct_to_tuple(T const& t) {
  constexpr auto members = nonstatic_data_members_of(^T);
  return std::make_tuple(t.[: ...members :]...);
}

A range splice, [: ... r :], would accept as its argument a constant range of meta::info, r, and would behave as an unexpanded pack of splices. So the above expression

make_tuple(t.[: ... members :]...)

would evaluate as

make_tuple(t.[:members[0]:], t.[:members[1]:], ..., t.[:members[N-1]:])

This is a very useful facility indeed!

However, range splicing of dependent arguments is at least an order of magnitude harder to implement than ordinary splicing. We think that not including range splicing gives us a better chance of having reflection in C++26. Especially since, as this paper’s examples demonstrate, a lot can be done without them.

Another way to work around a lack of range splicing would be to implement with_size<N>(f), which would behave like f(integral_constant<size_t, 0>{}, integral_constant<size_t, 0>{}, ..., integral_constant<size_t, N-1>{}). Which is enough for a tolerable implementation:

template <typename T>
constexpr auto struct_to_tuple(T const& t) {
  constexpr auto members = nonstatic_data_members_of(^T);
  return with_size<members.size()>([&](auto... Is){
    return std::make_tuple(t.[: members[Is] :]...);
  });
}

(P1240 did propose range splicers.)

4.2.2 Syntax discussion

Early discussions of splice-like constructs (related to the TS design) considered using unreflexpr(...) for that purpose. [P1240R0] adopted that option for expression splicing, observing that a single splicing syntax could not viably be parsed (some disambiguation is needed to distinguish types and templates). S-7 eventually agreed with the [: ... :] syntax — with disambiguating tokens such as typename where needed — which is a little lighter and more distinctive.

We propose [: and :] be single tokens rather than combinations of [, ], and :. Among others, it simplifies the handling of expressions like arr[[:refl():]]. On the flip side, it requires a special rule like the one that was made to handle <:: to leave the meaning of arr[::N] unchanged and another one to avoid breaking a (somewhat useless) attribute specifier of the form [[using ns:]].

A syntax that is delimited on the left and right is useful here because spliced expressions may involve lower-precedence operators. However, there are other possibilities. For example, now that $ is available in the basic source character set, we might consider $<expr>. This is somewhat natural to those of us that have used systems where $ is used to expand placeholders in document templates. For example:

$select_type(3) *ptr = nullptr;

The prefixes typename and template are only strictly needed in some cases where the operand of the splice is a dependent expression. In our proposal, however, we only make typename optional in the same contexts where it would be optional for qualified names with dependent name qualifiers. That has the advantage to catch unfortunate errors while keeping a single rule and helping human readers parse the intended meaning of otherwise ambiguous constructs.

4.3 std::meta::info

The type std::meta::info can be defined as follows:

namespace std {
  namespace meta {
    using info = decltype(^int);
  }
}

In our initial proposal a value of type std::meta::info can represent:

• any (C++) type and type alias
• any function or member function
• any variable, static data member, or structured binding
• any non-static data member
• any constant value
• any template
• any namespace

Notably absent at this time are general non-constant expressions (that aren’t expression-ids referring to functions, variables or structured bindings). For example:

int x = 0;
void g() {
  [:^x:] = 42;     // Okay.  Same as: x = 42;
  x = [:^(2*x):];  // Error: "2*x" is a general non-constant expression.
  constexpr int N = 42;
  x = [:^(2*N):];  // Okay: "2*N" is a constant-expression.
}

Note that for ^(2*N) an implementation only has to capture the constant value of 2*N and not various other properties of the underlying expression (such as any temporaries it involves, etc.).

The type std::meta::info is a scalar type. Nontype template arguments of type std::meta::info are permitted. The entity being reflected can affect the linkage of a template instance involving a reflection. For example:

template<auto R> struct S {};

extern int x;
static int y;

S<^x> sx;  // S<^x> has external name linkage.
S<^y> sy;  // S<^y> has internal name linkage.

Namespace std::meta is associated with type std::meta::info: That allows the core meta functions to be invoked without explicit qualification. For example:

#include <meta>
struct S {};
std::string name2 = std::meta::name_of(^S);  // Okay.
std::string name1 = name_of(^S);             // Also okay.

4.4 Metafunctions

We propose a number of metafunctions declared in namespace std::meta to operator on reflection values. Adding metafunctions to an implementation is expected to be relatively “easy” compared to implementing the core language features described previously. However, despite offering a normal consteval C++ function interface, each on of these relies on “compiler magic” to a significant extent.

