Member pointer to index (WIP)

include/boost/pfr.hpp

@@ -14,8 +14,10 @@
 #include <boost/pfr/core_name.hpp>
 #include <boost/pfr/functions_for.hpp>
 #include <boost/pfr/functors.hpp>
+#include <boost/pfr/index_of.hpp>
 #include <boost/pfr/io.hpp>
 #include <boost/pfr/io_fields.hpp>
+#include <boost/pfr/is_constexpr_default_constructible.hpp>
 #include <boost/pfr/ops.hpp>
 #include <boost/pfr/ops_fields.hpp>
 #include <boost/pfr/tuple_size.hpp>

include/boost/pfr/detail/for_each_field.hpp

@@ -42,14 +42,14 @@ constexpr void for_each_field(T&& value, F&& func) {
 
     ::boost::pfr::detail::for_each_field_dispatcher(
         value,
-        [f = std::forward<F>(func)](auto&& t) mutable {
+        [&func](auto&& t) mutable {
             // MSVC related workaround. Its lambdas do not capture constexprs.
             constexpr std::size_t fields_count_val_in_lambda
                 = boost::pfr::detail::fields_count<std::remove_reference_t<T>>();
 
             ::boost::pfr::detail::for_each_field_impl(
                 t,
-                std::forward<F>(f),
+                std::forward<F>(func),
                 detail::make_index_sequence<fields_count_val_in_lambda>{},
                 std::is_rvalue_reference<T&&>{}
             );

include/boost/pfr/index_of.hpp

@@ -0,0 +1,85 @@
+// Copyright (c) 2025-2026 Antony Polukhin
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_PFR_INDEX_OF_HPP
+#define BOOST_PFR_INDEX_OF_HPP
+#pragma once
+
+#include <boost/pfr/detail/config.hpp>
+
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_PFR_INTERFACE_UNIT)
+
+#include <boost/pfr/detail/for_each_field.hpp>
+#include <boost/pfr/is_constexpr_default_constructible.hpp>
+
+
+#if !defined(BOOST_PFR_INTERFACE_UNIT)
+#include <type_traits>
+#include <memory>      // std::addressof
+#endif
+
+/// \file boost/pfr/index_of.hpp
+/// \copybrief boost::pfr::index_of
+
+namespace boost { namespace pfr {
+
+namespace detail {
+
+template <typename M>
+struct address_comparing_visitor {
+    template <class T>
+    constexpr void operator()(const T&, std::size_t) noexcept {}
+
+    constexpr void operator()(const M& field, std::size_t idx) noexcept {
+        const void* field_address = std::addressof(field);
+        if (target_address == field_address) {
+            result = idx;
+        }
+    }
+
+    const void* const target_address;
+    std::size_t result;
+};
+
+}
+
+BOOST_PFR_BEGIN_MODULE_EXPORT
+
+template <typename T, typename M>
+constexpr std::size_t index_of(M T::*mem_ptr, const T& value) noexcept {
+    if (mem_ptr == nullptr) {
+        return ~static_cast<std::size_t>(0);
+    }
+
+    detail::address_comparing_visitor<M> visitor{
+        std::addressof(value.*mem_ptr),
+        ~static_cast<std::size_t>(0)
+    };
+    ::boost::pfr::detail::for_each_field(value, visitor);
+
+    return visitor.result;
+}
+
+template <typename T, typename M>
+constexpr std::size_t index_of(M T::*mem_ptr) noexcept {
+    static_assert(
+        boost::pfr::is_constexpr_default_constructible_v<T>,
+        "====================> Boost.PFR: T should be default constructible in constant evaluations. "
+        "Either add `constexpr` to constructors of member types or use the boost::pfr::index_of() overload "
+        "with explicit `const T& value` parameter."
+    );
+
+    // boost::pfr::is_constexpr_default_constructible_v<T> gives a faint hope that the
+    // compiler would be able to optimize away the temporary `T{}`.
+    return ::boost::pfr::index_of(mem_ptr, T{});
+}
+
+BOOST_PFR_END_MODULE_EXPORT
+
+}} // namespace boost::pfr
+
+#endif  // #if !defined(BOOST_USE_MODULES) || defined(BOOST_PFR_INTERFACE_UNIT)
+
+#endif // BOOST_PFR_INDEX_OF_HPP

