C++ keywords: reflexpr (reflection TS)

 

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Keywords

alignas (C++11) alignof (C++11) and and_eq asm auto bitand bitor bool break case catch char char8_t (C++20) char16_t (C++11) char32_t (C++11) class compl concept (C++20) const consteval (C++20) constexpr (C++11) constinit (C++20) const_cast continue co_await (C++20) co_return (C++20) co_yield (C++20) decltype (C++11) default delete	do double dynamic_cast else enum explicit export extern false float for friend goto if inline int long mutable namespace new noexcept (C++11) not not_eq nullptr (C++11) operator or or_eq private protected public register	reinterpret_cast requires (C++20) return short signed sizeof static static_assert (C++11) static_cast struct switch template this thread_local (C++11) throw true try typedef typeid typename union unsigned using virtual void volatile wchar_t while xor xor_eq
Identifiers with special meaning
final (C++11) override (C++11)	import (C++20) module (C++20)

 

Usage

1. gets the member list of a class type, or the enumerator list of an enum type.
2. gets the name of type and member.
3. detects whether a data member is static or constexpr.
4. detects whether member function is virtual, public, protected or private.
5. get the row and column of the source code when the type defines.

Example

reflexpr provides us the meta info of the object via meta-object types. Note that std::reflect::get_data_members_t make programmers able to visit any class just like std::tuple.

Run this code

#include <string>
#include <vector>
 
struct S {
    int b;
    std::string s;
    std::vector<std::string> v;
};
 
// Reflection TS
#include <experimental/reflect>
using meta_S = reflexpr(S);
using mem = std::reflect::get_data_members_t<meta_S>;
using meta = std::reflect::get_data_members_t<mem>;
static_assert(std::reflect::is_public_v<meta>); // successful
 
int main() {}

We can also know the name info from reflexpr:

Run this code

#include <string>
#include <string_view>
#include <iostream>
// Reflection TS
#include <experimental/reflect>
template <typename Tp>
constexpr std::string_view nameof() {
 using TpInfo = reflexpr(Tp);
 using aliased_Info = std::experimental::reflect::get_aliased_t<TpInfo>;
 return std::experimental::reflect::get_name_v<aliased_Info>;
}
 
int main(){
 std::cout << nameof<std::string>() << '\n';
 static_assert(nameof<std::string>() == "basic_string"); // successful
}

This is an example of getting the scope of a type in the Reflection TS.

Run this code

namespace Foo{
 struct FooFoo{int FooFooFoo};
}
namespace Bar{
 using BarBar = ::Foo::FooFoo;
}
using BarBarInfo = reflexpr(::Bar::BarBar);
using BarBarScope = ::std::experimental::reflect::get_scope_t<BarBarInfo>; // Bar, not Foo
 
struct Spam{int SpamSpam;};
struct Grok{
 using GrokGrok = Spam::SpamSpam;
};
using GrokGrokInfo = reflexpr(::Grok::GrokGrok);
using GrokGrokScope = std::experimental::reflect::get_scope_t<GrokGrokInfo>; // Grok, not Spam