Антон Бикинеев, Reflection in C++Next

Reﬂection in C++Next
Anton Bikineev, 2017

Agenda
1. Reflection
2. Surviving proposals
• P0194R3: Static reflection
• P0590R0: A design for static reflection
3. Examples

Reﬂection or introspection?
Type introspection is the ability of a program to examine the
type or properties of an object at runtime.
Reﬂection is the ability of a computer program to examine,
introspect, and modify its own structure and behavior at
runtime.
(from Wikipedia)

Reﬂection or introspection?
Static type introspection is the ability of a program to examine
the type or properties of an object at compile-time.
Static reﬂection is the ability of a computer program to
examine, introspect, and modify its own structure and
behavior at compile-time.

Introspection? Type traits!
namespace std {
template <class T>
struct is_integral {
constexpr static bool value = /* */true;
};
template <class T> /* since c++14 */
constexpr bool is_integral_v = is_integral<T>::value;
template <class T>
struct add_pointer {
using type = T*;
};
template <class T>
using add_pointer_t = typename add_pointer<T>::type;
}

template <class Archive, class T>
void load(const Archive& ar, T& t) {
if constexpr (std::is_integral_v<T>)
load_integral(ar, t);
if constexpr (std::is_floating_point_v<T>)
load_fp(ar, t);
else if constexpr (std::is_trivially_copyable_v<T>)
load_memset(ar, t);
else
load_unoptimized(ar, t);
}

template <class T, class enable = void>
struct has_foo: std::false_type {};
template <class T>
struct has_foo<T,
std::void_t<decltype(std::declval<T>().foo())>>:
std::true_type {};

• works only with types!
• no way to traverse members!

Reﬂection
Static reﬂection is the ability of a computer program to
examine, introspect, and modify its own structure and
behavior at compile-time.

Metaprogramming deﬁnition
Metaprogramming is the writing of computer programs
that write or manipulate other programs (or themselves)
as their data, or that do part of the work at compile time
that would otherwise be done at run-time.
(from Wikipedia, some rev.)

C++ template metaprogramming
auto multiply_by_two(const std::array<int, 3>& array) {
auto result = array;
for (auto& i: result)
i *= 2;
return result;
}
std::array<int, 3> arr1{1, 2, 3};
auto arr2 = multiply_by_two(arr1);

C++ template metaprogramming
template <class T>
struct multiply_by_two;
template <class T, T... I>
struct multiply_by_two<std::integer_sequence<T, I...>> {
using type = std::integer_sequence<T, (I * 2)...>;
};
using seq1 = std::integer_sequence<int, 1, 2, 3>;
using seq2 = multiply_by_two<seq1>::type;

C++ constexpr metaprogramming
constexpr
auto multiply_by_two(const std::array<int, 3>& array) {
auto result = array;
for (auto& i: result)
i *= 2;
return result;
}
constexpr std::array<int, 3> arr1{1, 2, 3};
constexpr auto arr2 = multiply_by_two(arr1);

Reﬂection?

What we actually need…
struct foo {
int a;
double b;
string c;
};
struct soa_foo {
vector<int> as;
vector<double> bs;
vector<string> cs;
};

class Person
{
[[getter, setter]] std::string name;
[[getter, setter]] std::string email;
};

class Person
{
[[getter, setter, serialized]] std::string name;
[[getter, setter, serialized]] std::string email;
};

class [[polymorphically_serialized]] Person: IPerson
{
Person() { /* initializing code here */ }
[[getter, setter, serialized]] std::string name;
[[getter, setter, serialized]] std::string email;
};

Study Group 7: Reflection
• N4428: Type property queries
- 4th revision
- date: 2015-04-08
• P0255R0: Reflection via template pack expansion
- 5th revision
- date: 2016-02-12
• P0194R3: Static reflection
- 7th revision
- date: 2017-02-06
• P0590R0: A design for static reflection
- 1st revision
- date: 2017-02-05