4.4.1 Constant evaluation order

In C++23, “constant evaluation” produces pure values without observable side-effects and thus the order in which constant-evaluation occurs is immaterial. In fact, while the language is designed to permit constant evaluation to happen at compile time, an implementation is not strictly required to take advantage of that possibility.

Some of the proposed metafunctions, however, have side-effects that have an effect of the remainder of the program. For example, we provide a define_class metafunction that provides a definition for a given class. Clearly, we want the effect of calling that metafunction to be “prompt” in a lexical-order sense. For example:

#include <meta>
struct S;

void g() {
  static_assert(is_type(define_class(^S, {})));
  S s;  // S should be defined at this point.
}

Hence this proposal also introduces constraints on constant evaluation as follows…

First, we identify a subset of manifestly constant-evaluated expressions and conversions characterized by the fact that their evaluation must occur and must succeed in a valid C++ program: We call these plainly constant-evaluated. We require that a programmer can count on those evaluations occurring exactly once and completing at translation time.

Second, we sequence plainly constant-evaluated expressions and conversions within the lexical order. Specifically, we require that the evaluation of a plainly constant-evaluated expression or conversion occurs before the implementation checks the validity of source constructs lexically following that expression or conversion.

Those constraints are mostly intuitive, but they are a significant change to the underlying principles of the current standard in this respect.

[P2758R1] (“Emitting messages at compile time”) also has to deal with side effects during constant evaluation. However, those effects (“output”) are of a slightly different nature in the sense that they can be buffered until a manifestly constant-evaluated expression/conversion has completed. “Buffering” a class type completion is not practical (e.g., because other metafunctions may well depend on the completed class type). Still, we are not aware of incompatibilities between our proposal and [P2758R1].

4.4.2 Error-Handling in Reflection

One important question we have to answer is: How do we handle errors in reflection metafunctions? For example, what does std::meta::template_of(^int) do? ^int is a reflection of a type, but that type is not a specialization of a template, so there is no valid reflected template for us to return.

There are a few options available to us today:

1. This fails to be a constant expression (unspecified mechanism).
2. This returns an invalid reflection (similar to NaN for floating point) which carries source location info and some useful message. (This was the approach suggested in P1240.)
3. This returns std::expected<std::meta::info, E> for some reflection-specific error type E which carries source location info and some useful message (this could be just info but probably should not be).
4. This throws an exception of type E (which requires allowing exceptions to work during constexpr evaluation, such that an uncaught exception would fail to be a constant exception).

The immediate downside of (2), yielding a NaN-like reflection for template_of(^int) is what we do for those functions that need to return a range. That is, what does template_arguments_of(^int) return?

1. This fails to be a constant expression (unspecified mechanism).
2. This returns a std::vector<std::meta::info> containing one invalid reflection.
3. This returns a std::expected<std::vector<std::meta::info>, E>.
4. This throws an exception of type E.

Having range-based functions return a single invalid reflection would make for awkward error handling code. Using std::expected or exceptions for error handling allow for a consistent, more straightforward interface.

This becomes another situation where we need to decide an error handling mechanism between exceptions and not exceptions, although importantly in this context a lot of usual concerns about exceptions do not apply:

• there is no runtime (so concerns about runtime performance, object file size, etc. do not exist), and
• there is no runtime (so concerns about code evolving to add a new uncaught exception type do not apply)

There is one interesting example to consider to decide between std::expected and exceptions here:

template <typename T>
  requires (template_of(^T) == ^std::optional)
void foo();

If template_of returns an excepted<info, E>, then foo<int> is a substitution failure — expected<T, E> is equality-comparable to T, that comparison would evaluate to false but still be a constant expression.

If template_of returns info but throws an exception, then foo<int> would cause that exception to be uncaught, which would make the comparison not a constant expression. This actually makes the constraint ill-formed - not a substitution failure. In order to have foo<int> be a substitution failure, either the constraint would have to first check that T is a template or we would have to change the language rule that requires constraints to be constant expressions (we would of course still keep the requirement that the constraint is a bool).

The other thing to consider are compiler modes that disable exception support (like -fno-exceptions in GCC and Clang). Today, implementations reject using try, catch, or throw at all when such modes are enabled. With support for constexpr exceptions, implementations would have to come up with a strategy for how to support compile-time exceptions — probably by only allowing them in consteval functions (including constexpr function templates that were propagated to consteval).

