Getting field names with boost::pfr - c++

Question

Hi I'm using boost::pfr for basic reflection, it works fine, but the problem is it is only print or deal with the field values, like with boost::pfr::io it prints each member of the struct, but how can I print it as name value pairs, same issue with for_each_field, the functor only accepts values, but not names. How can I get the field names?
struct S {
int n;
std::string name;
};
S o{1, "foo"};
std::cout << boost::pfr::io(o);
// Outputs: {1, "foo"}, how can I get n = 1, name = "foo"?

Answer 1

If you think adapting a struct is not too intrusive (it doesn't change your existing definitions, and you don't even need to have it in a public header):
BOOST_FUSION_ADAPT_STRUCT(S, n, name)
Then you can concoct a general operator<< for sequences:
namespace BF = boost::fusion;
template <typename T,
typename Enable = std::enable_if_t<
// BF::traits::is_sequence<T>::type::value>
std::is_same_v<BF::struct_tag, typename BF::traits::tag_of<T>::type>>>
std::ostream& operator<<(std::ostream& os, T const& v)
{
bool first = true;
auto visitor = [&]<size_t I>() {
os << (std::exchange(first, false) ? "" : ", ")
<< BF::extension::struct_member_name<T, I>::call()
<< " = " << BF::at_c<I>(v);
};
// visit members
[&]<size_t... II>(std::index_sequence<II...>)
{
return ((visitor.template operator()<II>(), ...);
}
(std::make_index_sequence<BF::result_of::size<T>::type::value>{});
return os;
}
(Prior to c++20 this would require some explicit template types instead of the lambdas, perhaps making it more readable. I guess I'm lazy...)
Here's a live demo: Live On Compiler Explorer
n = 1, name = foo
Bonus: Correctly quoting string-like types
Live On Compiler Explorer
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/for_each.hpp>
#include <boost/fusion/include/at_c.hpp>
#include <iostream>
#include <iomanip>
namespace MyLib {
struct S {
int n;
std::string name;
};
namespace BF = boost::fusion;
static auto inline pretty(std::string_view sv) { return std::quoted(sv); }
template <typename T,
typename Enable = std::enable_if_t<
not std::is_constructible_v<std::string_view, T const&>>>
static inline T const& pretty(T const& v)
{
return v;
}
template <typename T,
typename Enable = std::enable_if_t<
// BF::traits::is_sequence<T>::type::value>
std::is_same_v<BF::struct_tag, typename BF::traits::tag_of<T>::type>>>
std::ostream& operator<<(std::ostream& os, T const& v)
{
bool first = true;
auto visitor = [&]<size_t I>() {
os << (std::exchange(first, false) ? "" : ", ")
<< BF::extension::struct_member_name<T, I>::call()
<< " = " << pretty(BF::at_c<I>(v));
};
// visit members
[&]<size_t... II>(std::index_sequence<II...>)
{
return (visitor.template operator()<II>(), ...);
}
(std::make_index_sequence<BF::result_of::size<T>::type::value>{});
return os;
}
} // namespace MyLib
BOOST_FUSION_ADAPT_STRUCT(MyLib::S, n, name)
int main()
{
MyLib::S o{1, "foo"};
std::cout << o << "\n";
}
Outputs:
n = 1, name = "foo"

Answer 2

The library cannot offer any such functionality because it is currently impossible to obtain the name of a member of a class as value of an object.
If you want to output field names, you need to declare string objects mapped with the members and implement a operator<< which uses these strings manually.
To do this a more sophisticated reflection library would probably offer macros to use in the definition of the members. Macros can expand their argument(s) into a declaration using the provided name as identifier while also producing code using the name as string literal (via the # macro replacement operator).

Answer 3

It's stupid but hey, with a stringifying macro per field it could be enough for you.
C++14, no additional library
#include <boost/pfr.hpp>
struct S
{
int n;
std::string name;
static char const* const s_memNames[2];
};
char const* const S::s_memNames[2] = {"n", "name"};
// utility
template< size_t I, typename TR >
char const* MemberName()
{
using T = std::remove_reference_t<TR>;
if (I < std::size(T::s_memNames))
return T::s_memNames[I];
return nullptr;
}
// test:
#include <iostream>
using std::cout;
template< size_t I, typename T >
void StreamAt(T&& inst)
{
char const* n = MemberName<I,T>();
auto& v = boost::pfr::get<I>(inst);
cout << "(" << n << " = " << v << ")";
}
int main()
{
S s{2, "boo"};
boost::pfr::for_each_field(s, [&](const auto&, auto I)
{
StreamAt<decltype(I)::value>(s);
cout << "\n";
});
}
output:
(n = 2)
(name = boo)
(previous version of the suggestion, this one has more fluff so less interesting)
#include <boost/pfr.hpp>
// library additions:
static char const* g_names[100];
template< size_t V >
struct Id : std::integral_constant<size_t, V > {};
template< size_t I, typename T >
using TypeAt = boost::pfr::tuple_element_t<I, T>;
template<std::size_t Pos, class Struct>
constexpr int Ni() // name index
{
return std::tuple_element_t<Pos, typename std::remove_reference_t<Struct>::NamesAt >::value;
}
struct StaticCaller
{
template< typename Functor >
StaticCaller(Functor f) { f();}
};
///
/// YOUR CODE HERE
struct S
{
using NamesAt = std::tuple<Id<__COUNTER__>, Id<__COUNTER__>>; // add this
int n;
std::string name;
static void Init() // add this
{
g_names[Ni<0,S>()] = "n";
g_names[Ni<1,S>()] = "name";
}
};
StaticCaller g_sc__LINE__(S::Init); // add this
// utilities
template< size_t I, typename T >
auto GetValueName(T&& inst)
{
return std::make_pair(boost::pfr::get<I>(inst), g_names[Ni<I,T>()]);
}
// test:
#include <iostream>
using std::cout;
template< size_t I, typename T >
void StreamAt(T&& inst)
{
auto const& [v,n] = GetValueName<I>(inst);
cout << "(" << v << ", " << n << ")";
}
int main()
{
S s{2, "boo"};
boost::pfr::for_each_field(s, [&](const auto&, auto I)
{
StreamAt<decltype(I)::value>(s);
cout << "\n";
});
}
output
(2, n)
(boo, name)