Static reﬂection (P0194R3)
by Matúš Chochlík,
Axel Naumann,
David Sankel

P0194R3: Static reflection
• core-language changes:
- operator $reflect(x);
- where x is an id-expression;
- returns a unique generated type for an entity x;
- this type satisfies one or more concepts defined
in the library;
- you operate on these types (metaobjects) by
calling meta- operations.
• library changes:
- std::reflect::Concept<m>
- std::reflect::Operation<m>

P0194R3: Static reﬂection.
Simple example
std::string person = "John";
using person_m = $reflect(person);
static_assert(std::reflect::Variable<person_m>());
std::cout << std::reflect::get_base_name_v<person_m>;

Simple example
using type_m = std::reflect::get_type_t<person_m>;
static_assert(std::reflect::Type<type_m>());

Simple example
using type_m = std::reflect::get_type_t<person_m>;
static_assert(std::reflect::Type<type_m>());
using real_type =
std::reflect::get_reflected_type_t<type_m>;
static_assert(std::is_same_v<real_type, std::string>);

Meta-object concepts
namespace std::experimental::reflect {
template <class T> concept bool Object();
template <class T> concept bool ObjectSequence();
template <Object T> concept bool Named();
template <Object T> concept bool Alias();
template <Object T> concept bool RecordMember();
template <Object T> concept bool Enumerator();
template <Object T> concept bool Variable();
template <Object T> concept bool ScopeMember();
template <Object T> concept bool Typed();
template <Object T> concept bool Namespace();
template <Object T> concept bool GlobalScope();
template <Object T> concept bool Class();
template <Object T> concept bool Enum();
template <Object T> concept bool Record();
template <Object T> concept bool Scope();
template <Object T> concept bool Type();
template <Object T> concept bool Constant();
template <Object T> concept bool Base();
}

Class-like things
template <Object T> concept bool Record();
- a union or a class
template <Object T> concept bool Class();
- a class or a struct
template <Object T> concept bool Base();
- base classes
template <Object T> concept bool RecordMember();
- data member and member types

Scopes
template <Object T> concept bool Namespace();
- a namespace
template <Object T> concept bool Scope();
- a namespace, class or enumeration scope
template <Object T> concept bool ScopeMember();
- something in a scope
template <Object T> concept bool GlobalScope();
- the global namespace, $reflect(::)

Enums
template <Object T> concept bool Enum();
- an enum
template <Object T> concept bool Enumerator();
- an enumerator

Types
template <Object T> concept bool Typed();
- something that has a type, e.g. member-variable
template <Object T> concept bool Type();
- a type

Expressions
template <Object T> concept bool Variable();
- a variable
template <Object T> concept bool Constant();
- a constant-expression, like an enumerator

Other
template <Object T> concept bool Named();
- something with a name;
template <Object T> concept bool Alias();
- an alias, like a typedef
template <Object T> concept bool ObjectSequence();
- object-sequence, type-list…

P0194R3:
Meta-object operations by
examples

Enum-to-string
template <class Enum>
std::string to_string(Enum e) {
using namespace std::reflect;
using e_m = $reflect(Enum);
static_assert(std::reflect::Enum<e_m>());
std::string result;
for_each<get_enumerators_t<e_m>>([&](auto m) {
using en_m = decltype(m);
if (get_constant_v<en_m> == e)
result = get_base_name_v<en_m>;
});
return result;
}

Generic equal
template <class T>
bool generic_equal(const T& a, const T& b) {
using T_m = $reflect(T);
static_assert(std::reflect::Class<e_m>());
bool result = true;
for_each<get_data_members_t<T_m>>([&](auto m) {
using m_t = decltype(m);
result &= a.*get_pointer_v<m_t> ==
b.*get_pointer_v<m_t>;
});
return result;
}

Generic equal with unreﬂect
template <class T>
using T_m = $reflect(T);
static_assert(std::reflect::Class<e_m>());
bool result = true;
for_each<get_data_members_t<T_m>>([&](auto m) {
result &= a.$unreflect(m) ==
b.$unreflect(m);
});
return result;
}