Despite these concerns (and the requirement of a whole new language feature), we believe that exceptions will be the more user-friendly choice for error handling here, simply because exceptions are more ergonomic to use than std::expected (even if we adopt language features that make this type easier to use - like pattern matching and a control flow operator).

4.4.3 Handling Aliases

Consider

using A = int;

In C++ today, A and int can be used interchangeably and there is no distinction between the two types. With reflection as proposed in this paper, that will no longer be the case. ^A yields a reflection of an alias to int, while ^int yields a reflection of int. ^A == ^int evaluates to false, but there will be a way to strip aliases - so dealias(^A) == ^int evaluates to true.

This opens up the question of how various other metafunctions handle aliases and it is worth going over a few examples:

using A = int;
using B = std::unique_ptr<int>;
template <class T> using C = std::unique_ptr<T>;

This paper is proposing that:

• is_type(^A) is true. ^A is an alias, but it’s an alias to a type, and if this evaluated as false then everyone would have to dealias everything all the time.
• has_template_arguments(^B) is false while has_template_arguments(^C<int>) is true. Even though B is an alias to a type that itself has template arguments (unique_ptr<int>), B itself is simply a type alias and does not. This reflects the actual usage.
• Meanwhile, template_arguments_of(^C<int>) yields {^int} while template_arguments_of(^std::unique_ptr<int>) yields {^int, ^std::default_deleter<int>}. This is C has its own template arguments that can be reflected on.

4.4.4 Freestanding implementations

Several important metafunctions, such as std::meta::_nonstatic_data_members_of, return a std::vector value. Unfortunately, that means that they are currently not usable in a freestanding environment. That is an highly undesirable limitation that we believe should be addressed by imbuing freestanding implementations with a more restricted std::vector (e.g., one that can only allocate at compile time).

4.4.5 Synopsis

Here is a synopsis for the proposed library API. The functions will be explained below.

namespace std::meta {
  // name and location
  consteval auto name_of(info r) -> string_view;
  consteval auto qualified_name_of(info r) -> string_view;
  consteval auto display_name_of(info r) -> string_view;
  consteval auto source_location_of(info r) -> source_location;

  // type queries
  consteval auto type_of(info r) -> info;
  consteval auto parent_of(info r) -> info;
  consteval auto dealias(info r) -> info;

  // template queries
  consteval auto template_of(info r) -> info;
  consteval auto template_arguments_of(info r) -> vector<info>;

  // member queries
  template<typename ...Fs>
    consteval auto members_of(info class_type, Fs ...filters) -> vector<info>;
  template<typename ...Fs>
    consteval auto bases_of(info class_type, Fs ...filters) -> vector<info>;
  consteval auto static_data_members_of(info class_type) -> vector<info>;
  consteval auto nonstatic_data_members_of(info class_type) -> vector<info>;
  consteval auto subobjects_of(info class_type) -> vector<info>;
  consteval auto enumerators_of(info enum_type) -> vector<info>;

  // substitute
  consteval auto substitute(info templ, span<info const> args) -> info;

  // reflect_invoke
  consteval auto reflect_invoke(info target, span<info const> args) -> info;

   // value_of
  template<typename T>
    consteval auto value_of(info) -> T;

  // test_type
  consteval auto test_type(info templ, info type) -> bool;
  consteval auto test_types(info templ, span<info const> types) -> bool;

  // other type predicates (see the wording)
  consteval auto is_public(info r) -> bool;
  consteval auto is_protected(info r) -> bool;
  consteval auto is_private(info r) -> bool;
  consteval auto is_accessible(info r) -> bool;
  consteval auto is_virtual(info r) -> bool;
  consteval auto is_pure_virtual(info entity) -> bool;
  consteval auto is_override(info entity) -> bool;
  consteval auto is_deleted(info entity) -> bool;
  consteval auto is_defaulted(info entity) -> bool;
  consteval auto is_explicit(info entity) -> bool;
  consteval auto is_bit_field(info entity) -> bool;
  consteval auto has_static_storage_duration(info r) -> bool;
  consteval auto has_internal_linkage(info r) -> bool;
  consteval auto has_external_linkage(info r) -> bool;
  consteval auto has_linkage(info r) -> bool;
  consteval auto is_class_member(info entity) -> bool;
  consteval auto is_namespace_member(info entity) -> bool;
  consteval auto is_nonstatic_data_member(info entity) -> bool;
  consteval auto is_static_member(info entity) -> bool;
  consteval auto is_base(info entity) -> bool;
  consteval auto is_namespace(info entity) -> bool;
  consteval auto is_function(info entity) -> bool;
  consteval auto is_variable(info entity) -> bool;
  consteval auto is_type(info entity) -> bool;
  consteval auto is_alias(info entity) -> bool;
  consteval auto is_incomplete_type(info entity) -> bool;
  consteval auto is_template(info entity) -> bool;
  consteval auto is_function_template(info entity) -> bool;
  consteval auto is_variable_template(info entity) -> bool;
  consteval auto is_class_template(info entity) -> bool;
  consteval auto is_alias_template(info entity) -> bool;
  consteval auto is_concept(info entity) -> bool;
  consteval auto has_template_arguments(info r) -> bool;
  consteval auto is_constructor(info r) -> bool;
  consteval auto is_destructor(info r) -> bool;
  consteval auto is_special_member(info r) -> bool;

