Make for_each_field() constexpr for c++17

boostorg#127
alexkaratarakis · Jul 13, 2023 · 73b9af4 · 73b9af4
1 parent 2fa9036
commit 73b9af4
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Showing 4 changed files with 19 additions and 8 deletions.
diff --git a/include/boost/pfr/core.hpp b/include/boost/pfr/core.hpp
@@ -220,7 +220,7 @@ constexpr auto structure_tie(T&&, std::enable_if_t< std::is_rvalue_reference<T&&
 ///     assert(sum == 42);
 /// \endcode
 template <class T, class F>
-void for_each_field(T&& value, F&& func) {
+constexpr void for_each_field(T&& value, F&& func) {
     constexpr std::size_t fields_count_val = boost::pfr::detail::fields_count<std::remove_reference_t<T>>();
 
     ::boost::pfr::detail::for_each_field_dispatcher(

diff --git a/include/boost/pfr/detail/core17.hpp b/include/boost/pfr/detail/core17.hpp
@@ -57,7 +57,7 @@ constexpr auto tie_as_tuple(T& val) noexcept {
 }
 
 template <class T, class F, std::size_t... I>
-void for_each_field_dispatcher(T& t, F&& f, std::index_sequence<I...>) {
+constexpr void for_each_field_dispatcher(T& t, F&& f, std::index_sequence<I...>) {
     static_assert(
         !std::is_union<T>::value,
         "====================> Boost.PFR: For safety reasons it is forbidden to reflect unions. See `Reflection of unions` section in the docs for more info."

diff --git a/include/boost/pfr/detail/for_each_field_impl.hpp b/include/boost/pfr/detail/for_each_field_impl.hpp
@@ -20,18 +20,18 @@ template <std::size_t Index>
 using size_t_ = std::integral_constant<std::size_t, Index >;
 
 template <class T, class F, class I, class = decltype(std::declval<F>()(std::declval<T>(), I{}))>
-void for_each_field_impl_apply(T&& v, F&& f, I i, long) {
+constexpr void for_each_field_impl_apply(T&& v, F&& f, I i, long) {
     std::forward<F>(f)(std::forward<T>(v), i);
 }
 
 template <class T, class F, class I>
-void for_each_field_impl_apply(T&& v, F&& f, I /*i*/, int) {
+constexpr void for_each_field_impl_apply(T&& v, F&& f, I /*i*/, int) {
     std::forward<F>(f)(std::forward<T>(v));
 }
 
 #if !defined(__cpp_fold_expressions) || __cpp_fold_expressions < 201603
 template <class T, class F, std::size_t... I>
-void for_each_field_impl(T& t, F&& f, std::index_sequence<I...>, std::false_type /*move_values*/) {
+constexpr void for_each_field_impl(T& t, F&& f, std::index_sequence<I...>, std::false_type /*move_values*/) {
      const int v[] = {0, (
          detail::for_each_field_impl_apply(sequence_tuple::get<I>(t), std::forward<F>(f), size_t_<I>{}, 1L),
          0
@@ -41,7 +41,7 @@ void for_each_field_impl(T& t, F&& f, std::index_sequence<I...>, std::false_type
 
 
 template <class T, class F, std::size_t... I>
-void for_each_field_impl(T& t, F&& f, std::index_sequence<I...>, std::true_type /*move_values*/) {
+constexpr void for_each_field_impl(T& t, F&& f, std::index_sequence<I...>, std::true_type /*move_values*/) {
      const int v[] = {0, (
          detail::for_each_field_impl_apply(sequence_tuple::get<I>(std::move(t)), std::forward<F>(f), size_t_<I>{}, 1L),
          0
@@ -50,12 +50,12 @@ void for_each_field_impl(T& t, F&& f, std::index_sequence<I...>, std::true_type
 }
 #else
 template <class T, class F, std::size_t... I>
-void for_each_field_impl(T& t, F&& f, std::index_sequence<I...>, std::false_type /*move_values*/) {
+constexpr void for_each_field_impl(T& t, F&& f, std::index_sequence<I...>, std::false_type /*move_values*/) {
      (detail::for_each_field_impl_apply(sequence_tuple::get<I>(t), std::forward<F>(f), size_t_<I>{}, 1L), ...);
 }
 
 template <class T, class F, std::size_t... I>
-void for_each_field_impl(T& t, F&& f, std::index_sequence<I...>, std::true_type /*move_values*/) {
+constexpr void for_each_field_impl(T& t, F&& f, std::index_sequence<I...>, std::true_type /*move_values*/) {
      (detail::for_each_field_impl_apply(sequence_tuple::get<I>(std::move(t)), std::forward<F>(f), size_t_<I>{}, 1L), ...);
 }
 #endif

diff --git a/test/core/run/for_each_field.cpp b/test/core/run/for_each_field.cpp
@@ -56,6 +56,17 @@ struct simple {
 
 struct empty{};
 
+#if BOOST_PFR_USE_CPP17
+constexpr std::size_t get_field_count_through_for_each_field()
+{
+    std::size_t counter = 0;
+    boost::pfr::for_each_field(simple{}, [&counter](auto&& /*val*/, std::size_t /*i*/) {
+        ++ counter;
+    });
+    return counter;
+}
+static_assert(3 == get_field_count_through_for_each_field());
+#endif
 
 int main () {
     std::size_t control = 0;