Made sure that apply can never move or modify the tuple values; fixes #439 (#440)

Author: liuzicheng1987

docs/named_tuple.md

@@ -103,15 +103,9 @@ auto person = rfl::Field<"first_name", std::string>("Bart") *
               rfl::Field<"last_name", std::string>("Simpson");
 
 person.apply([](const auto& f) {
-  auto field_name = f.name();
-  const auto& value = *f.value();
-});
-
-person.apply([]<typename Field>(Field& f) {
   // The field name can also be obtained as a compile-time constant.
-  constexpr auto field_name = Field::name();
-  using field_pointer_type = typename Field::Type;
-  field_pointer_type* value = f.value();
+  auto field_name = f.name();
+  const auto& value = f.value();
 });
 ```
 

include/rfl/NamedTuple.hpp

@@ -185,17 +185,7 @@ class NamedTuple {
 
   /// Invokes a callable object once for each field in order.
   template <typename F>
-  void apply(F&& _f) {
-    const auto apply_to_field =
-        [&_f]<typename... AFields>(AFields&&... fields) {
-          ((_f(std::forward<AFields>(fields))), ...);
-        };
-    rfl::apply(apply_to_field, fields());
-  }
-
-  /// Invokes a callable object once for each field in order.
-  template <typename F>
-  void apply(F&& _f) const {
+  void apply(F&& _f) const& {
     const auto apply_to_field = [&_f](const auto&... fields) {
       ((_f(fields)), ...);
     };

tests/json/test_apply_on_named_tuple.cpp

@@ -0,0 +1,28 @@
+#include <cassert>
+#include <iostream>
+#include <rfl.hpp>
+#include <rfl/json.hpp>
+#include <string>
+#include <type_traits>
+#include <vector>
+
+#include "write_and_read.hpp"
+
+namespace test_apply_on_named_tuple {
+
+TEST(json, test_apply_on_named_tuple) {
+  auto person = rfl::Field<"first_name", std::string>("Bart") *
+                rfl::Field<"last_name", std::string>("Simpson");
+
+  const auto print = [](const auto& f) {
+    const auto& name = f.name();
+    const auto& value = f.value();
+    std::cout << name << ", " << value << std::endl;
+  };
+
+  person.apply(print);
+
+  EXPECT_EQ(rfl::json::write(person),
+            R"({"first_name":"Bart","last_name":"Simpson"})");
+}
+}  // namespace test_apply_on_named_tuple