  // reflect_value
  template<typename T>
    consteval auto reflect_value(T value) -> info;

  // define_class
  struct data_member_options_t;
  consteval auto data_member_spec(info class_type,
                                  data_member_options_t options = {}) -> info;
  consteval auto define_class(info class_type, span<info const>) -> info;

  // data layout
  consteval auto offset_of(info entity) -> size_t;
  consteval auto size_of(info entity) -> size_t;
  consteval auto bit_offset_of(info entity) -> size_t;
  consteval auto bit_size_of(info entity) -> size_t;
  consteval auto alignment_of(info entity) -> size_t;
}

4.4.6 name_of, display_name_of, source_location_of

namespace std::meta {
  consteval auto name_of(info r) -> string_view;
  consteval auto qualified_name_of(info r) -> string_view;
  consteval auto display_name_of(info r) -> string_view;
  consteval auto source_location_of(info r) -> source_location;
}

Given a reflection r that designates a declared entity X, name_of(r) and qualified_name_of(r) return a string_view holding the unqualified and qualified name of X, respectively. For all other reflections, an empty string_view is produced. For template instances, the name does not include the template argument list. The contents of the string_view consist of characters of the basic source character set only (an implementation can map other characters using universal character names).

Given a reflection r, display_name_of(r) returns a unspecified non-empty string_view. Implementations are encouraged to produce text that is helpful in identifying the reflected construct.

Given a reflection r, source_location_of(r) returns an unspecified source_location. Implementations are encouraged to produce the correct source location of the item designated by the reflection.

4.4.7 type_of, parent_of, dealias

namespace std::meta {
  consteval auto type_of(info r) -> info;
  consteval auto parent_of(info r) -> info;
  consteval auto dealias(info r) -> info;
}

If r is a reflection designating a typed entity, type_of(r) is a reflection designating its type. If r is already a type, type_of(r) is not a constant expression. This can be used to implement the C typeof feature (which works on both types and expressions and strips qualifiers):

consteval auto do_typeof(std::meta::info r) -> std::meta::info {
  return remove_cvref(is_type(r) ? r : type_of(r));
}

#define typeof(e) [: do_typeof(^e) :]

If r designates a member of a class or namespace, parent_of(r) is a reflection designating its immediately enclosing class or namespace.

If r designates an alias, dealias(r) designates the underlying entity. Otherwise, dealias(r) produces r. dealias is recursive - it strips all aliases:

using X = int;
using Y = X;
static_assert(dealias(^int) == ^int);
static_assert(dealias(^X) == ^int);
static_assert(dealias(^Y) == ^int);

4.4.8 template_of, template_arguments_of

namespace std::meta {
  consteval auto template_of(info r) -> info;
  consteval auto template_arguments_of(info r) -> vector<info>;
}

If r is a reflection designated a specialization of some template, then template_of(r) is a reflection of that template and template_arguments_of(r) is a vector of the reflections of the template arguments. In other words, the preconditions on both is that has_template_arguments(r) is true.

