get_struct_field.cpp

#include <type_traits>
#include <utility>
#include <tuple>

struct filler { template< typename type > operator type (); };

template< typename aggregate, 
          typename index_sequence = std::index_sequence<>, 
          typename = void >
struct aggregate_arity
        : index_sequence
{

};

template< typename aggregate, 
          std::size_t ...indices >
struct aggregate_arity< aggregate, 
                        std::index_sequence< indices... >, 
                        std::void_t< decltype(aggregate{(void(indices), std::declval< filler >())..., std::declval< filler >()}) > >
    : aggregate_arity< aggregate, 
                       std::index_sequence< indices..., sizeof...(indices) > >
{

};

template< std::size_t index, typename type >
constexpr
decltype(auto)
get(type & value) noexcept
{
    constexpr std::size_t arity = aggregate_arity< std::remove_cv_t< type > >::size();
    if constexpr (arity == 1) {        
        auto & [p1] = value;
        if constexpr (index == 0) {
            return (p1);
        } else {
            return;
        }
    } else if constexpr (arity == 2) {
        auto & [p1, p2] = value;
        if constexpr (index == 0) {
            return (p1);
        } else if constexpr (index == 1) {
            return (p2);
        } else {
            return;
        }
    } else if constexpr (arity == 3) {
        auto & [p1, p2, p3] = value;
        if constexpr (index == 0) {
            return (p1);
        } else if constexpr (index == 1) {
            return (p2);
        } else if constexpr (index == 2) {
            return (p3);
        } else {
            return;
        }
    } else /* extend it by yourself for higher arities */ {
        return;
    }
}

// main.cpp
#include <cstdlib>
#include <cassert>

namespace
{

using S = struct { int i; char c; bool b; };

S s{1, '2', true};
    
decltype(auto) i = get< 0 >(s);
decltype(auto) c = get< 1 >(s);
decltype(auto) b = get< 2 >(s);
    
static_assert(std::is_same< decltype(i), int & >{});
static_assert(std::is_same< decltype(c), char & >{});
static_assert(std::is_same< decltype(b), bool & >{});

static_assert(&i == &s.i);
static_assert(&c == &s.c);
static_assert(&b == &s.b);
    
}

int
main()
{
    assert(i == 1);
    assert(c == '2');
    assert(b == true);
    return EXIT_SUCCESS;
}