Templates parametrized by namespace
namespace foo {
void func1(const std::string&);
void func2(int);
int func3(int);
}
namespace bar {
void func1(const std::string&);
void func2(int);
int func3(int);
}

Templates parametrized by namespace
template <class MN>
void algorithm(const string& str, int i) {
// [foo|bar]::func1(str)
$unreflect(MN)::func1(str);
// [foo|bar]::func2([foo|bar]::func3(i))
$unreflect(MN)::func2($unreflect(MN)::func3(i));
}
void func(const string& str, int i, bool want_foo) {
if (want_foo)
algorithm<$reflect(foo)>(str, i);
else
algorithm<$reflect(bar)>(str, i);
}

String to identiﬁer transformation
struct foo {
int bar;
};
constexpr const char* bar_name =
get_base_name_v<$reflect(foo::bar)>;
int get_???bar_name???(const foo&);

struct foo {
int bar;
};
constexpr const char* bar_name =
get_base_name_v<$reflect(foo::bar)>;
int $identifier(ct_concat(“get_”, bar_name))(const foo&);

struct foo {
int a;
double b;
string c;
};
struct soa_foo {
vector<int> as;
vector<double> bs;
vector<string> cs;
};

• supported:
- variables;
- data members;
- member types;
- enumerators;
- template instantiations;
- aliases

• not supported:
- declarations in namespaces (except for types);
- functions (yet);
- class templates (yet)

Pros and cons
• Pros:
- reﬂection of different entities is supported;
- very lazy instantiation, only queried data is
instantiated;
- “you don’t pay for what you don’t use” at
compile-time;
- a good base for building higher-level and
domain-speciﬁc libraries;
- very extensible

Pros and cons
• Cons:
- awkward and verbose usage;
- no attribute ideas

A design for static reﬂection
(P0590R0)
by Andrew Sutton,
Herb Sutter

P0590R0: A design for static reﬂection
• core-language changes:
- operator $x;
- where x is an id-expression;
- returns a unique object of a generated type for
an entity x;
- the object represents its meta information;
- you operate on these objects by calling
member-functions
• library changes:
- std::meta::reflection_type<X>

P0590R0: A design for static reﬂection.
Simple example
auto person_m = $person;
static_assert(std::meta::is_variable_v<
decltype(person_m)>);
std::cout << person_m.name();

Simple example
auto type_m = person_m.type();
static_assert(std::meta::is_type_v<decltype(type_m)>);

Simple example
auto type_m = person_m.type();
static_assert(std::meta::is_type_v<decltype(type_m)>);
using real_type = ???;

P0194R3 vs P0590R0
Enum-to-string
using e_m = $reflect(Enum);
static_assert(std::reflect::Enum<e_m>());
std::string result;
for_each<get_enumerators_t<e_m>>([&](auto m) {
using en_m = decltype(m);
if (get_constant_v<en_m> == e)
result = get_base_name_v<en_m>;
});
return result;
}

P0194R3 vs P0590R0
Enum-to-string
using namespace std::meta;
auto e_m = $Enum;
static_assert(std::meta::is_enum_v<decltype(e_m)>);
std::string result;
for_each(e_m.enumerators(), [&](auto m) {
if (m.value() == e)
result = m.name();
});
return result;
}

P0194R3 vs P0590R0
Enum-to-string
using namespace std::meta;
auto e_m = $Enum;
static_assert(std::meta::is_enum_v<decltype(e_m)>);
std::string result;
for (auto e: e_m.enumerators())
if (m.value() == e)
result = m.name();
return result;
}

P0194R3 vs P0590R0
Generic equal
template <class T>
bool result = true;
for (auto member: $T.member_variables()) {
auto ptr = member.pointer();
result &= a.*ptr == b.*ptr;
}
return result;
}

Pros and cons
• Pros:
- reﬂection of different entities is supported;
- better syntax;
- a good base for building higher-level and
domain-speciﬁc libraries;
- extensible

Pros and cons
• Cons:
- some extra data may still be instantiated by
compiler;
- no attribute ideas

Thank you for your attention!

Антон Бикинеев, Reflection in C++Next

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