For example:

std::vector<int> v = {1, 2, 3};
static_assert(template_of(type_of(^v)) == ^std::vector);
static_assert(template_arguments_of(type_of(^v))[0] == ^int);

4.4.9 members_of, static_data_members_of, nonstatic_data_members_of, bases_of, enumerators_of, subobjects_of

namespace std::meta {
  template<typename ...Fs>
    consteval auto members_of(info class_type, Fs ...filters) -> vector<info>;

  template<typename ...Fs>
    consteval auto bases_of(info class_type, Fs ...filters) -> vector<info>;

  consteval auto static_data_members_of(info class_type) -> vector<info> {
    return members_of(class_type, is_variable);
  }

  consteval auto nonstatic_data_members_of(info class_type) -> vector<info> {
    return members_of(class_type, is_nonstatic_data_member);
  }

  consteval auto subobjects_of(info class_type) -> vector<info> {
    auto subobjects = bases_of(class_type);
    subobjects.append_range(nonstatic_data_members_of(class_type));
    return subobjects;
  }

  consteval auto enumerators_of(info enum_type) -> vector<info>;
}

The template members_of returns a vector of reflections representing the direct members of the class type represented by its first argument. Any nonstatic data members appear in declaration order within that vector. Anonymous unions appear as a nonstatic data member of corresponding union type. If any Filters... argument is specified, a member is dropped from the result if any filter applied to that members reflection returns false. E.g., members_of(^C, std::meta::is_type) will only return types nested in the definition of C and members_of(^C, std::meta::is_type, std::meta::is_variable) will return an empty vector since a member cannot be both a type and a variable.

The template bases_of returns the direct base classes of the class type represented by its first argument, in declaration order.

enumerators_of returns the enumerator constants of the indicated enumeration type in declaration order.

4.4.10 substitute

namespace std::meta {
  consteval auto substitute(info templ, span<info const> args) -> info;
}

Given a reflection for a template and reflections for template arguments that match that template, substitute returns a reflection for the entity obtained by substituting the given arguments in the template. If the template is a concept template, the result is a reflection of a constant of type bool.

For example:

constexpr auto r = substitute(^std::vector, std::vector{^int});
using T = [:r:]; // Ok, T is std::vector<int>

This process might kick off instantiations outside the immediate context, which can lead to the program being ill-formed.

Note that the template is only substituted, not instantiated. For example:

template<typename T> struct S { typename T::X x; };

constexpr auto r = substitute(^S, std::vector{^int});  // Okay.
typename[:r:] si;  // Error: T::X is invalid for T = int.

4.4.11 reflect_invoke

namespace std::meta {
  consteval auto reflect_invoke(info target, span<info const> args) -> info;
}

This metafunction produces a reflection of the value returned by a call expression.

Letting F be the entity reflected by target, and A_0, ..., A_n be the sequence of entities reflected by the values held by args: if the expression F(A_0, ..., A_N) is a well-formed constant expression evaluating to a type that is not void, and if every value in args is a reflection of a constant value, then reflect_invoke(target, args) evaluates to a reflection of the constant value F(A_0, ..., A_N).

For all other invocations, reflect_invoke(target, args) is ill-formed.

4.4.12 value_of<T>

namespace std::meta {
  template<typename T> consteval auto value_of(info) -> T;
}

If r is a reflection for a constant-expression or a constant-valued entity of type T, value_of<T>(r) evaluates to that constant value.

If r is a reflection for a variable of non-reference type T, value_of<T&>(r) and value_of<T const&>(r) are lvalues referring to that variable. If the variable is usable in constant expressions [expr.const], value_of<T>(r) evaluates to its value.

If r is a reflection for a variable of reference type T usable in constant-expressions, value_of<T>(r) evaluates to that reference.

If r is a reflection of an enumerator constant of type E, value_of<E>(r) evaluates to the value of that enumerator.

If r is a reflection of a non-bit-field non-reference non-static member of type M in a class C, value_of<M C::*>(r) is the pointer-to-member value for that nonstatic member.

For other reflection values r, value_of<T>(r) is ill-formed.

The function template value_of may feel similar to splicers, but unlike splicers it does not require its operand to be a constant-expression itself. Also unlike splicers, it requires knowledge of the type associated with the entity reflected by its operand.

4.4.13 test_type, test_types

namespace std::meta {
  consteval auto test_type(info templ, info type) -> bool {
    return test_types(templ, {type});
  }

  consteval auto test_types(info templ, span<info const> types) -> bool {
    return value_of<bool>(substitute(templ, types));
  }
}

This utility translates existing metaprogramming predicates (expressed as constexpr variable templates or concept templates) to the reflection domain. For example:

struct S {};
static_assert(test_type(^std::is_class_v, ^S));

An implementation is permitted to recognize standard predicate templates and implement test_type without actually instantiating the predicate template. In fact, that is recommended practice.