include/boost/pfr/is_constexpr_default_constructible.hpp

@@ -0,0 +1,49 @@
+// Copyright (c) 2025-2026 Antony Polukhin
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_PFR_IS_CONSTEXPR_DEFAULT_CONSTRUCTIBLE_HPP
+#define BOOST_PFR_IS_CONSTEXPR_DEFAULT_CONSTRUCTIBLE_HPP
+#pragma once
+
+#include <boost/pfr/detail/config.hpp>
+
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_PFR_INTERFACE_UNIT)
+
+#if !defined(BOOST_PFR_INTERFACE_UNIT)
+#include <type_traits>
+#endif
+
+namespace boost { namespace pfr {
+
+namespace detail {
+    template <class T, int = (static_cast<void>(T{}), 0)>
+    constexpr std::true_type is_constexpr_default_constructible(long) noexcept {
+        return {};
+    }
+
+    template <class T>
+    constexpr std::false_type is_constexpr_default_constructible(int) noexcept {
+        return {};
+    }
+}  // namespace detail
+
+BOOST_PFR_BEGIN_MODULE_EXPORT
+
+template <class T>
+struct is_constexpr_default_constructible
+    : decltype(detail::is_constexpr_default_constructible<T>(1L))
+{};
+
+template <class T>
+constexpr bool is_constexpr_default_constructible_v
+    = decltype(detail::is_constexpr_default_constructible<T>(1L))::value;
+
+BOOST_PFR_END_MODULE_EXPORT
+
+}}  // namespace boost::pfr
+
+#endif  // #if !defined(BOOST_USE_MODULES) || defined(BOOST_PFR_INTERFACE_UNIT)
+
+#endif  // BOOST_PFR_IS_CONSTEXPR_DEFAULT_CONSTRUCTIBLE_HPP

test/core/run/is_constexpr_constructible.cpp

@@ -0,0 +1,41 @@
+// Copyright (c) 2025-2026 Antony Polukhin
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/pfr/is_constexpr_default_constructible.hpp>
+
+#include <string>
+
+struct Yes {
+    int a;
+    double b;
+    char c;
+};
+
+struct Yes2 {
+    int a;
+    double b;
+    char c;
+    Yes d[42];
+};
+
+class MyClass{
+public:
+    MyClass();
+    ~MyClass();
+};
+
+struct No {
+    int a;
+    double b;
+    char c;
+    std::string str;
+    MyClass mc;
+};
+
+int main() {
+    static_assert(is_constexpr_default_constructible_v<Yes>, "");
+    static_assert(is_constexpr_default_constructible_v<Yes2>, "");
+    static_assert(!is_constexpr_default_constructible_v<No>, "");
+}

test/core/run/member_to_index.cpp

@@ -0,0 +1,58 @@
+// Copyright (c) 2025-2026 Antony Polukhin
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/pfr/index_of.hpp>
+
+#include <limits>
+#include <string>
+#include <vector>
+
+struct Sample {
+	std::string x;
+	std::vector<int> y;
+	std::string z;
+};
+
+struct TrivialSample {
+	int x;
+	short y;
+	bool z;
+};
+
+template <class T>
+int test_if_constexpr_default_constructible(std::false_type) {
+    return 0;
+}
+
+template <class T>
+int test_if_constexpr_default_constructible(std::true_type) {
+    if (boost::pfr::index_of(&T::x) != 0) return 31;
+    if (boost::pfr::index_of(&T::y) != 1) return 32;
+    if (boost::pfr::index_of(&T::z) != 2) return 33;
+    return 0;
+}
+
+int main() {
+#if BOOST_PFR_USE_CPP17 || BOOST_PFR_USE_CPP26
+    static_assert(boost::pfr::index_of(&TrivialSample::x) == 0);
+    static_assert(boost::pfr::index_of(&TrivialSample::y) == 1);
+    static_assert(boost::pfr::index_of(&TrivialSample::z) == 2);
+#endif
+
+    if (boost::pfr::index_of(&Sample::x, {}) != 0) return 1;
+    if (boost::pfr::index_of(&Sample::y, {}) != 1) return 2;
+    if (boost::pfr::index_of(&Sample::z, {}) != 2) return 3;
+
+    decltype(&Sample::x) mem_ptr = nullptr;
+    if (boost::pfr::index_of(mem_ptr, {}) != (std::numeric_limits<std::size_t>::max)()) return 4;
+
+    if (boost::pfr::index_of(&TrivialSample::x) != 0) return 11;
+    if (boost::pfr::index_of(&TrivialSample::y) != 1) return 12;
+    if (boost::pfr::index_of(&TrivialSample::z) != 2) return 13;
+
+    return test_if_constexpr_default_constructible<Sample>(
+        boost::pfr::is_constexpr_default_constructible<Sample>{}
+    );
+}