From fe4857ac8be9f53524eb80983046813fd40f6dc3 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Mon, 26 Jan 2026 15:07:39 +0300
Subject: [PATCH 1/2] Allow stateful visitors in for_each_field()

---
 include/boost/pfr/core.hpp                  |  2 +-
 include/boost/pfr/detail/for_each_field.hpp |  4 ++--
 test/core/run/for_each_field.cpp            | 11 +++++++++++
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/include/boost/pfr/core.hpp b/include/boost/pfr/core.hpp
index d621f273..a4f230cd 100644
--- a/include/boost/pfr/core.hpp
+++ b/include/boost/pfr/core.hpp
@@ -255,7 +255,7 @@ constexpr auto structure_tie(T&&, std::enable_if_t< std::is_rvalue_reference<T&&
 /// \endcode
 template <class T, class F>
 constexpr void for_each_field(T&& value, F&& func) {
-    return ::boost::pfr::detail::for_each_field(std::forward<T>(value), std::forward<F>(func));
+    ::boost::pfr::detail::for_each_field(std::forward<T>(value), std::forward<F>(func));
 }
 
 /// \brief std::tie-like function that allows assigning to tied values from aggregates.
diff --git a/include/boost/pfr/detail/for_each_field.hpp b/include/boost/pfr/detail/for_each_field.hpp
index 33b3ec1f..c2d92ed3 100644
--- a/include/boost/pfr/detail/for_each_field.hpp
+++ b/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&&>{}
             );
diff --git a/test/core/run/for_each_field.cpp b/test/core/run/for_each_field.cpp
index 031636b4..30001145 100644
--- a/test/core/run/for_each_field.cpp
+++ b/test/core/run/for_each_field.cpp
@@ -56,6 +56,13 @@ struct simple {
 
 struct empty{};
 
+struct stateful_counting_visitor {
+    std::size_t count = 0;
+
+    template <class T>
+    void operator()(const T&) { ++count; }
+};
+
 #if BOOST_PFR_USE_CPP17
 constexpr std::size_t get_field_count_through_for_each_field() {
     std::size_t counter = 0;
@@ -132,6 +139,10 @@ int main () {
     });
     BOOST_TEST_EQ("", ss.str());
     ss.str("");
+    
+    stateful_counting_visitor counting_visitor;
+    boost::pfr::for_each_field(simple{}, counting_visitor);
+    BOOST_TEST_EQ(3, counting_visitor.count);
 
     return boost::report_errors();
 }

From 582bf51a7a50bcf1ca34b1612df6ae21da283d21 Mon Sep 17 00:00:00 2001
From: Antony Polukhin <antoshkka@gmail.com>
Date: Mon, 26 Jan 2026 18:26:28 +0300
Subject: [PATCH 2/2] fix stateful visitation for for_each_field_with_name

---
 include/boost/pfr/detail/core_name20_static.hpp |  6 +++---
 test/core_name/run/for_each_field_with_name.cpp | 10 ++++++++++
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/boost/pfr/detail/core_name20_static.hpp b/include/boost/pfr/detail/core_name20_static.hpp
index 4c615904..ba03cac9 100644
--- a/include/boost/pfr/detail/core_name20_static.hpp
+++ b/include/boost/pfr/detail/core_name20_static.hpp
@@ -245,14 +245,14 @@ template <class T, class F>
 constexpr void for_each_field_with_name(T&& value, F&& func) {
     return boost::pfr::detail::for_each_field(
         std::forward<T>(value),
-        [f = std::forward<F>(func)](auto&& field, auto index) mutable {
+        [&func](auto&& field, auto index) mutable {
             using IndexType = decltype(index);
             using FieldType = decltype(field);
             constexpr auto name = boost::pfr::detail::get_name<std::remove_reference_t<T>, IndexType::value>();
             if constexpr (std::is_invocable_v<F, std::string_view, FieldType, IndexType>) {
-                f(name, std::forward<FieldType>(field), index);
+                std::forward<F>(func)(name, std::forward<FieldType>(field), index);
             } else {
-                f(name, std::forward<FieldType>(field));
+                std::forward<F>(func)(name, std::forward<FieldType>(field));
             }
         });
 }
diff --git a/test/core_name/run/for_each_field_with_name.cpp b/test/core_name/run/for_each_field_with_name.cpp
index c17d9c19..b071e04e 100644
--- a/test/core_name/run/for_each_field_with_name.cpp
+++ b/test/core_name/run/for_each_field_with_name.cpp
@@ -15,6 +15,12 @@ struct SimpleStruct {
     std::string str;
 };
 
+struct stateful_counting_visitor {
+    std::size_t count = 0;
+
+    template <class T>
+    void operator()(std::string_view /*name*/, const T&) { ++count; }
+};
 
 int main () {
     std::map<std::string, std::string> m;
@@ -27,5 +33,9 @@ int main () {
     BOOST_TEST_EQ(m["c"], "e");
     BOOST_TEST_EQ(m["str"], "test");
 
+    stateful_counting_visitor counting_visitor;
+    boost::pfr::for_each_field_with_name(SimpleStruct{}, counting_visitor);
+    BOOST_TEST_EQ(2, counting_visitor.count);
+
     return boost::report_errors();
 }