4.4.14 reflect_value

namespace std::meta {
  template<typename T> consteval auto reflect_value(T value) -> info;
}

This metafunction produces a reflection representing the constant value of the operand.

4.4.15 data_member_spec, define_class

namespace std::meta {
  struct data_member_options_t {
    optional<string_view> name;
    bool is_static = false;
    optional<int> alignment;
    optional<int> width;
  };
  consteval auto data_member_spec(info type,
                                  data_member_options_t options = {}) -> info;
  consteval auto define_class(info class_type, span<info const>) -> info;
}

data_member_spec returns a reflection of a description of a data member of given type. Optional alignment, bit-field-width, static-ness, and name can be provided as well. If no name is provided, the name of the data member is unspecified. If is_static is true, the data member is declared static.

define_class takes the reflection of an incomplete class/struct/union type and a range of reflections of data member descriptions and it completes the given class type with data members as described (in the given order). The given reflection is returned. For now, only data member reflections are supported (via data_member_spec) but the API takes in a range of info anticipating expanding this in the near future.

For example:

union U;
static_assert(is_type(define_class(^U, {
  data_member_spec(^int),
  data_member_spec(^char),
  data_member_spec(^double),
})));

// U is now defined to the equivalent of
// union U {
//   int _0;
//   char _1;
//   double _2;
// };

template<typename T> struct S;
constexpr auto U = define_class(^S<int>, {
  data_member_spec(^int, {.name="i", .align=64}),
  data_member_spec(^int, {.name="j", .align=64}),
});

// S<int> is now defined to the equivalent of
// template<> struct S<int> {
//   alignas(64) int i;
//   alignas(64) int j;
// };

When defining a union, if one of the alternatives has a non-trivial destructor, the defined union will still have a destructor provided - that simply does nothing. This allows implementing variant without having to further extend support in define_class for member functions.

4.4.16 Data Layout Reflection

namespace std::meta {
  consteval auto offset_of(info entity) -> size_t;
  consteval auto size_of(info entity) -> size_t;

  consteval auto bit_offset_of(info entity) -> size_t;
  consteval auto bit_size_of(info entity) -> size_t;

  consteval auto alignment_of(info entity) -> size_t;
}

4.4.17 Other Type Traits

There is a question of whether all the type traits should be provided in std::meta. For instance, a few examples in this paper use std::meta::remove_cvref(t) as if that exists. Technically, the functionality isn’t strictly necessary - since it can be provided indirectly:

std::meta::remove_cvref(type)

std::meta::substitute(^std::remove_cvref_t, {type})

std::meta::is_const(type)

std::meta::value_of<bool>(std::meta::substitute(^std::is_const_v, {type}))
std::meta::test_type(^std::is_const_v, type)

Having std::meta::meow for every trait std::meow is more straightforward and will likely be faster to compile, though means we will have a much larger library API. There are quite a few traits in 21 [meta] - but it should be easy enough to specify all of them. So we’re doing it.

5 Proposed Wording

5.1 Language

5.1.1 [lex.phases] Phases of translation

Modify the wording for phases 7-8 of 5.2 [lex.phases] as follows:

7 Whitespace characters separating tokens are no longer significant. Each preprocessing token is converted into a token (5.6). The resulting tokens constitute a translation unit and are syntactically and semantically analyzed and translated. Plainly constant-evaluated expressions ([expr.const]) appearing outside template declarations are evaluated in lexical order. Diagnosable rules (4.1.1 [intro.compliance.general]) that apply to constructs whose syntactic end point occurs lexically after the syntactic end point of a plainly constant-evaluated expression X are considered in a context where X has been evaluated. […]

8 […] All the required instantiations are performed to produce instantiation units. Plainly constant-evaluated expressions ([expr.const]) appearing in those instantiation units are evaluated in lexical order as part of the instantiation process. Diagnosable rules (4.1.1 [intro.compliance.general]) that apply to constructs whose syntactic end point occurs lexically after the syntactic end point of a plainly constant-evaluated expression X are considered in a context where X has been evaluated. […]

5.1.2 [lex.pptoken] Preprocessing tokens

Add a bullet after 5.4 [lex.pptoken] bullet (3.2):

…

— Otherwise, if the next three characters are <:: and the subsequent character is neither : nor >, the < is treated as a preprocessing token by itself and not as the first character of the alternative token <:.

— Otherwise, if the next three characters are [:: and the subsequent character is not :, the [ is treated as a preprocessing token by itself and not as the first character of the preprocessing token [:.

…